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From: "cvs-commit at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org> To: gcc-bugs@gcc.gnu.org Subject: [Bug target/11877] gcc should use xor trick with -Os Date: Mon, 21 Jun 2021 07:56:39 +0000 [thread overview] Message-ID: <bug-11877-4-yFj39SqyK7@http.gcc.gnu.org/bugzilla/> (raw) In-Reply-To: <bug-11877-4@http.gcc.gnu.org/bugzilla/> 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 <sayle@gcc.gnu.org>: https://gcc.gnu.org/g:9cedbaab8e048b90ceb9ceef0d851385fae67cde commit r12-1668-g9cedbaab8e048b90ceb9ceef0d851385fae67cde Author: Roger Sayle <roger@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 <roger@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.
next parent reply other threads:[~2021-06-21 7:56 UTC|newest] Thread overview: 16+ messages / expand[flat|nested] mbox.gz Atom feed top [not found] <bug-11877-4@http.gcc.gnu.org/bugzilla/> 2021-06-21 7:56 ` cvs-commit at gcc dot gnu.org [this message] 2021-06-22 8:18 ` cvs-commit at gcc dot gnu.org 2021-07-10 8:24 ` roger at nextmovesoftware dot com 2021-07-26 18:54 ` pinskia at gcc dot gnu.org 2021-07-26 18:57 ` pinskia at gcc dot gnu.org 2021-07-26 22:20 ` pinskia at gcc dot gnu.org [not found] <bug-11877-5724@http.gcc.gnu.org/bugzilla/> 2006-01-05 20:22 ` dann at godzilla dot ics dot uci dot edu 2003-08-10 15:47 [Bug target/11877] New: " debian-gcc at lists dot debian dot org 2003-08-10 16:03 ` [Bug target/11877] " pinskia at gcc dot gnu dot org 2003-08-23 1:15 ` dhazeghi at yahoo dot com 2003-12-31 21:39 ` kazu at cs dot umass dot edu 2004-01-04 6:55 ` kazu at cs dot umass dot edu 2004-01-04 7:50 ` pinskia at gcc dot gnu dot org 2004-02-01 17:24 ` kazu at cs dot umass dot edu 2004-03-25 19:16 ` kazu at cs dot umass dot edu 2005-08-12 5:28 ` pinskia at gcc dot gnu dot org 2005-08-12 5:28 ` pinskia at gcc dot gnu dot org
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