Extended Asm (Using the GNU Compiler Collection (GCC))

#### 6.11.2 Extended Asm - Assembler Instructions with C Expression Operands ¶

With extended `asm` you can read and write C variables from assembler and perform jumps from assembler code to C labels. Extended `asm` syntax uses colons (‘`:`’) to delimit the operand parameters after the assembler template:

    asm asm-qualifiers ( AssemblerTemplate
                     : OutputOperands
                     [ : InputOperands
                     [ : Clobbers ] ])

    asm asm-qualifiers ( AssemblerTemplate
                          : OutputOperands
                          : InputOperands
                          : Clobbers
                          : GotoLabels)

where in the last form, asm-qualifiers contains `goto` (and in the first form, not).

The `asm` keyword is a GNU extension. When writing code that can be compiled with `-ansi` and the various `-std` options, use `__asm__` instead of `asm` (see Alternate Keywords).

#### Qualifiers ¶

`volatile`

  The typical use of extended `asm` statements is to manipulate input values to produce output values. However, your `asm` statements may also produce side effects. If so, you may need to use the `volatile` qualifier to disable certain optimizations. See Volatile.

`inline`

  If you use the `inline` qualifier, then for inlining purposes the size of the `asm` statement is taken as the smallest size possible (see Size of an `asm`).

`goto`

  This qualifier informs the compiler that the `asm` statement may perform a jump to one of the labels listed in the GotoLabels. See GotoLabels.

#### Parameters ¶

AssemblerTemplate

  This is a literal string that is the template for the assembler code. It is a combination of fixed text and tokens that refer to the input, output, and goto parameters. See AssemblerTemplate.

OutputOperands

  A comma-separated list describing the C variables modified by the instructions in the AssemblerTemplate. An empty list is permitted. See OutputOperands.

InputOperands

  A comma-separated list describing the C expressions read by the instructions in the AssemblerTemplate. An empty list is permitted. See InputOperands.

Clobbers

  A comma-separated list of registers or other values changed by the AssemblerTemplate, beyond those listed as outputs. An empty list is permitted. See Clobbers and Scratch Registers.

GotoLabels

  When you are using the `goto` form of `asm`, this section contains the list of all C labels to which the code in the AssemblerTemplate may jump. See GotoLabels.

  `asm` statements may not perform jumps into other `asm` statements, only to the listed GotoLabels. GCC’s optimizers do not know about other jumps; therefore they cannot take account of them when deciding how to optimize.

The total number of input + output + goto operands is limited to 30.

#### Remarks ¶

The `asm` statement allows you to include assembly instructions directly within C code. This may help you to maximize performance in time-sensitive code or to access assembly instructions that are not readily available to C programs.

Similarly to basic `asm`, extended `asm` statements may be used both inside a C function or at file scope (“top-level”), where you can use this technique to emit assembler directives, define assembly language macros that can be invoked elsewhere in the file, or write entire functions in assembly language. Extended `asm` statements outside of functions may not use any qualifiers, may not specify clobbers, may not use `%`, `+` or `&` modifiers in constraints and can only use constraints which don’t allow using any register.

Functions declared with the `naked` attribute require basic `asm` (see Common Attributes).

While the uses of `asm` are many and varied, it may help to think of an `asm` statement as a series of low-level instructions that convert input parameters to output parameters. So a simple (if not particularly useful) example for i386 using `asm` might look like this:

    int src = 1;
    int dst;

    asm ("mov %1, %0\n\t"
        "add $1, %0"
        : "=r" (dst)
        : "r" (src));

    printf("%d\n", dst);

This code copies `src` to `dst` and add 1 to `dst`.

#### 6.11.2.1 Volatile ¶

GCC’s optimizers sometimes discard `asm` statements if they determine there is no need for the output variables. Also, the optimizers may move code out of loops if they believe that the code will always return the same result (i.e. none of its input values change between calls). Using the `volatile` qualifier disables these optimizations. `asm` statements that have no output operands and `asm goto` statements, are implicitly volatile.

This i386 code demonstrates a case that does not use (or require) the `volatile` qualifier. If it is performing assertion checking, this code uses `asm` to perform the validation. Otherwise, `dwRes` is unreferenced by any code. As a result, the optimizers can discard the `asm` statement, which in turn removes the need for the entire `DoCheck` routine. By omitting the `volatile` qualifier when it isn’t needed you allow the optimizers to produce the most efficient code possible.

    void DoCheck(uint32_t dwSomeValue)
    {
       uint32_t dwRes;

       // Assumes dwSomeValue is not zero.
       asm ("bsfl %1,%0"
         : "=r" (dwRes)
         : "r" (dwSomeValue)
         : "cc");

       assert(dwRes > 3);
    }

The next example shows a case where the optimizers can recognize that the input (`dwSomeValue`) never changes during the execution of the function and can therefore move the `asm` outside the loop to produce more efficient code. Again, using the `volatile` qualifier disables this type of optimization.

    void do_print(uint32_t dwSomeValue)
    {
       uint32_t dwRes;

       for (uint32_t x=0; x < 5; x++)
       {
          // Assumes dwSomeValue is not zero.
          asm ("bsfl %1,%0"
            : "=r" (dwRes)
            : "r" (dwSomeValue)
            : "cc");

          printf("%u: %u %u\n", x, dwSomeValue, dwRes);
       }
    }

The following example demonstrates a case where you need to use the `volatile` qualifier. It uses the x86 `rdtsc` instruction, which reads the computer’s time-stamp counter. Without the `volatile` qualifier, the optimizers might assume that the `asm` block will always return the same value and therefore optimize away the second call.

    uint64_t msr;

    asm volatile ( "rdtsc\n\t"    // Returns the time in EDX:EAX.
            "shl $32, %%rdx\n\t"  // Shift the upper bits left.
            "or %%rdx, %0"        // 'Or' in the lower bits.
            : "=a" (msr)
            :
            : "rdx");

    printf("msr: %llx\n", msr);

    // Do other work...

