x86_64 reference implementation of f16 fma uses wrong instruction

while reading through the new sqrt code I noticed we use the f32 fma instruction where we should be using the f16 fma:

Lines 240 to 253 in a6fa80d

    
           if is_x86_feature_detected!("avx512fp16") { 
        
               unsafe { 
        
                   let status = with_fp_env!( 
        
                       rm, 
        
                       "vfmadd213ss {x}, {y}, {z}", 
        
                       x = inout(xmm_reg) self, 
        
                       y = in(xmm_reg) mul, 
        
                       z = in(xmm_reg) add, 
        
                   ); 
        
                   Some(status.and(self)) 
        
               } 
        
           } else { 
        
               no_fp_env(rm, || self.mul_add(mul, add)) 
        
           }

that should be vfmadd213sh

Oh, darn, I don't have an avx512 machine to test this on.

Do you happen to know when .arch_extension is needed? I thought it technically was here but the assembler seems to accept without it.

Do you happen to know when .arch_extension is needed?

seems to only be on aarch64/arm, from searching for arch_extension in gas's manual: https://sourceware.org/binutils/docs/as/AS-Index.html

Oh, darn, I don't have an avx512 machine to test this on.

idk how much it helps, but Compiler Explorer seems to use an avx512 runner about half the time, you can click the refresh button until you get it

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x86_64 reference implementation of f16 fma uses wrong instruction #44

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	if is_x86_feature_detected!("avx512fp16") {
	unsafe {
	let status = with_fp_env!(
	rm,
	"vfmadd213ss {x}, {y}, {z}",
	x = inout(xmm_reg) self,
	y = in(xmm_reg) mul,
	z = in(xmm_reg) add,
	);
	Some(status.and(self))
	}
	} else {
	no_fp_env(rm, \|\| self.mul_add(mul, add))
	}

x86_64 reference implementation of f16 fma uses wrong instruction #44

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tgross35 commented on Oct 11, 2026

programmerjake commented on Oct 11, 2026

programmerjake commented on Oct 11, 2026

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