Re: [PATCH] tree-optimization/91838 - fix FAIL of g++.dg/opt/pr91838.C

[Jakub Jelinek <jakub@redhat.com>]

On Thu, Jul 27, 2023 at 12:00:56PM +0000, Richard Biener wrote:
> The following fixes the lack of simplification of a vector shift
> by an out-of-bounds shift value.  For scalars this is done both
> by CCP and VRP but vectors are not handled there.  This results
> in PR91838 differences in outcome dependent on whether a vector
> shift ISA is available and thus vector lowering does or does not
> expose scalar shifts here.
> 
> The following adds a match.pd pattern to catch uniform out-of-bound
> shifts, simplifying them to zero when not sanitizing shift amounts.
> 
> Bootstrapped and tested on x86_64-unknown-linux-gnu.
> 
> OK?
> 
> Thanks,
> Richard.
> 
> 	PR tree-optimization/91838
> 	* match.pd (([rl]shift @0 out-of-bounds) -> zero): New pattern.

The !(flag_sanitize & SANITIZE_SHIFT_EXPONENT)
should be !sanitize_flags_p (SANITIZE_SHIFT_EXPONENT)
or maybe even
GIMPLE || !sanitize_flags_p (SANITIZE_SHIFT_EXPONENT)
because the shift ubsan instrumentation is done on GENERIC, so it can be
optimized on GIMPLE even with ubsan.

Otherwise LGTM.

	Jakub