------- Comment #8 from rask at gcc dot gnu dot org 2007-11-27 18:04 -------
Created an attachment (id=14647)
--> (http://gcc.gnu.org/bugzilla/attachment.cgi?id=14647&action=view)
Patch to enhance cse.c
The attached patch can optimize this testcase:
void foo (int);
int bar2 (int a, int b)
{
int c = a - b;
if (a < b)
{
foo (c);
}
return c;
}
Before:
bar2:
pushl %ebx # 33 *pushsi2 [length = 1]
subl $8, %esp # 34 pro_epilogue_adjust_stack_1/1 [length
= 3]
movl 16(%esp), %edx # 2 *movsi_1/1 [length = 4]
movl 20(%esp), %eax # 3 *movsi_1/1 [length = 4]
movl %edx, %ebx # 32 *movsi_1/1 [length = 2]
subl %eax, %ebx # 7 *subsi_1/1 [length = 2]
cmpl %eax, %edx # 8 *cmpsi_1_insn/1 [length = 2]
jge .L2 # 9 *jcc_1 [length = 2]
movl %ebx, (%esp) # 11 *movsi_1/2 [length = 3]
call foo # 12 *call_0 [length = 5]
.L2:
movl %ebx, %eax # 19 *movsi_1/1 [length = 2]
addl $8, %esp # 37 pro_epilogue_adjust_stack_1/1 [length
= 3]
popl %ebx # 38 popsi1 [length = 1]
ret # 39 return_internal [length = 1]
After:
bar2:
pushl %ebx # 33 *pushsi2 [length = 1]
subl $8, %esp # 34 pro_epilogue_adjust_stack_1/1 [length
= 3]
movl 16(%esp), %eax # 2 *movsi_1/1 [length = 4]
movl %eax, %ebx # 32 *movsi_1/1 [length = 2]
subl 20(%esp), %ebx # 8 *subsi_2/2 [length = 4]
jns .L2 # 9 *jcc_1 [length = 2]
movl %ebx, (%esp) # 11 *movsi_1/2 [length = 3]
call foo # 12 *call_0 [length = 5]
.L2:
movl %ebx, %eax # 19 *movsi_1/1 [length = 2]
addl $8, %esp # 37 pro_epilogue_adjust_stack_1/1 [length
= 3]
popl %ebx # 38 popsi1 [length = 1]
ret # 39 return_internal [length = 1]
One of the difficulties is that by the time the code gets to the cse1 pass, the
statement "c = a - b" might have been moved below the "a < b" test, making it
harder to optimize. The "bar2" testcase was crafted so this doesn't happen. But
given the need to potentially replace both the sub/cmp insn and the jump insn,
it would be better to move this optimization to combine.
--
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=3507
