cgen, checker if and $if: CPU instructions support

[enghitalo]

Describe the feature

__builtin_cpu_init

This function runs the CPU detection code to check the type of CPU and the features supported. This built-in function needs to be invoked along with the built-in functions to check CPU type and features, __builtin_cpu_is and __builtin_cpu_supports, only when used in a function that is executed before any constructors are called. The CPU detection code is automatically executed in a very high priority constructor.

For example, this function has to be used in ifunc resolvers that check for CPU type using the built-in functions __builtin_cpu_is and __builtin_cpu_supports, or in constructors on targets that don’t support constructor priority.

static void (*resolve_memcpy (void)) (void)
{
  // ifunc resolvers fire before constructors, explicitly call the init
  // function.
  __builtin_cpu_init ();
  if (__builtin_cpu_supports ("ssse3"))
    return ssse3_memcpy; // super fast memcpy with ssse3 instructions.
  else
    return default_memcpy;
}

void *memcpy (void *, const void *, size_t)
     __attribute__ ((ifunc ("resolve_memcpy")));

__builtin_cpu_supports

if (__builtin_cpu_supports ("popcnt"))
  {
     asm("popcnt %1,%0" : "=r"(count) : "rm"(n) : "cc");
  }
else
  {
     count = generic_countbits (n); //generic implementation.
  }

Use Case

$if sse2 {
    // some faster implementation to do something
} $else {
    // default code
}

if support_sse2 {
    // some faster implementation to do something
} else {
    // default code
}

Proposed Solution

I guess:

1. changes at vlib/v/gen/c/cheaders.v to verify if compiler supports __builtin_cpu_supports

#define HAS_BUILTIN_CPU_SUPPORTS 1

2. changes at vlib/v/ast/comptime_valid_idents.v -> valid_comptime_if_cpu_features to supports instructions listed in __builtin_cpu_supports

3. changes at vlib/v/checker/comptime.v -> comptime_if_cond to supports instructions listed in __builtin_cpu_supports

4. changes at doc/docs.md -> #### $if condition

| OS                             | Compilers        | Platforms                     | Other                                         | CPU features                  |
|--------------------------------|------------------|-------------------------------|-----------------------------------------------|-------------------------------|
| `windows`, `linux`, `macos`    | `gcc`, `tinyc`   | `amd64`, `arm64`, `aarch64`   | `debug`, `prod`, `test`                       | `sse2`, `avx`, `avx2`, `neon`	|
| `darwin`, `ios`, `bsd`         | `clang`, `mingw` | `i386`, `arm32`               | `js`, `glibc`, `prealloc`                     | `mmx`, `sha`, `aes`, `fma`, 	|
| `freebsd`, `openbsd`, `netbsd` | `msvc`           | `rv64`, `rv32`                | `no_bounds_checking`, `freestanding`          | `f16c`, `bmi1`, `bmi2`, `adx`	|
| `android`, `mach`, `dragonfly` | `cplusplus`      | `x64`, `x32`                  | `no_segfault_handler`, `no_backtrace`         | `rdseed`, `rdtscp`			|
| `gnu`, `hpux`, `haiku`, `qnx`  |                  | `little_endian`, `big_endian` | `no_main`, `fast_math`, `apk`, `threads`      |								|
| `solaris`, `termux`            |                  |                               | `js_node`, `js_browser`, `js_freestanding`    |								|
| `serenity`, `vinix`, `plan9`   |                  |                               | `interpreter`, `es5`, `profile`, `wasm32`     |								|
|                                |                  |                               | `wasm32_emscripten`, `wasm32_wasi`            |								|
|                                |                  |                               | `native`, `autofree`                          |								|

Other Information

No response

Acknowledgements

• I may be able to implement this feature request
• This feature might incur a breaking change

Version used

latest

Environment details (OS name and version, etc.)

all

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[kbkpbot]

Maybe cpuinfo can give you some help.