[PATCH 6/6] x86: optimize {,V}PEXTR{D,Q} with immediate of 0

[Jiang, Haochen]

> >> Such are equivalent to simple moves, which are up to 3 bytes shorter to
> >> encode (and perhaps also cheaper to execute).
> >
> > The optimization is problematic. Instead of movd/q, we should optimize to
> > psrldq, which has lower latency and it is exactly what GCC is doing for 0,1,2,3
> > in immediate.
> >
> > psrldq has only 1 latency, while movd has 3 latency.
> 
> Wait. While the compiler may use PSRLDQ here, based on knowing
> assumptions
> made elsewhere, the assembler can't: The replacement insn must generate the
> exact same result in the destination register. PSRLDQ with an immediate of
> 0 (which effectively you're suggesting to use here) doesn't alter the
> destination register at all, though. When really we want the upper bits of
> the register cleared.

pextrd/q also doesn't clear them at all. For vpextrd/q and vpsrldq, they will
both clear higher bits. So they will be the same.

But you are right, we do have some assumptions, it will be both registers,
movd/q does not have that restriction.

> 
> By implication I question the use of PSRLDQ (should it really happen) when
> the immediate is 0: If the upper bits are of no interest, a yet simpler
> MOVDQ{A,U} could then be used. Or, if source and destination match, no insn
> would need emitting at all.
> 
> > Also, I suppose the optimization related to latency should not be done in
> > assembler.
> 
> Why? We have -O, -O1, and -O2 alongside -Os for a reason.

I am quite conservative on the optimization in assembler. If we are also going to
optimize those hand-written code, the optimization could work.

However, when they hand write some code, are we supposed to change them?
For -Os, we could give them all the optimizations we have, but for -O, I am not
that sure.

And I suppose we might add too much burden for the assembler if we are going
to add too much optimizations related to latency. It will become another compiler.
Are we supposed to copy all the optimizations from compiler? IMO, optimization to
codesize is ok, but for latency, I am a little concerned.

Thx,
Haochen

> 
> Jan