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From: "vincenzo.innocente at cern dot ch" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug tree-optimization/55645] New: skipping unlike branch in vectorized loops using movmsk or equivalent
Date: Tue, 11 Dec 2012 06:43:00 -0000
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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55645

             Bug #: 55645
           Summary: skipping unlike branch in vectorized loops using
                    movmsk or equivalent
    Classification: Unclassified
           Product: gcc
           Version: 4.8.0
            Status: UNCONFIRMED
          Severity: enhancement
          Priority: P3
         Component: tree-optimization
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: vincenzo.innocente@cern.ch


I'm wondering if the vectorization engine could accommodate some mechanism to
skip unlike branches using a global test based on movmsk or similar

below a trivial example including a possible SLP implementation that happens to
compile with 4.8 as
c++ -std=c++11 -Ofast -mavx2 -S divergent.cc; less divergent.s

float a[1024];
float b[1024];
float c[1024];

 #define likely(x) (__builtin_expect(x, true))

// possible syntax
void compute() {
  for (int i=0;i!=1024;++i) {
    if likely(a[i]<b[i])  // very often
    c[i]=a[i]+b[i];
    else // rare
    c[i]=a[i]-b[i];
  }
}


// hand-made implementation that compile with 4.8 today
#include <x86intrin.h>

typedef float __attribute__( ( vector_size( 32 ) ) ) float32x8_t;
typedef int __attribute__( ( vector_size( 32 ) ) ) int32x8_t;


float32x8_t va[1024];
float32x8_t vb[1024];
float32x8_t vc[1024];

void computeV() {
  for (int i=0;i!=1024;++i) {
    float32x8_t mask = va[i]<vb[i];
    if likely(_mm256_movemask_ps(mask) == 255) {
    vc[i]=va[i]+vb[i];
      } else {
      vc[i]= va[i]<vb[i] ? va[i]+vb[i] : va[i]-vb[i];
    }
  }
}