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 even don't try store+load.
Am I missing something?
---
With best regards, Konstantin
