[Bug c/54360] New: missed optimalization: unnecessary indirect call

[Bug c/54360] New: missed optimalization: unnecessary indirect call

[neleai at seznam dot cz]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54360

             Bug #: 54360
           Summary: missed optimalization: unnecessary indirect call
    Classification: Unclassified
           Product: gcc
           Version: 4.8.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: neleai@seznam.cz


Consider following program:

void bar(int *x);
int x(){
  struct {
    void (*b)(int *);
    int a;
  } foo;
  foo.b=bar;
  foo.b(&(foo.a));
  foo.b(&(foo.a));
}
int y(){
    void (*b)(int *);
    int a;
    b=bar;
    b(&a);
    b(&a);
}

In function x gcc eliminaties first indirect call but not second. In y both
calls are eliminated.  

.file "s.c"
  .text
  .p2align 4,,15
  .globl  x
  .type x, @function
x:
.LFB0:
  .cfi_startproc
  subq  $24, %rsp
  .cfi_def_cfa_offset 32
  leaq  8(%rsp), %rdi
  movq  $bar, (%rsp)
  call  bar
  leaq  8(%rsp), %rdi
  call  *(%rsp)
  addq  $24, %rsp
  .cfi_def_cfa_offset 8
  ret
  .cfi_endproc
.LFE0:
  .size x, .-x
  .p2align 4,,15
  .globl  y
  .type y, @function
y:
.LFB1:
  .cfi_startproc
  subq  $24, %rsp
  .cfi_def_cfa_offset 32
  leaq  12(%rsp), %rdi
  call  bar
  leaq  12(%rsp), %rdi
  call  bar
  addq  $24, %rsp
  .cfi_def_cfa_offset 8
  ret
  .cfi_endproc
.LFE1:
  .size y, .-y
  .ident  "GCC: (Debian 20120820-1) 4.8.0 20120820 (experimental) [trunk
revision 190537]"
  .section  .note.GNU-stack,"",@progbits

[Bug c/54360] missed optimalization: unnecessary indirect call

[jakub at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54360

Jakub Jelinek <jakub at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |jakub at gcc dot gnu.org

--- Comment #1 from Jakub Jelinek <jakub at gcc dot gnu.org> 2012-08-23 15:00:25 UTC ---
That is because bar could do:
#include <stddef.h>
void baz (int *);
void bar (int *x)
{
  struct S { void (*b) (int *); int a; } *p;
  p = (struct S) ((char *) x - offsetof (struct S, a));
  p->b = baz;
}
It can't do something similar in case of y.

[Bug c/54360] missed optimization: unnecessary indirect call

[rguenth at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54360

Richard Guenther <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|                            |INVALID

--- Comment #2 from Richard Guenther <rguenth at gcc dot gnu.org> 2012-09-03 09:53:37 UTC ---
.