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

[Alex Susu via llvm-dev]

   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