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10.6.1 The Clobber Parameter ¶

One of the dangers of intermixing assembly language and a compiled language such as Ada is that the compiler needs to be aware of which registers are being used by the assembly code. In some cases, such as the earlier examples, the constraint string is sufficient to indicate register usage (e.g., "a" for the eax register). But, more generally, the compiler needs an explicit identification of the registers that are used by the Inline Assembly statements.

Using a register that the compiler doesn’t know about could be a side effect of an instruction (like mull, which stores its result into both eax and edx). It can also arise from explicit register usage within your assembly code; for example:

Asm ("movl %0, %%ebx" & LF & HT &
     "movl %%ebx, %1",
     Outputs => Unsigned_32'Asm_Output ("=g", Var_Out),
     Inputs  => Unsigned_32'Asm_Input  ("g", Var_In));

where the compiler (since it does not analyze the Asm template string) does not know you are using the ebx register.

In such cases you need to supply the Clobber parameter to Asm, to identify the registers used by your assembly code:

Asm ("movl %0, %%ebx" & LF & HT &
     "movl %%ebx, %1",
     Outputs => Unsigned_32'Asm_Output ("=g", Var_Out),
     Inputs  => Unsigned_32'Asm_Input  ("g", Var_In),
     Clobber => "ebx");

The Clobber parameter is a static string expression specifying the register(s) you are using. Note that register names are ‘not’ prefixed by a percent sign. Also, if more than one register is used, you separate their names by commas; e.g., "eax, ebx"

The Clobber parameter has several additional uses:

• Use ‘register’ name cc to indicate that flags might have changed
• Use ‘register’ name memory if you changed a memory location