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

[Philipp Tomsich]

Jeff,

On Tue, 17 Nov 2020 at 00:38, Jeff Law <law@redhat.com> wrote:

>
> 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 :-)
>

The shift is undefined behavior on the language level (for C) and a warning
will be generated, if such a shift is encountered; additionally, the shift
will be
replaced with the value 0.

However, in the above case, the shift is generated only in the middle end:
At 136t.walloca, I still have:

>   # RANGE [-1, 0]
>   _1 = iftmp.1_2 + -1;
>   _6 = a_5(D) << _1;

Whereas at 137t.pre, this is changed into:

> Found partial redundancy for expression {lshift_expr,a_5(D),_1} (0006)
> Inserted _9 = a_5(D) << -1;


In other words, the change to VRP canonicalizes what a lshift_expr with an
shift-amount outside of the type width means... it doesn't assume anything
about the original language.
Do we assume that a LSHIFT_EXPR has the same semantics as for a
C-language shift-left? If so, then pre should not generate the LSHIFT_EXPR
for _9... or we might even catch this later in path isolation (as undefined
behavior, insert a __builtin_trap() and emit a warning)?

Note that in his comment to patch 2/2, Jim has noted that user code for
RISC-V may assume a truncation of the shift-operand...

Philipp.