Re: Register Allocation

[Andrew MacLeod <amacleod@redhat.com>]

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On Fri, 2004-03-26 at 15:16, Vladimir N. Makarov wrote:
> John Lu wrote:
> 
> >Hi,
> >
> >I've been working on a port based on gcc-3.3 and I've noticed that
> >better assembly code is generated if the C source has
> >separate variables declared for distinct live ranges.
> >
> >  
> >
> Pesudo-register renaming is made by -fweb.  It was written by Jan 
> Hubicka.  Probably, this option should be set up by default (although 
> there is no practically improvement for SPECInt2000 for P4, some tests 
> are really faster).  Another positive thing of usage this option is 
> better scheduling because of less anti-dependences.
> 

tree-ssa will be doing this type of thing automatically shortly. Its a
one line patch to enable it. (attached). I haven't turned it on yet
because it can make a real mess of the debug information and we'd like
to fix that :-).

Andrew

PS. tree-ssa optimizes that program to:

foo1 (p1, p2)
{
  int i.15;
  int total;
  int i;

<bb 0>:
  total = 0;
  i = 0;

<L0>:;
  total = *((int *)((unsigned int)i * 4) + p1) + total;
  i = i + 1;
  if (i <= 99) goto <L0>; else goto <L12>;

<L12>:;
  i.15 = 0;

<L6>:;
  total = *((int *)((unsigned int)i.15 * 4) + p1) + total;
  i.15 = i.15 + 2;
  if (i.15 <= 99) goto <L6>; else goto <L5>;

<L5>:;
  return total;
}


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	* tree-ssa (rewrite_out_of_ssa): Disable coalescing of variables not
	related by a copy that share the same root variable.


Index: tree-outof-ssa.c
===================================================================
RCS file: /cvs/gcc/gcc/gcc/Attic/tree-outof-ssa.c,v
retrieving revision 1.1.2.1
diff -c -p -r1.1.2.1 tree-outof-ssa.c
*** tree-outof-ssa.c	19 Mar 2004 02:07:25 -0000	1.1.2.1
--- tree-outof-ssa.c	24 Mar 2004 19:56:28 -0000
*************** rewrite_out_of_ssa (void)
*** 1970,1977 ****
  {
    var_map map;
    int var_flags = 0;
!   int ssa_flags = (SSANORM_REMOVE_ALL_PHIS | SSANORM_USE_COALESCE_LIST
! 		   | SSANORM_COALESCE_PARTITIONS);
  
    eliminate_virtual_phis ();
  
--- 1970,1976 ----
  {
    var_map map;
    int var_flags = 0;
!   int ssa_flags = (SSANORM_REMOVE_ALL_PHIS | SSANORM_USE_COALESCE_LIST);
  
    eliminate_virtual_phis ();