https://gcc.gnu.org/g:d3c11a21eea2fd1ebb90cdb55684c0182407105c
commit r16-5547-gd3c11a21eea2fd1ebb90cdb55684c0182407105c Author: Jeff Law <[email protected]> Date: Mon Nov 24 06:05:27 2025 -0700 [PR rtl-optimization/122782] Fix out of range shift causing bootstrap failure with ubsan As noted in the PR, we're doing a bogus shift in the new code triggering a ubsan failure. This code works up through DImode and needs to reject attempts at handling wider modes. Yes, it could be extended to those wider modes and I expect we will as int128 becomes more common, but it doesn't seem worth the effort right now. This patch adds the same kind of test we're using elsewhere to guard against the bogus shift. While I haven't been able to reproduce the ubsan bootstrap failure, I can see the bogus shift under the debugger and I can see they no longer occur after this patch. This has been bootstrapped and regression tested on x86 and riscv. It's also been through all the crosses. Pushing to the trunk momentarily. jeff PR rtl-optimization/122782 gcc/ * ext-dce.cc (ext_dct_process_uses): Guard against undefined shifts by properly checking modes on the input object. Diff: --- gcc/ext-dce.cc | 5 ++++- 1 file changed, 4 insertions(+), 1 deletion(-) diff --git a/gcc/ext-dce.cc b/gcc/ext-dce.cc index e6189f973bfc..911daef9f4b0 100644 --- a/gcc/ext-dce.cc +++ b/gcc/ext-dce.cc @@ -804,11 +804,14 @@ ext_dce_process_uses (rtx_insn *insn, rtx obj, Key on the right shift and use (for now) simplistic tests to find the corresponding left shift. */ + scalar_mode outer_mode; if ((code == LSHIFTRT || code == ASHIFTRT) && CONST_INT_P (XEXP (src, 1)) && (INTVAL (XEXP (src, 1)) == BITS_PER_WORD - 8 || INTVAL (XEXP (src, 1)) == BITS_PER_WORD - 16 - || INTVAL (XEXP (src, 1)) == BITS_PER_WORD - 32)) + || INTVAL (XEXP (src, 1)) == BITS_PER_WORD - 32) + && is_a <scalar_mode> (GET_MODE (src), &outer_mode) + && GET_MODE_BITSIZE (outer_mode) <= HOST_BITS_PER_WIDE_INT) { /* So we have a right shift that could correspond to the second in a pair impementing QI, HI or SI -> DI
