Re: [PATCH] PR tree-optimization/64992: (B << 2) != 0 is B when B is Boolean.

[Richard Biener <richard.guenther@gmail.com>]

On Mon, Aug 8, 2022 at 11:06 AM Roger Sayle <roger@nextmovesoftware.com> wrote:
>
>
> This patch resolves both PR tree-optimization/64992 and PR
> tree-optimization/98956 which are missed optimization enhancement
> request, for which Andrew Pinski already has a proposed solution
> (related to a fix for PR tree-optimization/98954).  Yesterday,
> I proposed an alternate improved patch for PR98954, which although
> superior in most respects, alas didn't address this case [which
> doesn't include a BIT_AND_EXPR], hence this follow-up fix.
>
> For many functions, F(B), of a (zero-one) Boolean value B, the
> expression F(B) != 0 can often be simplified to just B.  Hence
> "(B * 5) != 0" is B, "-B != 0" is B, "bswap(B) != 0" is B,
> "(B >>r 3) != 0" is B.  These are all currently optimized by GCC,
> with the strange exception of left shifts by a constant (possibly
> due to the undefined/implementation defined behaviour when the
> shift constant is larger than the first operand's precision).
> This patch adds support for this particular case, when the shift
> constant is valid.
>
> This patch has been tested on x86_64-pc-linux-gnu with make bootstrap
> and make -k check, both with and without --target_board=unix{-m32},
> with no new failures.  Ok for mainline?

+/* (X << C) != 0 can be simplified to X, when X is zero_one_valued_p.  */
+(simplify
+  (ne (lshift zero_one_valued_p@0 INTEGER_CST@1) integer_zerop@2)
+  (if (tree_fits_shwi_p (@1)
+       && tree_to_shwi (@1) > 0
+       && tree_to_shwi (@1) < TYPE_PRECISION (TREE_TYPE (@0)))
+    (convert @0)))

while we deliberately do not fold int << 34 since the result is undefined
there is IMHO no reason to not fold the above for any (even non-constant)
shift value.  We have guards with TYPE_OVERFLOW_SANITIZED in
some cases but I think that's not appropriate here, there's one
flag_sanitize check, maybe there's a special bit for SHIFT overflow we
can use.  Why is (X << 0) != 0 excempt in the condition?

>
> 2022-08-08  Roger Sayle  <roger@nextmovesoftware.com>
>
> gcc/ChangeLog
>         PR tree-optimization/64992
>         PR tree-optimization/98956
>         * match.pd (ne (lshift @0 @1) 0): Simplify (X << C) != 0 to X
>         when X is zero_one_valued_p and the shift constant C is valid.
>         (eq (lshift @0 @1) 0): Likewise, simplify (X << C) == 0 to !X
>         when X is zero_one_valued_p and the shift constant C is valid.
>
> gcc/testsuite/ChangeLog
>         PR tree-optimization/64992
>         * gcc.dg/pr64992.c: New test case.
>
>
> Thanks in advance,
> Roger
> --
>