[Bug c/37238] New: gcc miscompiles simple memcpy loop

[felix-gcc at fefe dot de]

This code:

void byte_copy(void* out, size_t len, const void* in) {
  char* s=out;
  const char* t=in;
  const char* u=t+len;
  for (;;) {
    if (t==u) break; *s=*t; ++s; ++t;
    if (t==u) break; *s=*t; ++s; ++t;
    if (t==u) break; *s=*t; ++s; ++t;
    if (t==u) break; *s=*t; ++s; ++t;
  }
}

gcc produces wrong code with -O1 or higher, but correct code with -O0.
Here is some simple test code:

#include <assert.h>
#include <string.h>

char buf[4096];
char text[128];

int main() {
  memset(buf,0,sizeof(buf));
  strcpy(text,"this is a test!\n");
  byte_copy(buf,16,text);
  assert(!memcmp(buf,"this is a test!\n\0",18));
  return 0;
}


-- 
           Summary: gcc miscompiles simple memcpy loop
           Product: gcc
           Version: 4.3.1
            Status: UNCONFIRMED
          Severity: critical
          Priority: P3
         Component: c
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: felix-gcc at fefe dot de
 GCC build triplet: x86_64-unknown-linux-gnu
  GCC host triplet: x86_64-unknown-linux-gnu
GCC target triplet: x86_64-unknown-linux-gnu


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=37238