> On 8 Aug 2026, at 00:51, H.J. Lu <[email protected]> wrote: > > 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
Sorry, I had some other match.pd fold patterns in my tree and there was a phase mismatch in my commit order. These scans and functions should not be in this patch. I’ve removed them with g:1c37c631abe1de02e9b843a264b9923c12e9a68d Thanks for catching it. Kyrill > > -- > H.J.