    // Reprint the timestamp
    asm volatile ( "rdtsc\n\t"    // Returns the time in EDX:EAX.
            "shl $32, %%rdx\n\t"  // Shift the upper bits left.
            "or %%rdx, %0"        // 'Or' in the lower bits.
            : "=a" (msr)
            :
            : "rdx");

    printf("msr: %llx\n", msr);

GCC’s optimizers do not treat this code like the non-volatile code in the earlier examples. They do not move it out of loops or omit it on the assumption that the result from a previous call is still valid.

Note that the compiler can move even `volatile asm` instructions relative to other code, including across jump instructions. For example, on many targets there is a system register that controls the rounding mode of floating-point operations. Setting it with a `volatile asm` statement, as in the following PowerPC example, does not work reliably.

    asm volatile("mtfsf 255, %0" : : "f" (fpenv));
    sum = x + y;

The compiler may move the addition back before the `volatile asm` statement. To make it work as expected, add an artificial dependency to the `asm` by referencing a variable in the subsequent code, for example:

    asm volatile ("mtfsf 255,%1" : "=X" (sum) : "f" (fpenv));
    sum = x + y;

Under certain circumstances, GCC may duplicate (or remove duplicates of) your assembly code when optimizing. This can lead to unexpected duplicate symbol errors during compilation if your `asm` code defines symbols or labels. Using ‘`%=`’ (see AssemblerTemplate) may help resolve this problem.

#### 6.11.2.2 Assembler Template ¶

An assembler template is a literal string containing assembler instructions. In C++ with `-std=gnu++11` or later, the assembler template can also be a constant expression inside parentheses (see C++11 Constant Expressions instead of String Literals).

The compiler replaces tokens in the template that refer to inputs, outputs, and goto labels, and then outputs the resulting string to the assembler. The string can contain any instructions recognized by the assembler, including directives. GCC does not parse the assembler instructions themselves and does not know what they mean or even whether they are valid assembler input. However, it does count the statements (see Size of an `asm`).

You may place multiple assembler instructions together in a single `asm` string, separated by the characters normally used in assembly code for the system. A combination that works in most places is a newline to break the line, plus a tab character to move to the instruction field (written as ‘`\n\t`’). Some assemblers allow semicolons as a line separator. However, note that some assembler dialects use semicolons to start a comment.

Do not expect a sequence of `asm` statements to remain perfectly consecutive after compilation, even when you are using the `volatile` qualifier. If certain instructions need to remain consecutive in the output, put them in a single multi-instruction `asm` statement.

Accessing data from C programs without using input/output operands (such as by using global symbols directly from the assembler template) may not work as expected. Similarly, calling functions directly from an assembler template requires a detailed understanding of the target assembler and ABI.

Since GCC does not parse the assembler template, it has no visibility of any symbols it references. This may result in GCC discarding those symbols as unreferenced unless they are also listed as input, output, or goto operands.

#### Special format strings ¶

In addition to the tokens described by the input, output, and goto operands, these tokens have special meanings in the assembler template:

‘`%%`’

  Outputs a single ‘`%`’ into the assembler code.

‘`%=`’

  Outputs a number that is unique to each instance of the `asm` statement in the entire compilation. This option is useful when creating local labels and referring to them multiple times in a single template that generates multiple assembler instructions.

‘`%{`’
‘`%|`’
‘`%}`’

  Outputs ‘`{`’, ‘`|`’, and ‘`}`’ characters (respectively) into the assembler code. When unescaped, these characters have special meaning to indicate multiple assembler dialects, as described below.

#### Multiple assembler dialects in `asm` templates ¶

On targets such as x86, GCC supports multiple assembler dialects. The `-masm` option controls which dialect GCC uses as its default for inline assembler. The target-specific documentation for the `-masm` option contains the list of supported dialects, as well as the default dialect if the option is not specified. This information may be important to understand, since assembler code that works correctly when compiled using one dialect will likely fail if compiled using another. See x86 Options.

If your code needs to support multiple assembler dialects (for example, if you are writing public headers that need to support a variety of compilation options), use constructs of this form:

    { dialect0 | dialect1 | dialect2... }

This construct outputs `dialect0` when using dialect #0 to compile the code, `dialect1` for dialect #1, etc. If there are fewer alternatives within the braces than the number of dialects the compiler supports, the construct outputs nothing.

For example, if an x86 compiler supports two dialects (‘`att`’, ‘`intel`’), an assembler template such as this:

    "bt{l %[Offset],%[Base] | %[Base],%[Offset]}; jc %l2"

is equivalent to one of

    "btl %[Offset],%[Base] ; jc %l2"   /* att dialect */
    "bt %[Base],%[Offset]; jc %l2"     /* intel dialect */

Using that same compiler, this code:

    "xchg{l}\t{%%}ebx, %1"

corresponds to either

    "xchgl\t%%ebx, %1"                 /* att dialect */
    "xchg\tebx, %1"                    /* intel dialect */

There is no support for nesting dialect alternatives.

#### 6.11.2.3 Output Operands ¶

An `asm` statement has zero or more output operands indicating the names of C variables modified by the assembler code.

In this i386 example, `old` (referred to in the template string as `%0`) and `*Base` (as `%1`) are outputs and `Offset` (`%2`) is an input:

    bool old;

    __asm__ ("btsl %2,%1\n\t" // Turn on zero-based bit #Offset in Base.
             "sbb %0,%0"      // Use the CF to calculate old.
       : "=r" (old), "+rm" (*Base)
       : "Ir" (Offset)
       : "cc");

    return old;

Operands are separated by commas. Each operand has this format:

    [ [asmSymbolicName] ] constraint (cvariablename)

asmSymbolicName

  Specifies an optional symbolic name for the operand. The literal square brackets ‘`[]`’ around the asmSymbolicName are required both in the operand specification and references to the operand in the assembler template, i.e. ‘`%[Value]`’. The scope of the name is the `asm` statement that contains the definition. Any valid C identifier is acceptable, including names already defined in the surrounding code. No two operands within the same `asm` statement can use the same symbolic name.

