[Bug target/57954] New: AVX missing vxorps (zeroing) before vcvtsi2s %edx, slow down AVX code

["vincenzo.innocente at cern dot ch" <gcc-bugzilla at gcc dot gnu dot org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=57954

            Bug ID: 57954
           Summary: AVX missing vxorps (zeroing) before vcvtsi2s %edx,
                    slow down AVX code
           Product: gcc
           Version: 4.9.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: vincenzo.innocente at cern dot ch

in the following benchmark performances w/o vectorization are poor wrt to
expectations
I find out this is due to non zeroing a register before using it 

c++ -O2 -S polyAVX.cpp -mavx
 as -v --64 -o polyAVX.o polyAVX.s
GNU assembler version 2.23.1 (x86_64-redhat-linux-gnu) using BFD version (GNU
Binutils) 2.23.1
c++ -O2 polyAVX.o -march=corei7-avx ; time ./a.out
53896530759
15.418u 0.000s 0:15.43 99.8%    0+0k 0+0io 1pf+0w
patch polyAVX.s
49a50
>         vxorps          %xmm0,%xmm0,%xmm0
patching file polyAVX.s
as -v --64 -o polyAVX.o polyAVX.s
GNU assembler version 2.23.1 (x86_64-redhat-linux-gnu) using BFD version (GNU
Binutils) 2.23.1
c++ -O2 polyAVX.o -march=corei7-avx ; time ./a.out
10340756863
2.958u 0.000s 0:02.96 99.6%    0+0k 0+0io 1pf+0w

I am sure there are many other cases like this.
gcc version 4.9.0 20130718 (experimental) [trunk revision 201034] (GCC) 

cat polyAVX.cpp 
//template<typename T>
typedef float T;
inline T polyHorner(T y) {
  return  T(0x2.p0) + y * (T(0x2.p0) + y * (T(0x1.p0) + y * (T(0x5.55523p-4) +
y * (T(0x1.5554dcp-4) + y * (T(0x4.48f41p-8) + y * T(0xb.6ad4p-12)))))) ;
}

#include <x86intrin.h>
#include<iostream>

volatile unsigned long long rdtsc() {
    unsigned int taux=0;
    return __rdtscp(&taux);
  }

int main() {


  long long t=0;

    bool ret=true;
    float s =0;
    for (int k=0; k!=100; ++k) {
      float c =   1.f/10000000.f;
      t -=rdtsc();
      for (int i=1; i<10000001; ++i) s+= polyHorner((float(i)+float(k))*c);
      t    +=rdtsc();
    }
    ret &= s!=0;

  std::cout << t <<std::endl;

  return ret ? 0 : -1;


}