[llvm-dev] [RFC] New pass: LoopExitValues

[James Molloy via llvm-dev]

Hi,

Coremark really isn't a good enough test - have you run the LLVM test suite
with this patch, and what were the performance differences?

I'm still a bit confused about what pattern exactly this pass is supposed
to trigger on. I understand the mechanics, but I still can't quite see what
patterns it would be useful on. You've mentioned matrix multiply - how does
this pass alter the IR? What value is it avoiding being recomputed? How
does this pass affect register pressure?

Also, your example just removes a mov and an add - the push/pops are just
register allocation (unless your pass in fact *reduces* register pressure?)

A bit more clarification would be great.

Cheers,

James

On Tue, 1 Sep 2015 at 19:07 Steve King via llvm-dev <llvm-dev at lists.llvm.org>
wrote:

> On Mon, Aug 31, 2015 at 5:52 PM, Jake VanAdrighem
> <jvanadrighem at gmail.com> wrote:
> > Do you have some specific performance measurements?
>
> Averaging 4 runs of 10000 iterations each of Coremark on my X86_64
> desktop showed:
>
> -O2 performance: +2.9% faster with the L.E.V. pass
> -Os size: 1.5% smaller with the L.E.V. pass
>
> In the case of Coremark, the benefit comes mainly from the matrix
> portion benchmark, which uses nested loops.  Similarly, I used a
> matrix multiplication for the regression test as shown below.  The
> L.E.V. pass eliminated 4 instructions.
>
> void matrix_mul(unsigned int Size, unsigned int *Dst, unsigned int
> *Src, unsigned int Val) {
>   for (int Outer = 0; Outer < Size; ++Outer)
>     for (int Inner = 0; Inner < Size; ++Inner)
>        Dst[Outer * Size + Inner] = Src[Outer * Size + Inner] * Val;
> }
>
>
> With LoopExitValues
> -------------------------------
> matrix_mul:
>     testl %edi, %edi
>     je .LBB0_5
>     xorl %r9d, %r9d
>     xorl %r8d, %r8d
> .LBB0_2:
>     xorl %r11d, %r11d
> .LBB0_3:
>     movl %r9d, %r10d
>     movl (%rdx,%r10,4), %eax
>     imull %ecx, %eax
>     movl %eax, (%rsi,%r10,4)
>     incl %r11d
>     incl %r9d
>     cmpl %r11d, %edi
>     jne .LBB0_3
>     incl %r8d
>     cmpl %edi, %r8d
>     jne .LBB0_2
> .LBB0_5:
>     retq
>
>
>
> Without LoopExitValues:
> -----------------------------------
> matrix_mul:
>     pushq %rbx           # Eliminated by L.E.V. pass
> .Ltmp0:
> .Ltmp1:
>     testl %edi, %edi
>     je .LBB0_5
>     xorl %r8d, %r8d
>     xorl %r9d, %r9d
> .LBB0_2:
>     xorl %r10d, %r10d
>     movl %r8d, %eax              # Eliminated by L.E.V. pass
> .LBB0_3:
>     movl %eax, %r11d
>     movl (%rdx,%r11,4), %ebx
>     imull %ecx, %ebx
>     movl %ebx, (%rsi,%r11,4)
>     incl %r10d
>     incl %eax
>     cmpl %r10d, %edi
>     jne .LBB0_3
>     incl %r9d
>     addl %edi, %r8d            # Eliminated by L.E.V. pass
>     cmpl %edi, %r9d
>     jne .LBB0_2
> .LBB0_5:
>     popq %rbx                    # Eliminated by L.E.V. pass
>     retq
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