  When not using an asmSymbolicName, use the (zero-based) position of the operand in the list of operands in the assembler template. For example if there are three output operands, use ‘`%0`’ in the template to refer to the first, ‘`%1`’ for the second, and ‘`%2`’ for the third.

constraint

  A string constant specifying constraints on the placement of the operand; See Constraints for `asm` Operands, for details. In C++ with `-std=gnu++11` or later, the constraint can also be a constant expression inside parentheses (see C++11 Constant Expressions instead of String Literals).

  Output constraints must begin with either ‘`=`’ (a variable overwriting an existing value) or ‘`+`’ (when reading and writing). When using ‘`=`’, do not assume the location contains the existing value on entry to the `asm`, except when the operand is tied to an input; see Input Operands.

  After the prefix, there must be one or more additional constraints (see Constraints for `asm` Operands) that describe where the value resides. Common constraints include ‘`r`’ for register and ‘`m`’ for memory. When you list more than one possible location (for example, `"=rm"`), the compiler chooses the most efficient one based on the current context. If you list as many alternates as the `asm` statement allows, you permit the optimizers to produce the best possible code. If you must use a specific register, but your Machine Constraints do not provide sufficient control to select the specific register you want, local register variables may provide a solution (see Specifying Registers for Local Variables).

cvariablename

  Specifies a C lvalue expression to hold the output, typically a variable name. The enclosing parentheses are a required part of the syntax.

When the compiler selects the registers to use to represent the output operands, it does not use any of the clobbered registers (see Clobbers and Scratch Registers).

Output operand expressions must be lvalues. The compiler cannot check whether the operands have data types that are reasonable for the instruction being executed. For output expressions that are not directly addressable (for example a bit-field), the constraint must allow a register. In that case, GCC uses the register as the output of the `asm`, and then stores that register into the output.

Operands using the ‘`+`’ constraint modifier count as two operands (that is, both as input and output) towards the total maximum of 30 operands per `asm` statement.

Use the ‘`&`’ constraint modifier (see Constraint Modifier Characters) on all output operands that must not overlap an input. Otherwise, GCC may allocate the output operand in the same register as an unrelated input operand, on the assumption that the assembler code consumes its inputs before producing outputs. This assumption may be false if the assembler code actually consists of more than one instruction.

The same problem can occur if one output parameter (a) allows a register constraint and another output parameter (b) allows a memory constraint. The code generated by GCC to access the memory address in b can contain registers which might be shared by a, and GCC considers those registers to be inputs to the asm. As above, GCC assumes that such input registers are consumed before any outputs are written. This assumption may result in incorrect behavior if the `asm` statement writes to a before using b. Combining the ‘`&`’ modifier with the register constraint on a ensures that modifying a does not affect the address referenced by b. Otherwise, the location of b is undefined if a is modified before using b.

`asm` supports operand modifiers on operands (for example ‘`%k2`’ instead of simply ‘`%2`’). Generic Operand modifiers lists the modifiers that are available on all targets. Other modifiers are hardware dependent. For example, the list of supported modifiers for x86 is found at x86 Operand modifiers.

If the C code that follows the `asm` makes no use of any of the output operands, use `volatile` for the `asm` statement to prevent the optimizers from discarding the `asm` statement as unneeded (see Volatile).

This code makes no use of the optional asmSymbolicName. Therefore it references the first output operand as `%0` (were there a second, it would be `%1`, etc). The number of the first input operand is one greater than that of the last output operand. In this i386 example, that makes `Mask` referenced as `%1`:

    uint32_t Mask = 1234;
    uint32_t Index;

      asm ("bsfl %1, %0"
         : "=r" (Index)
         : "r" (Mask)
         : "cc");

That code overwrites the variable `Index` (‘`=`’), placing the value in a register (‘`r`’). Using the generic ‘`r`’ constraint instead of a constraint for a specific register allows the compiler to pick the register to use, which can result in more efficient code. This may not be possible if an assembler instruction requires a specific register.

The following i386 example uses the asmSymbolicName syntax. It produces the same result as the code above, but some may consider it more readable or more maintainable since reordering index numbers is not necessary when adding or removing operands. The names `aIndex` and `aMask` are only used in this example to emphasize which names get used where. It is acceptable to reuse the names `Index` and `Mask`.

    uint32_t Mask = 1234;
    uint32_t Index;

      asm ("bsfl %[aMask], %[aIndex]"
         : [aIndex] "=r" (Index)
         : [aMask] "r" (Mask)
         : "cc");

Here are some more examples of output operands.

    uint32_t c = 1;
    uint32_t d;
    uint32_t *e = &c;

    asm ("mov %[e], %[d]"
       : [d] "=rm" (d)
       : [e] "rm" (*e));

Here, `d` may either be in a register or in memory. Since the compiler might already have the current value of the `uint32_t` location pointed to by `e` in a register, you can enable it to choose the best location for `d` by specifying both constraints.

#### 6.11.2.4 Flag Output Operands ¶

Some targets have a special register that holds the “flags” for the result of an operation or comparison. Normally, the contents of that register are either unmodified by the asm, or the `asm` statement is considered to clobber the contents.

On some targets, a special form of output operand exists by which conditions in the flags register may be outputs of the asm. The set of conditions supported are target specific, but the general rule is that the output variable must be a scalar integer, and the value is boolean. When supported, the target defines the preprocessor symbol `__GCC_ASM_FLAG_OUTPUTS__`.

Because of the special nature of the flag output operands, the constraint may not include alternatives.

Most often, the target has only one flags register, and thus is an implied operand of many instructions. In this case, the operand should not be referenced within the assembler template via `%0` etc, as there’s no corresponding text in the assembly language.

