https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115881
--- Comment #8 from GCC Commits <cvs-commit at gcc dot gnu.org> --- The trunk branch has been updated by Richard Sandiford <rsand...@gcc.gnu.org>: https://gcc.gnu.org/g:d63b6d8b494483b0049370ff0dfeee0e1d10e54b commit r15-2429-gd63b6d8b494483b0049370ff0dfeee0e1d10e54b Author: Richard Sandiford <richard.sandif...@arm.com> Date: Wed Jul 31 09:23:35 2024 +0100 recog: Disallow subregs in mode-punned value [PR115881] In g:9d20529d94b23275885f380d155fe8671ab5353a, I'd extended insn_propagation to handle simple cases of hard-reg mode punning. The punned "to" value was created using simplify_subreg rather than simplify_gen_subreg, on the basis that hard-coded subregs aren't generally useful after RA (where hard-reg propagation is expected to happen). This PR is about a case where the subreg gets pushed into the operands of a plus, but the subreg on one of the operands cannot be simplified. Specifically, we have to generate (subreg:SI (reg:DI sp) 0) rather than (reg:SI sp), since all references to the stack pointer must be via stack_pointer_rtx. However, code in x86 (reasonably) expects no subregs of registers to appear after RA, except for special cases like strict_low_part. This leads to an awkward situation where we can't ban subregs of sp (because of the strict_low_part use), can't allow direct references to sp in other modes (because of the stack_pointer_rtx requirement), and can't allow rvalue uses of the subreg (because of the "no subregs after RA" assumption). It all seems a bit of a mess... I sat on this for a while in the hope that a clean solution might become apparent, but in the end, I think we'll just have to check manually for nested subregs and punt on them. gcc/ PR rtl-optimization/115881 * recog.cc: Include rtl-iter.h. (insn_propagation::apply_to_rvalue_1): Check that the result of simplify_subreg does not include nested subregs. gcc/testsuite/ PR rtl-optimization/115881 * gcc.c-torture/compile/pr115881.c: New test.
