[Bug tree-optimization/79594] New: -Waggressive-loop-optimizations incomplete and/or inconsistentt

[vincenzo.innocente at cern dot ch]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=79594

            Bug ID: 79594
           Summary: -Waggressive-loop-optimizations incomplete and/or
                    inconsistentt
           Product: gcc
           Version: 7.0.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: vincenzo.innocente at cern dot ch
  Target Milestone: ---

given
 cat aggressiveLoop.cc
#include <cmath>
#include<limits>

float x[1024];
float y[1024];
float w[512];
float z[128];

float c,q;

void foo() {
  for (int i=0; i<1024; ++i) {
   auto zz=z[i];
   auto yy = y[i];
   if(x[i] > q)  yy = zz;
   y[i]=yy;
  }
}

void foo2() {
  for (int i=0; i<1024; ++i) {
   auto zz=z[i];
   auto yy = w[i];
   if(x[i] > q)  yy = zz;            
   x[i]=yy;
  }
}

void foo3() {
  for (int i=0; i<1024; ++i) {
   auto zz=z[i];
   auto yy = w[i];
   if(x[i] > q)  yy = zz;
   w[i]=yy;
  }
}

gcc version 7.0.1 20170205 (experimental) [trunk revision 245191] (GCC) 
reports
c++ -Wall -O2 aggressiveLoop.cc -S
aggressiveLoop.cc: In function 'void foo()':
aggressiveLoop.cc:13:9: warning: iteration 128 invokes undefined behavior
[-Waggressive-loop-optimizations]
    auto zz=z[i];
         ^~
aggressiveLoop.cc:12:18: note: within this loop
   for (int i=0; i<1024; ++i) {
                 ~^~~~~
aggressiveLoop.cc: In function 'void foo2()':
aggressiveLoop.cc:22:9: warning: iteration 128 invokes undefined behavior
[-Waggressive-loop-optimizations]
    auto zz=z[i];
         ^~
aggressiveLoop.cc:21:18: note: within this loop
   for (int i=0; i<1024; ++i) {
                 ~^~~~~
aggressiveLoop.cc: In function 'void foo3()':
aggressiveLoop.cc:34:8: warning: iteration 512 invokes undefined behavior
[-Waggressive-loop-optimizations]
    w[i]=yy;
    ~~~~^~~
aggressiveLoop.cc:30:18: note: within this loop
   for (int i=0; i<1024; ++i) {
                 ~^~~~~

while in foo2 there is also "auto yy = w[i];"
and in foo3 both assignments
  auto zz=z[i];
  auto yy = w[i];
will "invokes undefined behavior" at iterations 128 and 512...