ARM
AArch64

  The flag output constraints for the ARM family are of the form ‘`=@cccond`’ where cond is one of the standard conditions defined in the ARM ARM for `ConditionHolds`.

  `eq`

    Z flag set, or equal

  `ne`

    Z flag clear or not equal

  `cs`
  `hs`

    C flag set or unsigned greater than equal

  `cc`
  `lo`

    C flag clear or unsigned less than

  `mi`

    N flag set or “minus”

  `pl`

    N flag clear or “plus”

  `vs`

    V flag set or signed overflow

  `vc`

    V flag clear

  `hi`

    unsigned greater than

  `ls`

    unsigned less than equal

  `ge`

    signed greater than equal

  `lt`

    signed less than

  `gt`

    signed greater than

  `le`

    signed less than equal

  The flag output constraints are not supported in thumb1 mode.

x86 family

  The flag output constraints for the x86 family are of the form ‘`=@cccond`’ where cond is one of the standard conditions defined in the ISA manual for `jcc` or `setcc`.

  `a`

    “above” or unsigned greater than

  `ae`

    “above or equal” or unsigned greater than or equal

  `b`

    “below” or unsigned less than

  `be`

    “below or equal” or unsigned less than or equal

  `c`

    carry flag set

  `e`
  `z`

    “equal” or zero flag set

  `g`

    signed greater than

  `ge`

    signed greater than or equal

  `l`

    signed less than

  `le`

    signed less than or equal

  `o`

    overflow flag set

  `p`

    parity flag set

  `s`

    sign flag set

  `na`
  `nae`
  `nb`
  `nbe`
  `nc`
  `ne`
  `ng`
  `nge`
  `nl`
  `nle`
  `no`
  `np`
  `ns`
  `nz`

    “not” flag, or inverted versions of those above

s390

  The flag output constraint for s390 is ‘`=@cc`’. Only one such constraint is allowed. The variable has to be stored in a ‘`int`’ variable.

#### 6.11.2.5 Input Operands ¶

Input operands make values from C variables and expressions available to the assembly code.

Operands are separated by commas. Each operand has this format:

    [ [asmSymbolicName] ] constraint (cexpression)

asmSymbolicName

  Specifies an optional symbolic name for the operand. The literal square brackets ‘`[]`’ around the asmSymbolicName are required both in the operand specification and references to the operand in the assembler template, i.e. ‘`%[Value]`’. The scope of the name is the `asm` statement that contains the definition. Any valid C identifier is acceptable, including names already defined in the surrounding code. No two operands within the same `asm` statement can use the same symbolic name.

  When not using an asmSymbolicName, use the (zero-based) position of the operand in the list of operands in the assembler template. For example if there are two output operands and three inputs, use ‘`%2`’ in the template to refer to the first input operand, ‘`%3`’ for the second, and ‘`%4`’ for the third.

constraint

  A string constant specifying constraints on the placement of the operand; See Constraints for `asm` Operands, for details. In C++ with `-std=gnu++11` or later, the constraint can also be a constant expression inside parentheses (see C++11 Constant Expressions instead of String Literals).

  Input constraint strings may not begin with either ‘`=`’ or ‘`+`’. When you list more than one possible location (for example, ‘`"irm"`’), the compiler chooses the most efficient one based on the current context. If you must use a specific register, but your Machine Constraints do not provide sufficient control to select the specific register you want, local register variables may provide a solution (see Specifying Registers for Local Variables).

  Input constraints can also be digits (for example, `"0"`). This indicates that the specified input must be in the same place as the output constraint at the (zero-based) index in the output constraint list. When using asmSymbolicName syntax for the output operands, you may use these names (enclosed in brackets ‘`[]`’) instead of digits.

cexpression

  This is the C variable or expression being passed to the `asm` statement as input. The enclosing parentheses are a required part of the syntax.

When the compiler selects the registers to use to represent the input operands, it does not use any of the clobbered registers (see Clobbers and Scratch Registers).

If there are no output operands but there are input operands, place two consecutive colons where the output operands would go:

    __asm__ ("some instructions"
       : /* No outputs. */
       : "r" (Offset / 8));

Warning: Do not modify the contents of input-only operands (except for inputs tied to outputs). The compiler assumes that on exit from the `asm` statement these operands contain the same values as they had before executing the statement. It is not possible to use clobbers to inform the compiler that the values in these inputs are changing. One common work-around is to tie the changing input variable to an output variable that never gets used. Note, however, that if the code that follows the `asm` statement makes no use of any of the output operands, the GCC optimizers may discard the `asm` statement as unneeded (see Volatile).

`asm` supports operand modifiers on operands (for example ‘`%k2`’ instead of simply ‘`%2`’). Generic Operand modifiers lists the modifiers that are available on all targets. Other modifiers are hardware dependent. For example, the list of supported modifiers for x86 is found at x86 Operand modifiers.

In this example using the fictitious `combine` instruction, the constraint `"0"` for input operand 1 says that it must occupy the same location as output operand 0. Only input operands may use numbers in constraints, and they must each refer to an output operand. Only a number (or the symbolic assembler name) in the constraint can guarantee that one operand is in the same place as another. The mere fact that `foo` is the value of both operands is not enough to guarantee that they are in the same place in the generated assembler code.

    asm ("combine %2, %0"
       : "=r" (foo)
       : "0" (foo), "g" (bar));

Here is an example using symbolic names.

    asm ("cmoveq %1, %2, %[result]"
       : [result] "=r"(result)
       : "r" (test), "r" (new), "[result]" (old));

#### 6.11.2.6 Clobbers and Scratch Registers ¶

While the compiler is aware of changes to entries listed in the output operands, the inline `asm` code may modify more than just the outputs. For example, calculations may require additional registers, or the processor may overwrite a register as a side effect of a particular assembler instruction. In order to inform the compiler of these changes, list them in the clobber list. Clobber list items are either register names or the special clobbers (listed below). Each clobber list item is a string constant enclosed in double quotes and separated by commas. In C++ with `-std=gnu++11` or later, a clobber list item can also be a constant expression inside parentheses (see C++11 Constant Expressions instead of String Literals).

