[x86_64 PATCH] PR target/11877: Use xor to write zero to memory with -Os

Uros Bizjak ubizjak@gmail.com
Mon Jun 21 07:18:28 GMT 2021

On Sun, Jun 20, 2021 at 5:37 PM Roger Sayle <roger@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  <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.



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