Re: [Qemu-devel] [PATCH 7/8] target/ppc: Optimize emulation of vclzh and vclzb instructions

[Stefan Brankovic <stefan.brankovic@rt-rk.com>]

On 6.6.19. 22:38, Richard Henderson wrote:
> On 6/6/19 5:15 AM, Stefan Brankovic wrote:
>> Optimize Altivec instruction vclzh (Vector Count Leading Zeros Halfword).
>> This instruction counts the number of leading zeros of each halfword element
>> in source register and places result in the appropriate halfword element of
>> destination register.
> For halfword, you're generating 32 operations.  A loop over the halfwords,
> similar to the word loop I suggested for the last patch, does not reduce this
> total, since one has to adjust the clz32 result.
>
> For byte, you're generating 64 operations.
>
> These expansions are so big that without host vector support it's probably best
> to leave them out-of-line.
>
> I can imagine a byte clz expansion like
>
> 	t0 = input >> 4;
> 	t1 = input << 4;
> 	cmp = input == 0 ? -1 : 0;
> 	input = cmp ? t1 : input;
> 	output = cmp & 4;
>
> 	t0 = input >> 6;
> 	t1 = input << 2;
> 	cmp = input == 0 ? -1 : 0;
> 	input = cmp ? t1 : input;
> 	t0 = cmp & 2;
> 	output += t0;
>
> 	t1 = input << 1;
> 	cmp = input >= 0 ? -1 : 0;
> 	output -= cmp;
>
> 	cmp = input == 0 ? -1 : 0;
> 	output -= cmp;
>
> which would expand to 20 x86_64 vector instructions.  A halfword expansion
> would require one more round and thus 25 instructions.

I based this patch on performance results and my measurements say that 
tcg implementation is still significantly superior to helper 
implementation, regardless of somewhat large number of instructions.

I can attach both performance measurements results and disassembly of 
both helper and tcg implementations, if you want me to do this.

>
> I'll also note that ARM, Power8, and S390 all support this as a native vector
> operation; only x86_64 would require the above expansion.  It probably makes
> sense to add this operation to tcg.

I agree with this, but currently we don't have this implemented in tcg, 
so I worked with what I have.

Kind Regards,

Stefan

> r~