How to specify registers in inline asm?

Help
1

I’m trying to wrap cpuid instruction. Something like this:

const CpuId = extern struct {
    a: u32,
    b: u32,
    c: u32, 
    d: u32
};

export fn cpuid(leaf: u32) CpuId {
    @setRuntimeSafety(false);

    var a: u32 = undefined;
    var b: u32 = undefined;
    var c: u32 = undefined;
    var d: u32 = undefined;

    asm volatile(
        \\cpuid
        : [a]"=eax"(a),[b]"=ebx"(b),[c]"=ecx"(c),[d]"=edx"(d)
        : [id]"eax"(leaf)
    );

    return .{ .a = a, .b = b, .c = c, .d = d };
}

But i get absolutely dumb assembly:

0000000000000000 <cpuid>:
   0:   c5 f9 6e c7             vmovd  %edi,%xmm0
   4:   0f a2                   cpuid
   6:   c5 f9 7e d9             vmovd  %xmm3,%ecx
   a:   c5 f9 7e ce             vmovd  %xmm1,%esi
   e:   c5 f9 7e d2             vmovd  %xmm2,%edx
  12:   c5 f9 7e c0             vmovd  %xmm0,%eax
  16:   48 c1 e6 20             shl    $0x20,%rsi
  1a:   48 c1 e1 20             shl    $0x20,%rcx
  1e:   48 09 f0                or     %rsi,%rax
  21:   48 09 ca                or     %rcx,%rdx
  24:   c3                      ret

So, how can i tell the compiler which registers cpuid uses for output?

2

It turned out to be much easier. It’s enough to use curly braces to explicitly tell the compiler to use the specified registers. I’m not sure if this is a 100% guarantee, but it works for me.

asm volatile(
    \\cpuid
    : [a]"={eax}"(a),[b]"={ebx}"(b),[c]"={ecx}"(c),[d]"={edx}"(d)
    : [id]"{eax}"(leaf)
);