Re: [PATCH][committed][v2] match: fold two idioms built from the negation of a value

On Thu, Aug 6, 2026 at 10:32 PM <[email protected]> wrote:
>
> From: Kyrylo Tkachov <[email protected]>
>
> X | -X has every bit from the lowest set bit of X upwards, so adding X to
> it clears that bit, and masking with it isolates the padding needed to
> round X up:
>
>   X + (X | -X) -> X & (X - 1)
>   X + ((-X) & (C - 1)) -> (X + C - 1) & -C   for a power of two C
>
> The second is the alignment round up written with the padding computed
> first, which is how allocators tend to spell it.
>
> Neither needs a wrapping type.  X - 1 overflows only for the most
> negative value, where the source already does, and rounding X up is
> representable exactly when X + C - 1 is, because the largest multiple of
> C below the maximum leaves room for C - 1.  Restrict both rules to integral
> types.  The bitwise operations also accept fixed-point types, whose
> saturating arithmetic does not have these integer semantics.
>
>   int f (int x) { return x + ((-x) & 15); }
>
> aarch64 -O2:
>
>   before                          after
>     neg   w1, w0                    add   w0, w0, 15
>     and   w1, w1, 15                and   w0, w0, -16
>     add   w0, w1, w0
>
> The vector spelling folds too, a uniform vector constant is matched with
> uniform_integer_cst_p.
>
> Keep trapping and sanitized negations.  Also keep the source addition
> of X + (X | -X) under overflow sanitization because the replacement
> would remove one recoverable diagnostic.  Use :s on the consumed bitwise
> expression.  This prevents new work when it remains live, but allows the
> fold when the replacement arithmetic is already available.
>
> Reuse the matched uniform constant and test its wide value directly.
> This avoids rebuilding the same constant only to inspect it.
>
> Bootstrapped and tested on aarch64-none-linux-gnu.
> Committed as per Richard's approval.
>
> gcc/ChangeLog:
>
>         * match.pd (X + (X | -X)): New simplification.
>         (X + ((-X) & (C - 1))): Likewise.
>
> gcc/testsuite/ChangeLog:
>
>         * gcc.dg/tree-ssa/signbit-1.c: New test.
>         * gcc.dg/tree-ssa/alignup-2.c: New test.
>         * gcc.dg/tree-ssa/vector-alignup-1.c: New test.
>         * gcc.dg/tree-ssa/alignup-overflow-1.c: New test.
>         * gcc.dg/tree-ssa/alignup-overflow-2.c: New test.
>         * gcc.dg/tree-ssa/signbit-4.c: New test.
>

On Linux/x86-64, I got

FAIL: gcc.dg/tree-ssa/signbit-1.c scan-tree-dump-times optimized " \\| " 1
FAIL: gcc.dg/tree-ssa/signbit-1.c scan-tree-dump-times optimized " != 0" 3

-- 
H.J.