https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110701
--- Comment #8 from CVS Commits <cvs-commit at gcc dot gnu.org> --- The master branch has been updated by Roger Sayle <sa...@gcc.gnu.org>: https://gcc.gnu.org/g:263369b2f7f726a3d4b269678d2c13a9d06a041e commit r14-4398-g263369b2f7f726a3d4b269678d2c13a9d06a041e Author: Roger Sayle <ro...@nextmovesoftware.com> Date: Wed Oct 4 17:11:23 2023 +0100 PR rtl-optimization/110701: Fix SUBREG SET_DEST handling in combine. This patch is my proposed fix to PR rtl-optimization 110701, a latent bug in combine's record_dead_and_set_regs_1 exposed by recent improvements to simplify_subreg. The issue involves the handling of (normal) SUBREG SET_DESTs as in the instruction: (set (subreg:HI (reg:SI x) 0) (expr:HI y)) The semantics of this are that the bits specified by the SUBREG are set to the SET_SRC, y, and that the other bits of the SET_DEST are left/become undefined. To simplify explanation, we'll only consider lowpart SUBREGs (though in theory non-lowpart SUBREGS could be handled), and the fact that bits outside of the lowpart WORD retain their original values (treating these as undefined is a missed optimization rather than incorrect code bug, that only affects targets with less than 64-bit words). The bug is that combine simulates the behaviour of the above instruction, for calculating nonzero_bits and set_sign_bit_copies, in the function record_value_for_reg, by using the equivalent of: (set (reg:SI x) (subreg:SI (expr:HI y)) by calling gen_lowpart on the SET_SRC. Alas, the semantics of this revised instruction aren't always equivalent to the original. In the test case for PR110701, the original instruction (set (subreg:HI (reg:SI x), 0) (and:HI (subreg:HI (reg:SI y) 0) (const_int 340))) which (by definition) leaves the top bits of x undefined, is mistakenly considered to be equivalent to (set (reg:SI x) (and:SI (reg:SI y) (const_int 340))) where gen_lowpart's freedom to do anything with paradoxical SUBREG bits, has now cleared the high bits. The same bug also triggers when the SET_SRC is say (subreg:HI (reg:DI z)), where gen_lowpart transforms this into (subreg:SI (reg:DI z)) which defines bits 16-31 to be the same as bits 16-31 of z. The fix is that after calling record_value_for_reg, we need to mark the bits that should be undefined as undefined, in case gen_lowpart, which performs transforms appropriate for r-values, has changed the interpretation of the SUBREG when used as an l-value. 2023-10-04 Roger Sayle <ro...@nextmovesoftware.com> gcc/ChangeLog PR rtl-optimization/110701 * combine.cc (record_dead_and_set_regs_1): Split comment into pieces placed before the relevant clauses. When the SET_DEST is a partial_subreg_p, mark the bits outside of the updated portion of the destination as undefined. gcc/testsuite/ChangeLog PR rtl-optimization/110701 * gcc.target/i386/pr110701.c: New test case.
