Re: [PATCH] Canonicalize X&-Y as X*Y in match.pd when Y is [0,1].

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

On Tue, May 24, 2022 at 3:55 PM Roger Sayle <roger@nextmovesoftware.com> wrote:
>
>
> "For every pessimization, there's an equal and opposite optimization".
>
> In the review of my original patch for PR middle-end/98865, Richard
> Biener pointed out that match.pd shouldn't be transforming X*Y into
> X&-Y as the former is considered cheaper by tree-ssa's cost model
> (operator count).  A corollary of this is that we should instead be
> transforming X&-Y into the cheaper X*Y as a preferred canonical form
> (especially as RTL expansion now intelligently selects the appropriate
> implementation based on the target's costs).
>
> With this patch we now generate identical code for:
> int foo(int x, int y) { return -(x&1) & y; }
> int bar(int x, int y) { return (x&1) * y; }
>
> specifically on x86_64-pc-linux-gnu both use and/neg/and when
> optimizing for speed, but both use and/mul when optimizing for
> size.
>
> One minor wrinkle/improvement is that this patch includes three
> additional optimizations (that account for the change in canonical
> form) to continue to optimize PR92834 and PR94786.

Those are presumably the preceeding patterns which match

(convert? (negate@4 (convert? (cmp@5 @2 @3)))

for the multiplication operand - those should be zero_one_valued_p
maybe with the exception of the conversions?  So are the original
patterns still needed after the canonicalization to a multiplication?

Otherwise looks good to me.

Thanks,
Richard.

> 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?
>
>
> 2022-05-24  Roger Sayle  <roger@nextmovesoftware.com>
>
> gcc/ChangeLog
>         * match.pd (match_zero_one_valued_p): New predicate.
>         (mult @0 @1): Use zero_one_valued_p for optimization to the
>         expression "bit_and @0 @1".
>         (bit_and (negate zero_one_valued_p@0) @1): Optimize to MULT_EXPR.
>         (plus @0 (mult (minus @1 @0) zero_one_valued_p@2): New transform.
>         (minus @0 (mult (minus @0 @1) zero_one_valued_p@2): Likewise.
>         (bit_xor @0 (mult (bit_xor @0 @1) zero_one_valued_p@2): Likewise.
>
> gcc/testsuite/ChangeLog
>         * gcc.dg/pr98865.c: New test case.
>
>
> Thanks in advance,
> Roger
> --
>