https://gcc.gnu.org/bugzilla/show_bug.cgi?id=120837
--- Comment #18 from GCC Commits <cvs-commit at gcc dot gnu.org> --- The master branch has been updated by Jakub Jelinek <ja...@gcc.gnu.org>: https://gcc.gnu.org/g:e16820d4f7ab1d8a40f70beef722e6f8a4c2392c commit r16-2000-ge16820d4f7ab1d8a40f70beef722e6f8a4c2392c Author: Jakub Jelinek <ja...@redhat.com> Date: Fri Jul 4 07:50:12 2025 +0200 c-family: Tweak ptr +- (expr +- cst) FE optimization [PR120837] The following testcase is miscompiled with -fsanitize=undefined but we introduce UB into the IL even without that flag. The optimization ptr +- (expr +- cst) when expr/cst have undefined overflow into (ptr +- cst) +- expr is sometimes simply not valid, without careful analysis on what ptr points to we don't know if it is valid to do (ptr +- cst) pointer arithmetics. E.g. on the testcase, ptr points to start of an array (actually conditionally one or another) and cst is -1, so ptr - 1 is invalid pointer arithmetics, while ptr + (expr - 1) can be valid if expr is at runtime always > 1 and smaller than size of the array ptr points to + 1. Unfortunately, removing this 1992-ish optimization altogether causes FAIL: c-c++-common/restrict-2.c -Wc++-compat scan-tree-dump-times lim2 "Moving statement" 11 FAIL: gcc.dg/tree-ssa/copy-headers-5.c scan-tree-dump ch2 "is now do-while loop" FAIL: gcc.dg/tree-ssa/copy-headers-5.c scan-tree-dump-times ch2 " if " 3 FAIL: gcc.dg/vect/pr57558-2.c scan-tree-dump vect "vectorized 1 loops" FAIL: gcc.dg/vect/pr57558-2.c -flto -ffat-lto-objects scan-tree-dump vect "vectorized 1 loops" regressions (restrict-2.c also for C++ in all std modes). I've been thinking about some match.pd optimization for signed integer addition/subtraction of constant followed by widening integral conversion followed by multiplication or left shift, but that wouldn't help 32-bit arches. So, instead at least for now, the following patch keeps doing the optimization, just doesn't perform it in pointer arithmetics. pointer_int_sum itself actually adds the multiplication by size_exp, so ptr + expr is turned into ptr p+ expr * size_exp, so this patch will try to optimize ptr + (expr +- cst) into ptr p+ ((sizetype)expr * size_exp +- (sizetype)cst * size_exp) and ptr - (expr +- cst) into ptr p+ -((sizetype)expr * size_exp +- (sizetype)cst * size_exp) 2025-07-04 Jakub Jelinek <ja...@redhat.com> PR c/120837 * c-common.cc (pointer_int_sum): Rewrite the intop PLUS_EXPR or MINUS_EXPR optimization into extension of both intop operands, their separate multiplication and then addition/subtraction followed by rest of pointer_int_sum handling after the multiplication. * gcc.dg/ubsan/pr120837.c: New test.
