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
          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):

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

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.

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