[llvm-dev] Loop optimization

[Garba Peter via llvm-dev]

Thanks a lot for your help.

Indeed if I compile with -OZ the loop gets replaced with something like this:

  if ( *v3 < 0 )
    v4 = 0;

Is there a reason why it does not get applied with O3 ?
I mean this a huge improvement to the loop.

-----Original Message-----
From: Doerfert, Johannes [mailto:jdoerfert at anl.gov]
Sent: Dienstag, 13. August 2019 19:13
To: llvm-dev at lists.llvm.org; Garba Peter <peter.garba at gemalto.com>
Subject: Re: Loop optimization

Without testing it I'd say loop idiom recognition (Transforms/Scalar/LoopIdiomRecognize.h) is the one that should kick in and optimize this.

________________________________________
From: llvm-dev <llvm-dev-bounces at lists.llvm.org> on behalf of Garba Peter via llvm-dev <llvm-dev at lists.llvm.org>
Sent: Tuesday, August 13, 2019 11:54
To: llvm-dev at lists.llvm.org
Subject: [llvm-dev] Loop optimization

Hi,

I was wondering if there is an optimization pass in LLVM or Polly that can optimize this given loop:


int main(int argc, char* argv[]) {
                char s = 0;

                // Loop is setting s = argv[1][0];
                for (int i=0; i<argv[1][0]; i++) {
                    s++;
                }

                printf("s = %c\n", s);

                return 0;
}


into:

int main(int argc, char* argv[]) {
                // Optimized loop
                char s = argv[1][0];

                printf("s = %c\n", s);

                return 0;
}


Is there an optimization that can do this?
And if not, maybe someone knows what hinders the optimizer?!


Thanks,
Peter
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