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SubjectRE: [PATCH] KVM: x86: optimize check for valid PAT value
Date
From: Paolo Bonzini
> Sent: 15 April 2019 09:12
> On 11/04/19 11:06, David Laight wrote:
> > It may be possible to generate shorter code that executes just as
> > fast by generating a single constant and deriving the others from it.
> > - generate 4s - needed first
> > - shift right 2 to get 1s (in parallel with the xor)
> > - use lea to get 6s (in parallel with an lea to do the add)
> > - invert the 1s to get FEs (also in parallel with the add)
> > - xor the FEs with the 6s to get F8s (in parallel with the or)
> > - and/test for the result

That version needs an extra register move I hadn't allowed for.
It is also impossible to stop gcc folding constant expressions
without an asm nop on a register.

> FWIW, here is yet another way to do it:
>
> /* Change 6/7 to 4/5 */
> data &= ~((data & 0x0404040404040404ULL) >> 1);
> /* Only allow 0/1/4/5 now */
> return !(data & 0xFAFAFAFAFAFAFAFAULL);
>
> movabs $0x404040404040404, %rcx
> andq %rdx, %rcx
> shrq %rcx
> notq %rcx
> movabs $0xFAFAFAFAFAFAFA, %rax
> andq %rcx, %rdx
> test %rax, %rdx

Fewer opcode bytes, but 5 dependant instructions
(assuming the first constant can executed in parallel
with an earlier instruction).
I think my one was only 4 dependant instructions.

All these are far faster than the loop...

David

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