[llvm-dev] memcmp code fragment

[Daniel Berlin via llvm-dev]

i think you'd have to write some idiom recognizer here. The ones we have
won't do it.

I guess my other question would be how commonly this happens. If it's
common and matters a lot, awesome.
I wouldn't do it just to fix SPEC :P

(people who care about bzip2 speed probably use any of the faster bzip2
impls :P)

On Fri, May 19, 2017 at 12:46 PM, Sirish Pande via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> Hi,
>
> Look at the following code:
>
> Look at the following C code  seqence:
>
> unsigned char mainGtU ( unsigned int i1,
>                unsigned int i2,
>                unsigned char* block)
> {
>    unsigned char c1, c2;
>    c1 = block[i1]; c2 = block[i2];
>    if (c1 != c2) return (c1 > c2);
>    i1++; i2++;
>
>    c1 = block[i1]; c2 = block[i2];
>    if (c1 != c2) return (c1 > c2);
>    i1++; i2++;
>
> ..
> ..
> <repeat 12 times>
>
> In LLVM IR it will be following:
>
> define i8 @mainGtU(i32 %i1, i32 %i2, i8* readonly %block, i16* nocapture
> readnone %quadrant, i32 %nblock, i32* nocapture readnone %budget)
> local_unnamed_addr #0 {
> entry:
>   %idxprom = zext i32 %i1 to i64
>   %arrayidx = getelementptr inbounds i8, i8* %block, i64 %idxprom
>   %0 = load i8, i8* %arrayidx, align 1
>   %idxprom1 = zext i32 %i2 to i64
>   %arrayidx2 = getelementptr inbounds i8, i8* %block, i64 %idxprom1
>   %1 = load i8, i8* %arrayidx2, align 1
>   %cmp = icmp eq i8 %0, %1
>   br i1 %cmp, label %if.end, label %if.then
>
> if.then:                                          ; preds = %entry
>   %cmp7 = icmp ugt i8 %0, %1
>   br label %return
>
> if.end:                                           ; preds = %entry
>   %inc = add i32 %i1, 1
>   %inc10 = add i32 %i2, 1
>   %idxprom11 = zext i32 %inc to i64
>   %arrayidx12 = getelementptr inbounds i8, i8* %block, i64 %idxprom11
>   %2 = load i8, i8* %arrayidx12, align 1
>   %idxprom13 = zext i32 %inc10 to i64
>   %arrayidx14 = getelementptr inbounds i8, i8* %block, i64 %idxprom13
>   %3 = load i8, i8* %arrayidx14, align 1
>   %cmp17 = icmp eq i8 %2, %3
>   br i1 %cmp17, label %if.end25, label %if.then19
>
> if.then19:                                        ; preds = %if.end
>   %cmp22 = icmp ugt i8 %2, %3
>   br label %return
>
> ..
> ..
> <repeats 12 times>
>
> This code sequence can be collapsed into call to  memcmp and we can get
> rid of basic blocks. I have written a small peephole optimization for
> squenece of instructions that identifies
> branch termiantor, compare, load, gep etc and converts them to a call to
> memcmp. This small pass gave me improvement of 67% on SPEC2000 bzip2 on X86.
>
> Is there a better idea, other than small peephole pass on IR to optimize
> this code?
>
> Sirish
>
>
> _______________________________________________
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> llvm-dev at lists.llvm.org
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>
>
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