[Bug rtl-optimization/19464] New: [3.3/3.4/4.0 Regression] gcse causes poor register allocation

["belyshev at depni dot sinp dot msu dot ru" <gcc-bugzilla at gcc dot gnu dot org>]

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  <dan@cgsoftware.com>

	* 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