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

[Richard Biener <richard dot guenther at gmail dot com>]

On Fri, Aug 19, 2016 at 1:06 AM, Patrick Palka <patrick@parcs.ath.cx> wrote:
> 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

you can use element_precision (op0) == 1 instead.

Richard.

>                   && 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);
>>
>>
>>