Re: [PATCH][RTL ifcvt] PR rtl-optimization/66940: Avoid signed overflow in noce_get_alt_condition

[Kyrill Tkachov <kyrylo dot tkachov at foss dot arm dot com>]

On 23/05/16 12:27, Richard Biener wrote:
> On Mon, May 23, 2016 at 1:17 PM, Kyrill Tkachov
> <kyrylo.tkachov@foss.arm.com> wrote:
> > Hi all,
> >
> > In this PR we end up hitting a signed overflow in noce_get_alt_condition
> > when we try to
> > increment or decrement a HOST_WIDE_INT that might be HOST_WIDE_INT_MAX or
> > HOST_WIDE_INT_MIN.
> >
> > I've confirmed the overflow by adding an assert before the operation:
> > gcc_assert (desired_val != HOST_WIDE_INT_MAX);
> >
> > This patch fixes those cases by catching the cases when desired_val has the
> > extreme
> > value and avoids the transformation that function is trying to make.
> >
> > Bootstrapped and tested on arm, aarch64, x86_64.
> >
> > I've added the testcase that I used to trigger the assert mentioned above as
> > a compile test,
> > though I'm not sure how much value it has...
> >
> > Ok for trunk?
> If this isn't also a wrong-code issue (runtime testcase?) then why not perform
> the operation in unsigned HOST_WIDE_INT instead?

This part of the code transforms a comparison "x < CST" to "x <= CST - 1"
and similar transformations. Fro what I understand the LT,LE,GT,GE RTL comparison
operators operate on signed integers, so I'm not sure how valid it would be
to do all this on unsigned HOST_WIDE_INT.

Thanks,
Kyrill


> Richard.
>
> > Thanks,
> > Kyrill
> >
> > 2016-05-23  Kyrylo Tkachov  <kyrylo.tkachov@arm.com>
> >
> >      PR rtl-optimization/66940
> >      * ifcvt.c (noce_get_alt_condition): Check that incrementing or
> >      decrementing desired_val will not overflow before performing these
> >      operations.
> >
> > 2016-05-23  Kyrylo Tkachov  <kyrylo.tkachov@arm.com>
> >
> >      PR rtl-optimization/66940
> >      * gcc.c-torture/compile/pr66940.c: New test.