RISC-V Attributes (Using the GNU Compiler Collection (GCC))

#### 6.4.2.22 RISC-V Attributes ¶

These attributes are supported by the RISC-V back end:

`interrupt` ¶

  This attribute applies to functions.

  It indicates that the specified function is an interrupt handler. The compiler generates function entry and exit sequences suitable for use in an interrupt handler when this attribute is present.

  You can specify the kind of interrupt to be handled by adding an optional parameter to the interrupt attribute like this:

      void f (void) __attribute__ ((interrupt ("supervisor")));

  Permissible values for this parameter are `supervisor`, `machine`, and `rnmi`. If there is no parameter, then it defaults to `machine`.

`riscv_vector_cc` ¶

  This attribute applies to functions.

  Use this attribute to force the function to use the vector calling convention variant.

      void foo() __attribute__((riscv_vector_cc));
      [[riscv::vector_cc]] void foo(); // For C++11 and C23

`target (options)` ¶

  This attribute applies to functions.

  As discussed in Common Attributes, this attribute allows specification of target-specific compilation options.

  The following options are available for the RISC-V target. For the most part, these options mirror the behavior of similar command-line options (see RISC-V Options), but on a per-function basis.

  ‘`arch=`’ ¶

    Specifies the architecture version and architectural extensions to use for this function. The behavior and permissible arguments are the same as for the `-march=` command-line option, in addtion, it also support extension enablement list, a list of extension name and prefixed with `+`, like `arch=+zba` means enable `zba` extension. Multiple extension can be enabled by separating them with a comma. For example: `arch=+zba,+zbb`.

  ‘`tune=`’ ¶

    Specifies the core for which to tune the performance of this function. The behavior and permissible arguments are the same as for the `-mtune=` command-line option.

  ‘`cpu=`’ ¶

    Specifies the core for which to tune the performance of this function and also whose architectural features to use. The behavior and valid arguments are the same as for the `-mcpu=` command-line option.

  ‘`max-vectorization`’ ¶
  ‘`no-max-vectorization`’

    `max-vectorization` tells GCC’s vectorizer to treat all vector loops as being more profitable than the original scalar loops when optimizing the current function. `no-max-vectorization` disables this behavior. This corresponds to the behavior of the command-line options `-mmax-vectorization` and `-mno-max-vectorization`.

  The above target attributes can be specified as follows:

      __attribute__((target("attr-string")))
      int
      f (int a)
      {
        return a + 5;
      }

  where `attr-string` is one of the attribute strings specified above.

  Multiple target function attributes can be specified by separating them with a semicolon. For example:

      __attribute__((target("arch=+zba,+zbb;tune=rocket")))
      int
      foo (int a)
      {
        return a + 5;
      }

  is valid and compiles function `foo` with `zba` and `zbb` extensions and tunes it for `rocket`.