On Sun, Jun 20, 2021 at 5:37 PM Roger Sayle <ro...@nextmovesoftware.com> wrote: > > > 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.
Agreed. Peephole2 pass runs before sched2 (and x86 targets do not use sched1 pass), so there is a good chance these instructions stay together until the new peephole2 converts them. > 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. > > This patch has been tested on x86_64-pc-linux-gnu with a make bootstrap > and make -k check with no new failures. > > Ok for mainline? > > > 2021-06-20 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. OK. Thanks, Uros.