Clobber descriptions may not in any way overlap with an input or output operand. For example, you may not have an operand describing a register class with one member when listing that register in the clobber list. Variables declared to live in specific registers (see Variables in Specified Registers) and used as `asm` input or output operands must have no part mentioned in the clobber description. In particular, there is no way to specify that input operands get modified without also specifying them as output operands.

When the compiler selects which registers to use to represent input and output operands, it does not use any of the clobbered registers. As a result, clobbered registers are available for any use in the assembler code.

Another restriction is that the clobber list should not contain the stack pointer register. This is because the compiler requires the value of the stack pointer to be the same after an `asm` statement as it was on entry to the statement. However, previous versions of GCC did not enforce this rule and allowed the stack pointer to appear in the list, with unclear semantics. This behavior is deprecated and listing the stack pointer may become an error in future versions of GCC.

Here is a realistic example for the VAX showing the use of clobbered registers:

    asm volatile ("movc3 %0, %1, %2"
                       : /* No outputs. */
                       : "g" (from), "g" (to), "g" (count)
                       : "r0", "r1", "r2", "r3", "r4", "r5", "memory");

Also, there are three special clobber arguments:

`"cc"`

  The `"cc"` clobber indicates that the assembler code modifies the flags register. On some machines, GCC represents the condition codes as a specific hardware register; `"cc"` serves to name this register. On other machines, condition code handling is different, and specifying `"cc"` has no effect. But it is valid no matter what the target.

`"memory"`

  The `"memory"` clobber tells the compiler that the assembly code performs memory reads or writes to items other than those listed in the input and output operands (for example, accessing the memory pointed to by one of the input parameters). To ensure memory contains correct values, GCC may need to flush specific register values to memory before executing the `asm`. Further, the compiler does not assume that any values read from memory before an `asm` remain unchanged after that `asm`; it reloads them as needed. Using the `"memory"` clobber effectively forms a read/write memory barrier for the compiler.

  Note that this clobber does not prevent the processor from doing speculative reads past the `asm` statement. To prevent that, you need processor-specific fence instructions.

`"redzone"`

  The `"redzone"` clobber tells the compiler that the assembly code may write to the stack red zone, area below the stack pointer which on some architectures in some calling conventions is guaranteed not to be changed by signal handlers, interrupts or exceptions and so the compiler can store there temporaries in leaf functions. On targets which have no concept of the stack red zone, the clobber is ignored. It should be used e.g. in case the assembly code uses call instructions or pushes something to the stack without taking the red zone into account by subtracting red zone size from the stack pointer first and restoring it afterwards.

Flushing registers to memory has performance implications and may be an issue for time-sensitive code. You can provide better information to GCC to avoid this, as shown in the following examples. At a minimum, aliasing rules allow GCC to know what memory doesn’t need to be flushed.

Here is a fictitious sum of squares instruction, that takes two pointers to floating point values in memory and produces a floating point register output. Notice that `x`, and `y` both appear twice in the `asm` parameters, once to specify memory accessed, and once to specify a base register used by the `asm`. You won’t normally be wasting a register by doing this as GCC can use the same register for both purposes. However, it would be foolish to use both `%1` and `%3` for `x` in this `asm` and expect them to be the same. In fact, `%3` may well not be a register. It might be a symbolic memory reference to the object pointed to by `x`.

    asm ("sumsq %0, %1, %2"
         : "+f" (result)
         : "r" (x), "r" (y), "m" (*x), "m" (*y));

Here is a fictitious `*z++ = *x++ * *y++` instruction. Notice that the `x`, `y` and `z` pointer registers must be specified as input/output because the `asm` modifies them.

    asm ("vecmul %0, %1, %2"
         : "+r" (z), "+r" (x), "+r" (y), "=m" (*z)
         : "m" (*x), "m" (*y));

An x86 example where the string memory argument is of unknown length.

    asm("repne scasb"
        : "=c" (count), "+D" (p)
        : "m" (*(const char (*)[]) p), "0" (-1), "a" (0));

If you know the above will only be reading a ten byte array then you could instead use a memory input like: `"m" (*(const char (*)[10]) p)`.

Here is an example of a PowerPC vector scale implemented in assembly, complete with vector and condition code clobbers, and some initialized offset registers that are unchanged by the `asm`.

    void
    dscal (size_t n, double *x, double alpha)
    {
      asm ("/* lots of asm here */"
           : "+m" (*(double (*)[n]) x), "+&r" (n), "+b" (x)
           : "d" (alpha), "b" (32), "b" (48), "b" (64),
             "b" (80), "b" (96), "b" (112)
           : "cr0",
             "vs32","vs33","vs34","vs35","vs36","vs37","vs38","vs39",
             "vs40","vs41","vs42","vs43","vs44","vs45","vs46","vs47");
    }

Rather than allocating fixed registers via clobbers to provide scratch registers for an `asm` statement, an alternative is to define a variable and make it an early-clobber output as with `a2` and `a3` in the example below. This gives the compiler register allocator more freedom. You can also define a variable and make it an output tied to an input as with `a0` and `a1`, tied respectively to `ap` and `lda`. Of course, with tied outputs your `asm` can’t use the input value after modifying the output register since they are one and the same register. What’s more, if you omit the early-clobber on the output, it is possible that GCC might allocate the same register to another of the inputs if GCC could prove they had the same value on entry to the `asm`. This is why `a1` has an early-clobber. Its tied input, `lda` might conceivably be known to have the value 16 and without an early-clobber share the same register as `%11`. On the other hand, `ap` can’t be the same as any of the other inputs, so an early-clobber on `a0` is not needed. It is also not desirable in this case. An early-clobber on `a0` would cause GCC to allocate a separate register for the `"m" (*(const double (*)[]) ap)` input. Note that tying an input to an output is the way to set up an initialized temporary register modified by an `asm` statement. An input not tied to an output is assumed by GCC to be unchanged, for example `"b" (16)` below sets up `%11` to 16, and GCC might use that register in following code if the value 16 happened to be needed. You can even use a normal `asm` output for a scratch if all inputs that might share the same register are consumed before the scratch is used. The VSX registers clobbered by the `asm` statement could have used this technique except for GCC’s limit on the number of `asm` parameters.

