[llvm-dev] LoopIdiomRecognize is not recognizing the ctpop idiom

[Alex Susu via llvm-dev]

   Hello.
     Thanks for the reply.

     I managed to make LLVM recognize ctpop. To make LLVM apply the loop idiom we have to 
give with care arguments like -Os to opt, and especially choose an architecture that 
supports in hardware ctpop 
(http://lists.llvm.org/pipermail/llvm-commits/Week-of-Mon-20121119/156300.html mentions of 
x86, ARM, PowerPC having an instruction for population count). For example:
       clang -O3 -mllvm -disable-llvm-optzns -S -emit-llvm test.c
       opt -Os -debug -S -force-vector-width=1 -mcpu=corei7 test.ll -o test_opt.ll 
-loop-idiom # -Os is required (-O3 doesn't work). -mcpu is required, but we could put 
something else instead of corei7, like some ARM, PPC.


   Best regards,
     Alex


On 6/2/2018 7:04 PM, Craig Topper wrote:
> LLVM doesn't have any support for transforming the first loop into ctpop. We only
> recognize the second form in your popcount_Yang_i32 function.
>
> I don't know of any papers myself, I've just spent some time looking at the popcount
> recognition code recently.
>
> ~Craig
>
>
> On Sat, Jun 2, 2018 at 4:24 AM Alex Susu via llvm-dev <llvm-dev at lists.llvm.org
> <mailto:llvm-dev at lists.llvm.org>> wrote:
>
>        Hello.
>          Could you please tell me why am I not able to recognize (with LLVM built from the
>     SVN
>     code in Apr 25, 2018) the LLVM IR intrinsic ctpop (described at
>     https://llvm.org/docs/LangRef.html#llvm-ctpop-intrinsic) in the following program:
>            int PopCnt_Simple(int x) {
>                int numBits = 0;
>                int i;
>
>                //for (i = 0; i < 32; i++) {
>                for (i = 0; x != 0; i++) {
>                    if (x & 1)
>                        numBits++;
>                    x >>= 1;
>                }
>
>                return numBits;
>            }
>
>          I also did check the following code, getting inspired from the discussion at
>     http://lists.llvm.org/pipermail/llvm-commits/Week-of-Mon-20121119/156300.html:
>            int popcount_Yang_i32(int a) {
>                  int c = 0;
>                  while (a) {
>                      c++;
>                      //...  // both a & c would be used multiple times in or out of
>                      //loop
>                      a &= a - 1;
>                      //...
>                  }
>
>                  return c;
>            }
>
>
>          Is there any good paper discussing this type of loop idiom recognitions? I found
>     only
>     a vaguely related paper: "Automatic Recognition of Performance Idioms in Scientific
>     Applications", IPDPS 2011  (http://www.sdsc.edu/~allans/ipdps11.pdf).
>
>        Thank you very much,
>          Alex
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