https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114661
--- Comment #3 from GCC 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:df9451936c6c9e4faea371e3f188e1fc6b6d39e3 commit r15-2053-gdf9451936c6c9e4faea371e3f188e1fc6b6d39e3 Author: Roger Sayle <ro...@nextmovesoftware.com> Date: Tue Jul 16 07:58:28 2024 +0100 PR tree-optimization/114661: Generalize MULT_EXPR recognition in match.pd. This patch resolves PR tree-optimization/114661, by generalizing the set of expressions that we canonicalize to multiplication. This extends the optimization(s) contributed (by me) back in July 2021. https://gcc.gnu.org/pipermail/gcc-patches/2021-July/575999.html The existing transformation folds (X*C1)^(X<<C2) into X*C3 when allowed. A subtlety is that for non-wrapping integer types, we actually fold this into (int)((unsigned)X*C3) so that we don't introduce an undefined overflow that wasn't in the original. Unfortunately, this transformation confuses itself, as the type-cast multiplication isn't recognized when further combining bit operations. Fixed here by allowing optional useless type conversions in transforms to turn (int)((unsigned)X*C1)^(X<<C2) into (int)((unsigned)X*C3) so that match.pd and EVRP can continue to construct multiplications. For the example given in the PR: unsigned mul(unsigned char c) { if (c > 3) __builtin_unreachable(); return c << 18 | c << 15 | c << 12 | c << 9 | c << 6 | c << 3 | c; } GCC on x86_64 with -O2 previously generated: mul: movzbl %dil, %edi leal (%rdi,%rdi,8), %edx leal 0(,%rdx,8), %eax movl %edx, %ecx sall $15, %edx orl %edi, %eax sall $9, %ecx orl %ecx, %eax orl %edx, %eax ret with this patch we now generate: mul: movzbl %dil, %eax imull $299593, %eax, %eax ret 2024-07-16 Roger Sayle <ro...@nextmovesoftware.com> Richard Biener <rguent...@suse.de> gcc/ChangeLog PR tree-optimization/114661 * match.pd ((X*C1)|(X*C2) to X*(C1+C2)): Allow optional useless type conversions around multiplications, such as those inserted by this transformation. gcc/testsuite/ChangeLog PR tree-optimization/114661 * gcc.dg/pr114661.c: New test case.
