https://gcc.gnu.org/bugzilla/show_bug.cgi?id=11877
--- Comment #10 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:9cedbaab8e048b90ceb9ceef0d851385fae67cde commit r12-1668-g9cedbaab8e048b90ceb9ceef0d851385fae67cde Author: Roger Sayle <ro...@nextmovesoftware.com> Date: Mon Jun 21 08:54:50 2021 +0100 PR target/11877: Use xor to write zero to memory with -Os The following patch attempts to resolve PR target/11877 (without triggering PR/23102). On x86_64, writing an SImode or DImode zero to memory uses an instruction encoding that is larger than first clearing a register (using xor) then writing that to memory. Hence, after reload, the peephole2 pass can determine if there's a suitable free register, and if so, use that to shrink the code size with -Os. To improve code size, and avoid inserting a large number of xor instructions (PR target/23102), this patch makes use of peephole2's efficient pattern matching to use a single temporary for a run of consecutive writes. In theory, one could do better still with a new target-specific pass, gated on -Os, to shrink these instructions (like stv), but that's probably overkill for the little remaining space savings. Evaluating this patch on the CSiBE benchmark (v2.1.1) results in a 0.26% code size improvement (3715273 bytes down to 3705477) on x86_64 with -Os [saving 1 byte every 400]. 549 of 894 tests improve, two tests grow larger. Analysis of these 2 pathological cases reveals that although peephole2's match_scratch prefers to use a call-clobbered register (to avoid requiring a new stack frame), very rarely this interacts with GCC's shrink wrapping optimization, which may previously have avoided saving/restoring a call clobbered register, such as %eax, in the calling function. 2021-06-21 Roger Sayle <ro...@nextmovesoftware.com> gcc/ChangeLog PR target/11877 * config/i386/i386.md: New define_peephole2s to shrink writing 1, 2 or 4 consecutive zeros to memory when optimizing for size. gcc/testsuite/ChangeLog PR target/11877 * gcc.target/i386/pr11877.c: New test case. -- You are receiving this mail because: You reported the bug.
