Re: [PATCH] middle-end: Optimize ((X >> C1) & C2) != C3 for more cases.

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

On Sun, Aug 7, 2022 at 9:08 PM Roger Sayle <roger@nextmovesoftware.com> wrote:
>
>
> Following my middle-end patch for PR tree-optimization/94026, I'd promised
> Jeff Law that I'd clean up the dead-code in fold-const.cc now that these
> optimizations are handled in match.pd.  Alas, I discovered things aren't
> quite that simple, as the transformations I'd added avoided cases where
> C2 overlapped with the new bits introduced by the shift, but the original
> code handled any value of C2 provided that it had a single-bit set (under
> the condition that C3 was always zero).
>
> This patch upgrades the transformations supported by match.pd to cover
> any values of C2 and C3, provided that C1 is a valid bit shift constant,
> for all three shift types (logical right, arithmetic right and left).
> This then makes the code in fold-const.cc fully redundant, and adds
> support for some new (corner) cases not previously handled.  If the
> constant C1 is valid for the type's precision, the shift is now always
> eliminated (with C2 and C3 possibly updated to test the sign bit).
>
> Interestingly, the fold-const.cc code that I'm now deleting was originally
> added by me back in 2006 to resolve PR middle-end/21137.  I've confirmed
> that those testcase(s) remain resolved with this patch (and I'll close
> 21137 in Bugzilla).  This patch also implements most (but not all) of the
> examples mentioned in PR tree-optimization/98954, for which I have some
> follow-up patches.
>
> 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?

+         (with { wide_int smask = wi::arshift (sb, c1); }
+           (if ((c2 & smask) == 0)
+             (cmp (bit_and @0 { wide_int_to_tree (t0, c2 << c1); })
+                  { wide_int_to_tree (t0, c3 << c1); })
+             (if ((c3 & smask) == 0)
+               (cmp (bit_and @0 { wide_int_to_tree (t0, (c2 << c1) | sb); })
+                    { wide_int_to_tree (t0, c3 << c1); })
+               (if ((c2 & smask) != (c3 & smask))

you can use

   (switch
    (if ((c2 & smask) == 0)
     (...)
    (if ((c3 & smask) == 0)
     (..)
    (if ((c2 & smask) != (c3 & smask))
     (..)))

to make this better readable (switch is basically an if else-if
else-if ... clause).

OK with that change.

Thanks,
Richard.

>
> 2022-08-07  Roger Sayle  <roger@nextmovesoftware.com>
>
> gcc/ChangeLog
>         PR middle-end/21137
>         PR tree-optimization/98954
>         * fold-const.cc (fold_binary_loc): Remove optimizations to
>         optimize ((X >> C1) & C2) ==/!= 0.
>         * match.pd (cmp (bit_and (lshift @0 @1) @2) @3): Remove wi::ctz
>         check, and handle all values of INTEGER_CSTs @2 and @3.
>         (cmp (bit_and (rshift @0 @1) @2) @3): Likewise, remove wi::clz
>         checks, and handle all values of INTEGER_CSTs @2 and @3.
>
> gcc/testsuite/ChangeLog
>         PR middle-end/21137
>         PR tree-optimization/98954
>         * gcc.dg/fold-eqandshift-4.c: New test case.
>
>
> Thanks in advance,
> Roger
> --
>