    static void
    dgemv_kernel_4x4 (long n, const double *ap, long lda,
                      const double *x, double *y, double alpha)
    {
      double *a0;
      double *a1;
      double *a2;
      double *a3;

      __asm__
        (
         /* lots of asm here */
         "#n=%1 ap=%8=%12 lda=%13 x=%7=%10 y=%0=%2 alpha=%9 o16=%11\n"
         "#a0=%3 a1=%4 a2=%5 a3=%6"
         :
           "+m" (*(double (*)[n]) y),
           "+&r" (n),	// 1
           "+b" (y),	// 2
           "=b" (a0),	// 3
           "=&b" (a1),	// 4
           "=&b" (a2),	// 5
           "=&b" (a3)	// 6
         :
           "m" (*(const double (*)[n]) x),
           "m" (*(const double (*)[]) ap),
           "d" (alpha),	// 9
           "r" (x),		// 10
           "b" (16),	// 11
           "3" (ap),	// 12
           "4" (lda)	// 13
         :
           "cr0",
           "vs32","vs33","vs34","vs35","vs36","vs37",
           "vs40","vs41","vs42","vs43","vs44","vs45","vs46","vs47"
         );
    }

#### 6.11.2.7 Goto Labels ¶

`asm goto` allows assembly code to jump to one or more C labels. The GotoLabels section in an `asm goto` statement contains a comma-separated list of all C labels to which the assembler code may jump. GCC assumes that `asm` execution falls through to the next statement (if this is not the case, consider using the `__builtin_unreachable` intrinsic after the `asm` statement). Optimization of `asm goto` may be improved by using the `hot` and `cold` label attributes (see Common Attributes).

If the assembler code does modify anything, use the `"memory"` clobber to force the optimizers to flush all register values to memory and reload them if necessary after the `asm` statement.

Also note that an `asm goto` statement is always implicitly considered volatile.

Be careful when you set output operands inside `asm goto` only on some possible control flow paths. If you don’t set up the output on given path and never use it on this path, it is okay. Otherwise, you should use ‘`+`’ constraint modifier meaning that the operand is input and output one. With this modifier you will have the correct values on all possible paths from the `asm goto`.

To reference a label in the assembler template, prefix it with ‘`%l`’ (lowercase ‘`L`’) followed by its (zero-based) position in GotoLabels plus the number of input and output operands. Output operand with constraint modifier ‘`+`’ is counted as two operands because it is considered as one output and one input operand. For example, if the `asm` has three inputs, one output operand with constraint modifier ‘`+`’ and one output operand with constraint modifier ‘`=`’ and references two labels, refer to the first label as ‘`%l6`’ and the second as ‘`%l7`’).

Alternately, you can reference labels using the actual C label name enclosed in brackets. For example, to reference a label named `carry`, you can use ‘`%l[carry]`’. The label must still be listed in the GotoLabels section when using this approach. It is better to use the named references for labels as in this case you can avoid counting input and output operands and special treatment of output operands with constraint modifier ‘`+`’.

Here is an example of `asm goto` for i386:

    asm goto (
        "btl %1, %0\n\t"
        "jc %l2"
        : /* No outputs. */
        : "r" (p1), "r" (p2)
        : "cc"
        : carry);

    return 0;

    carry:
    return 1;

The following example shows an `asm goto` that uses a memory clobber.

    int frob(int x)
    {
      int y;
      asm goto ("frob %%r5, %1; jc %l[error]; mov (%2), %%r5"
                : /* No outputs. */
                : "r"(x), "r"(&y)
                : "r5", "memory"
                : error);
      return y;
    error:
      return -1;
    }

The following example shows an `asm goto` that uses an output.

    int foo(int count)
    {
      asm goto ("dec %0; jb %l[stop]"
                : "+r" (count)
                :
                :
                : stop);
      return count;
    stop:
      return 0;
    }

The following artificial example shows an `asm goto` that sets up an output only on one path inside the `asm goto`. Usage of constraint modifier ‘`=`’ instead of ‘`+`’ would be wrong as `factor` is used on all paths from the `asm goto`.

    int foo(int inp)
    {
      int factor = 0;
      asm goto ("cmp %1, 10; jb %l[lab]; mov 2, %0"
                : "+r" (factor)
                : "r" (inp)
                :
                : lab);
    lab:
      return inp * factor; /* return 2 * inp or 0 if inp < 10 */
    }

#### 6.11.2.8 Generic Operand Modifiers ¶

The following table shows the modifiers supported by all targets and their effects:

Modifier | Description | Example
--- | --- | ---
`c` | Require a constant operand and print the constant expression with no punctuation. | `%c0`
`cc` | Like ‘`%c`’ except try harder to print it with no punctuation. ‘`%c`’ can e.g. fail to print constant addresses in position independent code on some architectures. | `%cc0`
`n` | Like ‘`%c`’ except that the value of the constant is negated before printing. | `%n0`
`a` | Substitute a memory reference, with the actual operand treated as the address. This may be useful when outputting a “load address” instruction, because often the assembler syntax for such an instruction requires you to write the operand as if it were a memory reference. | `%a0`
`l` | Print the label name with no punctuation. | `%l0`

