[Bug c/81741] New: Misoptimisation : replacing a constant field read access by a function call

["patrick.pelissier at gmail dot com" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81741

            Bug ID: 81741
           Summary: Misoptimisation : replacing a constant field read
                    access by a function call
           Product: gcc
           Version: 7.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: patrick.pelissier at gmail dot com
  Target Milestone: ---

GCC 7.1.0 generates for the following code:


#include <string.h>

typedef struct string_s {
  unsigned long size, alloc;
  char *ptr;
} string_t[1];

# define M_ASSUME(x)                                    \
  (! __builtin_constant_p (!!(x) || !(x)) || (x) ?      \
   (void) 0 : __builtin_unreachable())

int f(string_t s)
{
  M_ASSUME(strlen(s->ptr) == s->size);
  return s->size;
}


the following code on an x86-64 platform (gcc -std=c99 -O2 -S test.c):


f:
        subq    $8, %rsp
        movq    16(%rdi), %rdi
        call    strlen
        addq    $8, %rsp
        ret


Notice that the field access s->size is replaced by strlen(s->ptr), which is
way slower.

GCC 4.9 doesn't have this issue.