On 8/5/2026 5:40 AM, [email protected] wrote:
From: Kyrylo Tkachov <[email protected]>
Neither an inclusive nor an exclusive or can carry, so an operand whose set
bits all lie below the shift count contributes nothing to the result:
int f (int a, int b) { return (a ^ (b & 1)) >> 1; }
aarch64 -O2 before:
and w1, w1, 1
eor w0, w1, w0
asr w0, w0, 1
after:
asr w0, w0, 1
The set bits are read from tree_nonzero_bits, so the rule also fires when
the operand is a boolean, a narrow value or anything else whose range the
middle end already knows. Found by mining the optimized dumps of real code,
where the shape comes from flag bits packed into the low bits of a word.
Bootstrapped and tested on aarch64-none-linux-gnu.
Ok for trunk?
Thanks,
Kyrill
gcc/ChangeLog:
* match.pd ((X | Y) >> C, (X ^ Y) >> C): New simplification.
gcc/testsuite/ChangeLog:
* gcc.dg/tree-ssa/shift-drops-bitop-1.c: New test.
GIven this can fire for !GIMPLE and we totally drop the Y term, don't we
need to either verify Y has no side effects or guard the transformation
on GIMPLE?
Jeff
