[PATCH] Fix one part of PR42108

[Richard Guenther <rguenther at suse dot de>]

This fixes one part of PR42108, the missed discovery of a full
redundant load.  The issue is that the SSA SCC value-numberer
does not visit loads and stores in a defined order.  The (or rather
one) fix is to properly canonicalize the virtual operand SSA names
we record in the expression hash tables.  The proper canonical
virtual operand is the def of the first dominating may-definition
(or a PHI node vdef, but we can as well choose a non-may-definition
without loss of precision and generality).

The patch possibly slows down SCCVN a bit for examples like

   # .MEM_2 = VDEF <.MEM_1(D)>
   may-def
 ....
   # VUSE <.MEM_120>
   ... = X;
   # VUSE <.MEM_120>
   ... = X;

where discovering the redundant load of X needs to canonicalize
their VUSE SSA name twice (previously we entered the expression
into the hashtable with .MEM_120 so it would be found immediately).
Now if we had

   # VUSE <.MEM_60>
   ... = X;

inbetween the may-def and the other loads we previously discovered
the full redundancy only if we first visited the load with .MEM_60
and only after that the loads with .MEM_120.  But nothing guarantees
this - this is the case the patch fixes.  There are about 0.5%
more redundant loads discovered in tramp3d with this patch.

This is a regression of the alias-improvements branch merge as
previously we had different virtual operands and thus in more
cases the canonical vuses were automagically chosen.

Bootstrapped and tested on x86_64-unknown-linux-gnu.  I have patched
one of our SPEC / C++ testers for more testing coverage.

Richard.

2009-12-18  Richard Guenther  <rguenther@suse.de>

	PR tree-optimization/42108
	* tree-ssa-sccvn.c (last_vuse_ptr): New variable.
	(vn_reference_lookup_2): Update last seen VUSE.
	(vn_reference_lookup_3): Avoid updating last seen VUSE after
	translating.
	(visit_reference_op_load): Use last seen VUSE from the first
	lookup when entering into the table.

	* gfortran.dg/pr42108.f90: New testcase.

Index: gcc/testsuite/gfortran.dg/pr42108.f90
===================================================================
*** gcc/testsuite/gfortran.dg/pr42108.f90	(revision 0)
--- gcc/testsuite/gfortran.dg/pr42108.f90	(revision 0)
***************
*** 0 ****
--- 1,27 ----
+ ! { dg-do compile }
+ ! { dg-options "-O2 -fdump-tree-fre" }
+ 
+ subroutine  eval(foo1,foo2,foo3,foo4,x,n,nnd)
+   implicit real*8 (a-h,o-z)
+   dimension foo3(n),foo4(n),x(nnd)
+   nw=0
+   foo3(1)=foo2*foo4(1)
+   do i=2,n
+     foo3(i)=foo2*foo4(i)
+     do  j=1,i-1
+       temp=0.0d0
+       jmini=j-i
+       do  k=i,nnd,n
+         temp=temp+(x(k)-x(k+jmini))**2
+       end do
+       temp = sqrt(temp+foo1)
+       foo3(i)=foo3(i)+temp*foo4(j)
+       foo3(j)=foo3(j)+temp*foo4(i)
+     end do
+   end do
+ end subroutine eval
+ 
+ ! There should be only one load from n left
+ 
+ ! { dg-final { scan-tree-dump-times "\\*n_" 1 "fre" } }
+ ! { dg-final { cleanup-tree-dump "fre" } }
Index: gcc/tree-ssa-sccvn.c
===================================================================
*** gcc/tree-ssa-sccvn.c	(revision 155346)
--- gcc/tree-ssa-sccvn.c	(working copy)
*************** vn_reference_lookup_1 (vn_reference_t vr
*** 984,989 ****
--- 984,991 ----
    return NULL_TREE;
  }
  
+ static tree *last_vuse_ptr;
+ 
  /* Callback for walk_non_aliased_vuses.  Adjusts the vn_reference_t VR_
     with the current VUSE and performs the expression lookup.  */
  
*************** vn_reference_lookup_2 (ao_ref *op ATTRIB
*** 994,999 ****
--- 996,1004 ----
    void **slot;
    hashval_t hash;
  
+   if (last_vuse_ptr)
+     *last_vuse_ptr = vuse;
+ 
    /* Fixup vuse and hash.  */
    vr->hashcode = vr->hashcode - iterative_hash_expr (vr->vuse, 0);
    vr->vuse = SSA_VAL (vuse);
*************** vn_reference_lookup_3 (ao_ref *ref, tree
*** 1161,1166 ****
--- 1166,1174 ----
  	return (void *)-1;
        *ref = r;
  
+       /* Do not update last seen VUSE after translating.  */
+       last_vuse_ptr = NULL;
+ 
        /* Keep looking for the adjusted *REF / VR pair.  */
        return NULL;
      }
*************** static bool
*** 1961,1967 ****
  visit_reference_op_load (tree lhs, tree op, gimple stmt)
  {
    bool changed = false;
!   tree result = vn_reference_lookup (op, gimple_vuse (stmt), true, NULL);
  
    /* If we have a VCE, try looking up its operand as it might be stored in
       a different type.  */
--- 1969,1981 ----
  visit_reference_op_load (tree lhs, tree op, gimple stmt)
  {
    bool changed = false;
!   tree last_vuse;
!   tree result;
! 
!   last_vuse = gimple_vuse (stmt);
!   last_vuse_ptr = &last_vuse;
!   result = vn_reference_lookup (op, gimple_vuse (stmt), true, NULL);
!   last_vuse_ptr = NULL;
  
    /* If we have a VCE, try looking up its operand as it might be stored in
       a different type.  */
*************** visit_reference_op_load (tree lhs, tree 
*** 2045,2051 ****
    else
      {
        changed = set_ssa_val_to (lhs, lhs);
!       vn_reference_insert (op, lhs, gimple_vuse (stmt));
      }
  
    return changed;
--- 2059,2065 ----
    else
      {
        changed = set_ssa_val_to (lhs, lhs);
!       vn_reference_insert (op, lhs, last_vuse);
      }
  
    return changed;