#### 6.11.2.9 AArch64 Operand Modifiers ¶

The following table shows the modifiers supported by AArch64 and their effects:

Modifier | Description
--- | ---
`w` | Print a 32-bit general-purpose register name or, given a constant zero operand, the 32-bit zero register (`wzr`).
`x` | Print a 64-bit general-purpose register name or, given a constant zero operand, the 64-bit zero register (`xzr`).
`b` | Print an FP/SIMD register name with a `b` (byte, 8-bit) prefix.
`h` | Print an FP/SIMD register name with an `h` (halfword, 16-bit) prefix.
`s` | Print an FP/SIMD register name with an `s` (single word, 32-bit) prefix.
`d` | Print an FP/SIMD register name with a `d` (doubleword, 64-bit) prefix.
`q` | Print an FP/SIMD register name with a `q` (quadword, 128-bit) prefix.
`Z` | Print an FP/SIMD register name as an SVE register (i.e. with a `z` prefix). This is a no-op for SVE register operands.

#### 6.11.2.10 x86 Operand Modifiers ¶

References to input, output, and goto operands in the assembler template of extended `asm` statements can use modifiers to affect the way the operands are formatted in the code output to the assembler. For example, the following code uses the ‘`h`’ and ‘`b`’ modifiers for x86:

    uint16_t  num;
    asm volatile ("xchg %h0, %b0" : "+a" (num) );

These modifiers generate this assembler code:

    xchg %ah, %al

The rest of this discussion uses the following code for illustrative purposes.

    int main()
    {
       int iInt = 1;

    top:

       asm volatile goto ("some assembler instructions here"
       : /* No outputs. */
       : "q" (iInt), "X" (sizeof(unsigned char) + 1), "i" (42)
       : /* No clobbers. */
       : top);
    }

With no modifiers, this is what the output from the operands would be for the ‘`att`’ and ‘`intel`’ dialects of assembler:

Operand | ‘`att`’ | ‘`intel`’
--- | --- | ---
`%0` | `%eax` | `eax`
`%1` | `$2` | `2`
`%3` | `$.L3` | `OFFSET FLAT:.L3`

The table below shows the list of supported modifiers and their effects.

Modifier | Description | Operand | ‘`att`’ | ‘`intel`’
--- | --- | --- | --- | ---
`A` | Print an absolute memory reference. | `%A0` | `*%rax` | `rax`
`b` | Print the QImode name of the register. | `%b0` | `%al` | `al`
`c` | Require a constant operand and print the constant expression with no punctuation. | `%c1` | `2` | `2`
`E` | Print the address in Double Integer (DImode) mode (8 bytes) when the target is 64-bit. Otherwise mode is unspecified (VOIDmode). | `%E1` | `%(rax)` | `[rax]`
`h` | Print the QImode name for a “high” register. | `%h0` | `%ah` | `ah`
`H` | Add 8 bytes to an offsettable memory reference. Useful when accessing the high 8 bytes of SSE values. For a memref in (%rax), it generates | `%H0` | `8(%rax)` | `8[rax]`
`k` | Print the SImode name of the register. | `%k0` | `%eax` | `eax`
`l` | Print the label name with no punctuation. | `%l3` | `.L3` | `.L3`
`p` | Print raw symbol name (without syntax-specific prefixes). | `%p2` | `42` | `42`
`P` | If used for a function, print the PLT suffix and generate PIC code. For example, emit `foo@PLT` instead of ’foo’ for the function foo(). If used for a constant, drop all syntax-specific prefixes and issue the bare constant. See `p` above.
`q` | Print the DImode name of the register. | `%q0` | `%rax` | `rax`
`w` | Print the HImode name of the register. | `%w0` | `%ax` | `ax`
`z` | Print the opcode suffix for the size of the current integer operand (one of `b`/`w`/`l`/`q`). | `%z0` | `l` |

`V` is a special modifier which prints the name of the full integer register without `%`.

#### 6.11.2.11 x86 Floating-Point `asm` Operands ¶

On x86 targets, there are several rules on the usage of stack-like registers in the operands of an `asm`. These rules apply only to the operands that are stack-like registers:

1. Given a set of input registers that die in an `asm`, it is necessary to know which are implicitly popped by the `asm`, and which must be explicitly popped by GCC.

   An input register that is implicitly popped by the `asm` must be explicitly clobbered, unless it is constrained to match an output operand.

2. For any input register that is implicitly popped by an `asm`, it is necessary to know how to adjust the stack to compensate for the pop. If any non-popped input is closer to the top of the reg-stack than the implicitly popped register, it would not be possible to know what the stack looked like—it’s not clear how the rest of the stack “slides up”.

   All implicitly popped input registers must be closer to the top of the reg-stack than any input that is not implicitly popped.

   It is possible that if an input dies in an `asm`, the compiler might use the input register for an output reload. Consider this example:

       asm ("foo" : "=t" (a) : "f" (b));

   This code says that input `b` is not popped by the `asm`, and that the `asm` pushes a result onto the reg-stack, i.e., the stack is one deeper after the `asm` than it was before. But, it is possible that reload may think that it can use the same register for both the input and the output.

   To prevent this from happening, if any input operand uses the ‘`f`’ constraint, all output register constraints must use the ‘`&`’ early-clobber modifier.

   The example above is correctly written as:

       asm ("foo" : "=&t" (a) : "f" (b));

3. Some operands need to be in particular places on the stack. All output operands fall in this category—GCC has no other way to know which registers the outputs appear in unless you indicate this in the constraints.

   Output operands must specifically indicate which register an output appears in after an `asm`. ‘`=f`’ is not allowed: the operand constraints must select a class with a single register.

4. Output operands may not be “inserted” between existing stack registers. Since no 387 opcode uses a read/write operand, all output operands are dead before the `asm`, and are pushed by the `asm`. It makes no sense to push anywhere but the top of the reg-stack.

   Output operands must start at the top of the reg-stack: output operands may not “skip” a register.

