Re: [PATCH] Fix up reduction-1{1,2} testcases (PR middle-end/68221)

[Thomas Schwinge <thomas at codesourcery dot com>]

Hi!

On Mon, 23 Nov 2015 12:13:07 +0100 (CET), Richard Biener <rguenther@suse.de> wrote:
> On Fri, 20 Nov 2015, Jakub Jelinek wrote:
> > If C/C++ array section reductions have non-zero (positive) bias, it is
> > implemented by declaring a smaller private array and subtracting the bias
> > from the start of the private array (because valid code may only dereference
> > elements from bias onwards).  But, this isn't something that is kosher in
> > C/C++ pointer arithmetics and the alias oracle seems to get upset on that.
> > So, the following patch fixes that by performing the subtraction on integral
> > type instead of p+ -bias.
> 
> So this still does use the biased pointer because you do not
> re-write accesses (where you could have applied the biasing to
> the indexes / offsets), right?  Thus the patch is merely obfuscation
> for GCC rather than making it kosher for C/C++ (you still have a
> pointer pointing outside of the private array object)?
> 
> I still hope to have a look where the alias oracle gets things
> wrong (well, if so by accident at least).

I understand this ("have a look where the alias oracle gets things
wrong") to have happened in Richi's trunk r230793,
<https://gcc.gnu.org/bugzilla/show_bug.cgi?id=68221#c5>?

I've tested that with the original POINTER_PLUS_EXPR code restored and
with Richi's r230793 applied, for x86_64 GNU/Linux there is no change for
the libgomp.c/reduction-11.c, libgomp.c/reduction-12.c,
libgomp.c++/reduction-11.C, libgomp.c++/reduction-12.C test cases
(already PASSed), but for 32-bit x86, they now PASS instead of FAILing.
That is, I tested with the following (part of r230672) reverted:

> > --- gcc/omp-low.c.jj	2015-11-20 12:56:17.000000000 +0100
> > +++ gcc/omp-low.c	2015-11-20 13:44:29.080374051 +0100
> > @@ -4444,11 +4444,13 @@ lower_rec_input_clauses (tree clauses, g
> >  
> >  	      if (!integer_zerop (bias))
> >  		{
> > -		  bias = fold_convert_loc (clause_loc, sizetype, bias);
> > -		  bias = fold_build1_loc (clause_loc, NEGATE_EXPR,
> > -					  sizetype, bias);
> > -		  x = fold_build2_loc (clause_loc, POINTER_PLUS_EXPR,
> > -				       TREE_TYPE (x), x, bias);
> > +		  bias = fold_convert_loc (clause_loc, pointer_sized_int_node,
> > +					   bias);
> > +		  yb = fold_convert_loc (clause_loc, pointer_sized_int_node,
> > +					 x);
> > +		  yb = fold_build2_loc (clause_loc, MINUS_EXPR,
> > +					pointer_sized_int_node, yb, bias);
> > +		  x = fold_convert_loc (clause_loc, TREE_TYPE (x), yb);
> >  		  yb = create_tmp_var (ptype, name);
> >  		  gimplify_assign (yb, x, ilist);
> >  		  x = yb;

OK to commit the following to trunk?

commit 92b0eebfcbe914d3addeb97d4bb33f76a44dbe60
Author: Thomas Schwinge <thomas@codesourcery.com>
Date:   Thu Nov 26 14:21:13 2015 +0100

    Restore original POINTER_PLUS_EXPR code
    
    	PR middle-end/68221
    	gcc/
    	* omp-low.c (lower_rec_input_clauses): If C/C++ array reduction
    	has non-zero bias, use pointer plus of negated bias instead of
    	subtracting it in integer type.
---
 gcc/omp-low.c |   12 +++++-------
 1 file changed, 5 insertions(+), 7 deletions(-)

diff --git gcc/omp-low.c gcc/omp-low.c
index 0b44588..927d9d9 100644
--- gcc/omp-low.c
+++ gcc/omp-low.c
@@ -4451,13 +4451,11 @@ lower_rec_input_clauses (tree clauses, gimple_seq *ilist, gimple_seq *dlist,
 
 	      if (!integer_zerop (bias))
 		{
-		  bias = fold_convert_loc (clause_loc, pointer_sized_int_node,
-					   bias);
-		  yb = fold_convert_loc (clause_loc, pointer_sized_int_node,
-					 x);
-		  yb = fold_build2_loc (clause_loc, MINUS_EXPR,
-					pointer_sized_int_node, yb, bias);
-		  x = fold_convert_loc (clause_loc, TREE_TYPE (x), yb);
+		  bias = fold_convert_loc (clause_loc, sizetype, bias);
+		  bias = fold_build1_loc (clause_loc, NEGATE_EXPR,
+					  sizetype, bias);
+		  x = fold_build2_loc (clause_loc, POINTER_PLUS_EXPR,
+				       TREE_TYPE (x), x, bias);
 		  yb = create_tmp_var (ptype, name);
 		  gimplify_assign (yb, x, ilist);
 		  x = yb;


GrÃÃe
 Thomas

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