Re: [PATCH] Restrict jump threading statement simplifier to scalar types (PR71077)

[Patrick Palka <patrick at parcs dot ath dot cx>]

On Thu, 18 Aug 2016, Richard Biener wrote:

> On August 18, 2016 8:25:18 PM GMT+02:00, Patrick Palka <patrick@parcs.ath.cx> wrote:
> >In comment #5 Yuri reports that r235653 introduces a runtime failure
> >for
> >176.gcc which I guess is caused by the combining step in
> >simplify_control_stmt_condition_1() not behaving properly on operands
> >of
> >type VECTOR_TYPE.  I'm a bit stumped as to why it mishandles
> >VECTOR_TYPEs because the logic should be generic enough to support them
> >as well.  But it was confirmed that restricting the combining step to
> >operands of scalar type fixes the runtime failure so here is a patch
> >that does this.  Does this look OK to commit after bootstrap +
> >regtesting on x86_64-pc-linux-gnu?
> 
> Hum, I'd rather understand what is going wrong.  Can you at least isolate a testcase?
> 
> Richard.

I don't have access to the SPEC benchmarks unfortunately.  Maybe Yuri
can isolate a test case?

But I think I found a theoretical bug which may or may not coincide with
the bug that Yuri is observing.  The part of the combining step that may
provide wrong results for VECTOR_TYPEs is the one that simplifies the
conditional (A & B) != 0 to true when given that A != 0 and B != 0 and
given that their TYPE_PRECISION is 1.

The TYPE_PRECISION test was intended to succeed only on scalars, but
IIUC it accidentally succeeds on one-dimensional vectors too.  So we may
be wrongly simplifying X & Y != <0> to true given that e.g.  X == <8>
and Y == <2>.  So this simplification should probably be restricted to
integral types like so:

diff --git a/gcc/tree-ssa-threadedge.c b/gcc/tree-ssa-threadedge.c
index 170e456..b8c8b70 100644
--- a/gcc/tree-ssa-threadedge.c
+++ b/gcc/tree-ssa-threadedge.c
@@ -648,14 +648,17 @@ simplify_control_stmt_condition_1 (edge e,
 	  if (res1 != NULL_TREE && res2 != NULL_TREE)
 	    {
 	      if (rhs_code == BIT_AND_EXPR
+		  && INTEGRAL_TYPE_P (TREE_TYPE (op0))
 		  && TYPE_PRECISION (TREE_TYPE (op0)) == 1
 		  && integer_nonzerop (res1)
 		  && integer_nonzerop (res2))
-- 
2.9.3.650.g20ba99f

Hope this makes sense.

> 
> >gcc/ChangeLog:
> >
> >	PR tree-optimization/71077
> >	* tree-ssa-threadedge.c (simplify_control_stmt_condition_1):
> >	Perform the combining step only if the operands have an integral
> >	or a pointer type.
> >---
> > gcc/tree-ssa-threadedge.c | 3 +++
> > 1 file changed, 3 insertions(+)
> >
> >diff --git a/gcc/tree-ssa-threadedge.c b/gcc/tree-ssa-threadedge.c
> >index 170e456..a97c00c 100644
> >--- a/gcc/tree-ssa-threadedge.c
> >+++ b/gcc/tree-ssa-threadedge.c
> >@@ -577,6 +577,9 @@ simplify_control_stmt_condition_1 (edge e,
> >   if (handle_dominating_asserts
> >       && (cond_code == EQ_EXPR || cond_code == NE_EXPR)
> >       && TREE_CODE (op0) == SSA_NAME
> >+      /* ??? Vector types are mishandled here.  */
> >+      && (INTEGRAL_TYPE_P (TREE_TYPE (op0))
> >+	  || POINTER_TYPE_P (TREE_TYPE (op0)))
> >       && integer_zerop (op1))
> >     {
> >       gimple *def_stmt = SSA_NAME_DEF_STMT (op0);
> 
> 
>