Re: [x86-64] RFC: Add nosse abi attribute

[Alexander Monakov via Gcc-patches]

On Tue, 11 Jul 2023, Michael Matz wrote:

> Hey,
> 
> On Tue, 11 Jul 2023, Alexander Monakov via Gcc-patches wrote:
> 
> > > > > * nosseclobber: claims (and ensures) that xmm8-15 aren't clobbered
> > > > 
> > > > This is the weak/active form; I'd suggest "preserve_high_sse".
> > > 
> > > But it preserves only the low parts :-)  You swapped the two in your 
> > > mind when writing the reply?
> > 
> > Ahhh. By "high SSE" I mean the high-numbered SSE regs, i.e. xmm8-15, not
> > the higher halves of (unspecified subset of) SSE regs.
> 
> Ah, gotcha :-)  It just shows that all these names are confusing.  Maybe 
> I'll just go with "attribute1" and "attribute2" and rely on docu.  (SCNR)

Heh, that reminds me that decimal digits are allowed in attribute names.
Let me offer "preserve_xmm_8_15" and "only_xmm_0_7" then.

One more thing to keep in mind is interaction with SSE-AVX transition.
If the function with a new attribute is using classic non-VEX-encoded SSE,
but its caller is using 256-bit ymm0-15, it will incur a substantial penalty
on Intel CPUs. There's no penalty on AMD (afaik) and no penalty for zmm16-31,
since those are inaccessible in non-EVEX code.

Alexander