https://gcc.gnu.org/bugzilla/show_bug.cgi?id=93738
--- Comment #22 from GCC Commits <cvs-commit at gcc dot gnu.org> --- The master branch has been updated by Jakub Jelinek <[email protected]>: https://gcc.gnu.org/g:9167c9eeea1ca55408a2f500d10ce521a296af0e commit r16-6765-g9167c9eeea1ca55408a2f500d10ce521a296af0e Author: Jakub Jelinek <[email protected]> Date: Wed Jan 14 13:21:57 2026 +0100 simplify-rtx: Fix up SUBREG and LSHIFTRT order canonicalization for AND with constant [PR123544] On Tue, Nov 04, 2025 at 12:59:03PM +0530, Kishan Parmar wrote: > PR rtl-optimization/93738 > * simplify-rtx.cc (simplify_binary_operation_1): Canonicalize > SUBREG(LSHIFTRT) into LSHIFTRT(SUBREG) when valid. This change regressed the following testcase on aarch64-linux. From what I can see, the PR93738 change has been written with non-paradoxical SUBREGs in mind but on this testcase on aarch64 we have a paradoxical SUBREG, in particular simplify_binary_operation_1 is called with AND, SImode, (subreg:SI (lshiftrt:HI (subreg:HI (reg/v:SI 108 [ x ]) 0) (const_int 8 [0x8])) 0) and op1 (const_int 32767 [0x7fff]) and simplifies that since the PR93738 optimization was added into (and:SI (lshiftrt:SI (reg/v:SI 108 [ x ]) (const_int 8 [0x8])) (const_int 32767 [0x7fff])) This looks wrong to me. Consider (reg/v:SI 108 [ x ]) 0) could have value 0x12345678U. The original expression takes lowpart 16-bits from that, i.e. 0x5678U, shifts that right logically by 8 bits, so 0x56U, makes a paradoxical SUBREG from that, i.e. 0x????0056U and masks that with 0x7fff, i.e. result is 0x56U. The new expression shifts 0x12345678U logically right by 8 bits, i.e. 0x123456U and masks it by 0x7fff, result 0x3456U. Thus, I think we need to limit to non-paradoxical SUBREGs. On the rlwimi-2.c testcase I see on powerpc64le-linux no differences in emitted assembly without/with the patch. 2026-01-14 Jakub Jelinek <[email protected]> PR rtl-optimization/123544 * simplify-rtx.cc (simplify_context::simplify_binary_operation_1) <case AND>: Don't canonicalize (subreg (lshiftrt (x cnt)) low) into (lshiftrt (subreg x low) cnt) if the SUBREG is paradoxical. * gcc.dg/pr123544.c: New test.
