https://gcc.gnu.org/bugzilla/show_bug.cgi?id=120447
--- Comment #11 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:5cb46d8fff07afee5ca828303544025e4a2e17b7 commit r16-1101-g5cb46d8fff07afee5ca828303544025e4a2e17b7 Author: Richard Sandiford <richard.sandif...@arm.com> Date: Wed Jun 4 13:36:51 2025 +0100 emit-rtl: Tweak validate_subreg ordered_p condition [PR120447] In the comment trail for PR119966, I'd said that the validate_subreg condition: /* The outer size must be ordered wrt the register size, otherwise we wouldn't know at compile time how many registers the outer mode occupies. */ if (!ordered_p (osize, regsize)) return false; "is also potentially relevant" for paradoxical subregs. But I'd forgotten an important caveat. If the inner size is smaller than a register, we know that the inner value will only occupy a single register. Although the paradoxical subreg might extend that single register to multiple registers by padding with undefined bits, the register size that matters for the extension is: REGMODE_NATURAL_SIZE (omode) rather than regsize's: REGMODE_NATURAL_SIZE (imode) The ordered check is still relevant if the inner value spans multiple registers. Enabling the check above for paradoxical subregs led to an ICE in the testcase, where we tried to generate a VNx4QI paradoxical subreg of a QI scalar. This was previously allowed, and AFAIK worked correctly. The patch doesn't have the effect of relaxing the condition for non-paradoxical subregs, since: known_le (osize, isize) && known_le (isize, regsize) => known_le (osize, regsize) => ordered_p (osize, regsize) So even before the patch for PR119966, the condition only existed for the maybe_gt (isize, regsize) case. The term "block" used in the comment is taken from the rtl.texi documentation of subregs. gcc/ PR rtl-optimization/120447 * emit-rtl.cc (validate_subreg): Restrict ordered_p test between osize and regsize to cases where the inner value occupies multiple blocks. gcc/testsuite/ PR rtl-optimization/120447 * gcc.dg/pr120447.c: New test.
