GCC with gcse enabled generates very poor code for this fragment:
------------------------------------------------------------------------------
int r0, r1, r2, r3, r4, r5;
void f (int n)
{
while (-- n)
{
r1 += r0;
r2 += r1;
r3 += r2;
r4 += r3;
r5 += r4;
r0 += r5;
}
}
------------------------------------------------------------------------------
This is code from 3.4.4 with -O2 -fomit-frame-pointer:
.L4:
movl 16(%esp), %ebp
movl 12(%esp), %esi
movl 8(%esp), %ecx
movl 4(%esp), %edx
movl (%esp), %eax
addl %edi, %ebp
addl %ebp, %esi
movl %ebp, 16(%esp)
addl %esi, %ecx
movl %esi, 12(%esp)
addl %ecx, %edx
movl %ecx, 8(%esp)
addl %edx, %eax
decl 20(%esp)
movl %edx, 4(%esp)
leal (%edi,%eax), %ebx
movl %eax, (%esp)
movl %ebx, %edi
jne .L4
And this is with -O2 -fomit-frame-pointer -fno-gcse:
.L4:
addl %edi, %ebp
addl %ebp, %ebx
addl %ebx, %ecx
addl %ecx, %edx
addl %edx, %eax
addl %eax, %edi
decl %esi
jne .L4
Same applies for 4.0.0, only one should use '-O2 -fomit-frame-pointer
-fno-tree-loop-im -fno-gcse' to get decent code.
This is a regression and was introduced by this change:
2001-07-16 Daniel Berlin <[EMAIL PROTECTED]>
* gcse.c: Update comment at top.
Update comment on mem handling.
mem_last_set, mem_first_set, mem_set_in_block: gone.
Declaration of reg_set_info: gone.
(oprs_unchanged_p): Don't use mem_*set_* anymore. They are
pointless with load_killed_in_block_p (they are *more*
conservative then it, not less, and less accurate).
(oprs_not_set_p): Ditto.
(alloc_gcse_mem): Don't allocate mem_set_in_block
(free_gcse_mem): Don't free it, either.
(record_last_mem_set_info): Update comment in front, remove
mem_*set_* stuff. Note the reason we don't handle stores directly
here.
(compute_hash_table): Update comments to reflect reality. Remove
mem_*set_* references.
(reset_opr_set_tables): Remove mem_*set_* references.
(mark_call): Ditto.
(mark_set): Ditto. Also remove double sets of bitmaps for REG's.
(mark_clobber): Ditto (on both parts, we double set here too).
(expr_killed_p): Remove mem_set_in_block test.
(compute_transp): Remove mem_set_in_block test.
--
Summary: [3.3/3.4/4.0 Regression] gcse causes poor register
allocation
Product: gcc
Version: 4.0.0
Status: UNCONFIRMED
Keywords: missed-optimization
Severity: normal
Priority: P2
Component: rtl-optimization
AssignedTo: unassigned at gcc dot gnu dot org
ReportedBy: belyshev at depni dot sinp dot msu dot ru
CC: gcc-bugs at gcc dot gnu dot org
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=19464
