[Bug tree-optimization/62156] New: memcmp doesn't see through memcpy at compile-time

[glisse at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=62156

            Bug ID: 62156
           Summary: memcmp doesn't see through memcpy at compile-time
           Product: gcc
           Version: 5.0
            Status: UNCONFIRMED
          Keywords: missed-optimization
          Severity: enhancement
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: glisse at gcc dot gnu.org

int f(){
  const char*a="abcdefghijkl";
  const char*b="acbdefghijkl";
  char*c=(char*)__builtin_malloc(42);
  __builtin_memcpy(c,b,13);
  return __builtin_memcmp(a,c,5);
}

We have no problem computing strlen(c) or memcmp(a,b,5) at compile-time, but we
don't manage to fold this. This type of code is what we get with std::string
when we inline everything.