GCC cannot move address calculation to store+load?

[Konstantin Vladimirov]

Hi,

Faced this problem in private backend, but it can be easily reproduced
on x86 GCC:

Sample code (test.c):

int a;

int foo(int *x, int y)
{
  a = x[(y << 1)];
  x[(y << 1)] = y;
  return 0;
}

Compile with gcc-4.7.2:

$ gcc --version
gcc (GCC) 4.7.2
Copyright (C) 2012 Free Software Foundation, Inc.

Command line is:

$ gcc -O2 -S -m32 -dp test.c

Yields code:

foo:
.LFB0:
  .cfi_startproc
  movl  8(%esp), %edx # 3 *movsi_internal/1 [length = 4]
  leal  0(,%edx,8), %eax  # 25  *leasi  [length = 7]
  addl  4(%esp), %eax # 9 *addsi_1/1  [length = 4]
  movl  (%eax), %ecx  # 10  *movsi_internal/1 [length = 2]
  movl  %ecx, a # 11  *movsi_internal/2 [length = 6]
  movl  %edx, (%eax)  # 12  *movsi_internal/2 [length = 2]
  xorl  %eax, %eax  # 29  *movsi_xor  [length = 2]
  ret # 28  simple_return_internal  [length = 1]

It is obvious, that it can be rewritten without prior leal much better:

foo:
.LFB0:
  .cfi_startproc
  movl  8(%esp), %eax # 3 *movsi_internal/1 [length = 4]
  movl  4(%esp), %edx # 19  *movsi_internal/1 [length = 4]
  movl  (%edx,%eax,8), %ecx  # 8 *movsi_internal/1 [length = 3]
  movl  %ecx, a # 11  *movsi_internal/2 [length = 6]
  movl  %eax, (%edx,%eax,8)  # 8 *movsi_internal/2 [length = 3]
  xorl  %eax, %eax  # 29  *movsi_xor  [length = 2]
  ret # 28  simple_return_internal  [length = 1]

(this assembler is handwritten, summary instruction count -1, summery length -5)

Key Idea is that common address here, that forms leal is profitable to
be not calculated standalone, but moved into address operands in store
and in load.

When we have only store or only load, this job is done by combining.

But it seems, that combine pass even don't try store+load.

Am I missing something?

P.S. Posted similar one to gcc-help, got no response, trying to repost
here (sorry for possible offtopic).

---
With best regards, Konstantin