[moosedev]

Yeah, use of SSE registers does not imply SIMD, since x87 is gone in x86-64, so even scalar FP has to use SSE registers. The asm snippets for v::operator*() in the "Optimization level 1" section use scalar SSE arithmetic only (mulss). (There's some use of movaps to move data around, but it's a stretch to call that SIMD.)

I think the "leverage" sentence you quoted and the "with SIMD taken care of" one shortly after are maybe a bit misleading, since the asm snippets there don't really demonstrate SIMD.

[amluto]

> since x87 is gone in x86-64, so even scalar FP has to use SSE registers.

No, it’s still there. What’s actually going on is that all x86-64 CPUs support SSE2, so there is little reason to use x87 in 64-bit code.

(You can use it for 80-bit precision. OTOH, for most purposes, 80-bit precision is actively harmful, and x87 is an incredible mess, so almost no one wants it.)

[ethelward]

> 80-bit precision is actively harmful

How comes? Unexpected clipping when converting back and forth to 64bits?

[amluto]

Exactly. The conversations happen at unexpected and unpredictable times depending on when the compiler needs to spill registers, which has surprising effects.

[moosedev]

You're right - thanks for the correction!