5. Some `asm` statements may need extra stack space for internal calculations. This can be guaranteed by clobbering stack registers unrelated to the inputs and outputs.

This `asm` takes one input, which is internally popped, and produces two outputs.

    asm ("fsincos" : "=t" (cos), "=u" (sin) : "0" (inp));

This `asm` takes two inputs, which are popped by the `fyl2xp1` opcode, and replaces them with one output. The `st(1)` clobber is necessary for the compiler to know that `fyl2xp1` pops both inputs.

    asm ("fyl2xp1" : "=t" (result) : "0" (x), "u" (y) : "st(1)");

#### 6.11.2.12 MSP430 Operand Modifiers ¶

The list below describes the supported modifiers and their effects for MSP430.

Modifier | Description
--- | ---
`A` | Select low 16-bits of the constant/register/memory operand.
`B` | Select high 16-bits of the constant/register/memory operand.
`C` | Select bits 32-47 of the constant/register/memory operand.
`D` | Select bits 48-63 of the constant/register/memory operand.
`H` | Equivalent to `B` (for backwards compatibility).
`I` | Print the inverse (logical `NOT`) of the constant value.
`J` | Print an integer without a `#` prefix.
`L` | Equivalent to `A` (for backwards compatibility).
`O` | Offset of the current frame from the top of the stack.
`Q` | Use the `A` instruction postfix.
`R` | Inverse of condition code, for unsigned comparisons.
`W` | Subtract 16 from the constant value.
`X` | Use the `X` instruction postfix.
`Y` | Subtract 4 from the constant value.
`Z` | Subtract 1 from the constant value.
`b` | Append `.B`, `.W` or `.A` to the instruction, depending on the mode.
`d` | Offset 1 byte of a memory reference or constant value.
`e` | Offset 3 bytes of a memory reference or constant value.
`f` | Offset 5 bytes of a memory reference or constant value.
`g` | Offset 7 bytes of a memory reference or constant value.
`p` | Print the value of 2, raised to the power of the given constant. Used to select the specified bit position.
`r` | Inverse of condition code, for signed comparisons.
`x` | Equivalent to `X`, but only for pointers.

#### 6.11.2.13 LoongArch Operand Modifiers ¶

The list below describes the supported modifiers and their effects for LoongArch.

Modifier | Description
--- | ---
`d` | Same as `c`.
`i` | Print the character ”`i`” if the operand is not a register.
`m` | Same as `c`, but the printed value is `operand - 1`.
`u` | Print a LASX register.
`w` | Print a LSX register.
`X` | Print a constant integer operand in hexadecimal.
`z` | Print the operand in its unmodified form, followed by a comma.

References to input and output operands in the assembler template of extended asm statements can use modifiers to affect the way the operands are formatted in the code output to the assembler. For example, the following code uses the ’w’ modifier for LoongArch:

    test-asm.c:

    #include <lsxintrin.h>

    __m128i foo (void)
    {
    __m128i  a,b,c;
    __asm__ ("vadd.d %w0,%w1,%w2\n\t"
       :"=f" (c)
       :"f" (a),"f" (b));

    return c;
    }

The compile command for the test case is as follows:

    gcc test-asm.c -mlsx -S -o test-asm.s

The assembly statement produces the following assembly code:

    vadd.d $vr0,$vr0,$vr1

This is a 128-bit vector addition instruction, `c` (referred to in the template string as %0) is the output, and `a` (%1) and `b` (%2) are the inputs. `__m128i` is a vector data type defined in the file `lsxintrin.h` (See LoongArch SX Vector Intrinsics). The symbol ’=f’ represents a constraint using a floating-point register as an output type, and the ’f’ in the input operand represents a constraint using a floating-point register operand, which can refer to the definition of a constraint (See Constraints for `asm` Operands) in gcc.

#### 6.11.2.14 RISC-V Operand Modifiers ¶

The list below describes the supported modifiers and their effects for RISC-V.

Modifier | Description
--- | ---
`z` | Print ”`zero`” instead of 0 if the operand is an immediate with a value of zero.
`i` | Print the character ”`i`” if the operand is an immediate.
`N` | Print the register encoding as integer (0 - 31).
`H` | Print the name of the next register for integer.

#### 6.11.2.15 SH Operand Modifiers ¶

The list below describes the supported modifiers and their effects for the SH family of processors.

Modifier | Description
--- | ---
`.` | Print ”`.s`” if the instruction needs a delay slot.
`,` | Print ”`LOCAL_LABEL_PREFIX`”.
`@` | Print ”`trap`”, ”`rte`” or ”`rts`” depending on the interrupt pragma used.
`#` | Print ”`nop`” if there is nothing to put in the delay slot.
`'` | Print likelihood suffix (”`/u`” for unlikely).
`>` | Print branch target if ”`-fverbose-asm`”.
`O` | Require a constant operand and print the constant expression with no punctuation.
`R` | Print the ”`LSW`” of a dp value - changes if in little endian.
`S` | Print the ”`MSW`” of a dp value - changes if in little endian.
`T` | Print the next word of a dp value - same as ”`R`” in big endian mode.
`M` | Print ”`.b` ”, ”`.w`”, ”`.l`”, ”`.s`”, ”`.d`”, suffix if operand is a MEM.
`N` | Print ”`r63`” if the operand is ”`const_int 0`”.
`d` | Print a ”`V2SF`” as ”`dN`” instead of ”`fpN`”.
`m` | Print the pair ”`base,offset`” or ”`base,index`” for LD and ST.
`U` | Like ”`%m`” for ”`LD`” and ”`ST`”, ”`HI`” and ”`LO`”.
`V` | Print the position of a single bit set.
`W` | Print the position of a single bit cleared.
`t` | Print a memory address which is a register.
`u` | Print the lowest 16 bits of ”`CONST_INT`”, as an unsigned value.
`o` | Print an operator.