[PATCH v1 1/2] Simplify shifts wider than the bitwidth of types

Jeff Law law@redhat.com
Mon Nov 16 23:38:20 GMT 2020


On 11/16/20 11:57 AM, Philipp Tomsich wrote:
> From: Philipp Tomsich <prt@gnu.org>
>
> While most shifts wider than the bitwidth of a type will be caught by
> other passes, it is possible that these show up for VRP.
> Consider the following example:
>   int func (int a, int b, int c)
>   {
>     return (a << ((b && c) - 1));
>   }
>
> This adds simplify_using_ranges::simplify_lshift_using_ranges to
> detect and rewrite such cases.  If the intersection of meaningful
> shift amounts for the underlying type and the value-range computed
> for the shift-amount (whether an integer constant or a variable) is
> empty, the statement is replaced with the zero-constant of the same
> precision as the result.
>
> gcc/ChangeLog:
>
>        * vr-values.h (simplify_using_ranges): Declare.
>        * vr-values.c (simplify_lshift_using_ranges): New function.
>        (simplify): Use simplify_lshift_using_ranges for LSHIFT_EXPR.

Umm, isn't this a shift wider than the bitwidth undefined behavior?  We
should be generating warnings for that, not trying to further optimize
it :-)


jeff




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