https://gcc.gnu.org/bugzilla/show_bug.cgi?id=121413
--- Comment #6 from GCC Commits <cvs-commit at gcc dot gnu.org> --- The releases/gcc-14 branch has been updated by Jakub Jelinek <ja...@gcc.gnu.org>: https://gcc.gnu.org/g:edc2388e802853ddc70b04a6de2b3c180a2a8442 commit r14-11941-gedc2388e802853ddc70b04a6de2b3c180a2a8442 Author: Jakub Jelinek <ja...@redhat.com> Date: Wed Aug 6 11:30:08 2025 +0200 bitint: Fix up INTEGER_CST PHI handling [PR121413] The following testcase is miscompiled on aarch64-linux. The problem is in the optimization to shorten large constants in PHI arguments. In a couple of places during bitint lowering we compute minimal precision of constant and if it is significantly smaller than the precision of the type, store smaller constant in memory and extend it at runtime (zero or all ones). Now, in most places that works fine, we handle the stored number of limbs by loading them from memory and then the rest is extended. In the PHI INTEGER_CST argument handling we do it differently, we don't form there any loops (because we insert stmt sequences on the edges). The problem is that we copy the whole _BitInt variable from memory to the PHI VAR_DECL + initialize the rest to = {} or memset to -1. It has min_prec = CEIL (min_prec, limb_prec) * limb_prec; precision, so e.g. on x86_64 there is no padding and it works just fine. But on aarch64 which has abi_limb_mode TImode and limb_mode DImode it doesn't in some cases. In the testcase the constant has 408 bits min precision, rounded up to limb_prec (64) is 448, i.e. 7 limbs. But aarch64 with TImode abi_limb_mode will actually allocate 8 limbs and the most significant limb is solely padding. As we want to extend the constant with all ones, copying the padding (from memory, so 0s) will result in 64 0 bits where 1 bits were needed. The following patch fixes it by detecting this case and setting min_prec to a multiple of abi limb precision so that it has no padding. 2025-08-06 Jakub Jelinek <ja...@redhat.com> PR tree-optimization/121413 * gimple-lower-bitint.cc (abi_limb_prec): New variable (bitint_precision_kind): Initialize it. (gimple_lower_bitint): Clear it at the start. For min_prec > limb_prec descreased precision vars for INTEGER_CST PHI arguments ensure min_prec is either prec or multiple of abi_limb_prec. * gcc.dg/torture/bitint-85.c: New test. (cherry picked from commit 70aff5112ec25f2391d8048d8c7994160d3cb008)
