Integrated: 8315554: C1: Replace "cmp reg, 0" with "test reg, reg" on x86
Aleksey Shipilev
shade at openjdk.org
Wed Sep 6 15:40:52 UTC 2023
On Fri, 1 Sep 2023 17:28:03 GMT, Aleksey Shipilev <shade at openjdk.org> wrote:
> Noticed this when looking at C1 profiling code. There are plenty of usages for `cmp reg, 0` in C1 x86 code, both in generic `LIR_Assembler::comp_op`, and in some profiling paths.
>
> `test reg, reg` is a denser idiom for this comparison. The difference between `cmp` and `test` on x86 seems to be only with AF (aux carry flag). For ubiquitous int/pointer comparisons, this distinction is irrelevant.
>
> C2 already does this transformation in .ad match rules.
>
> Code size improvements with `-Xcomp -XX:+CITime -XX:TieredStopAtLevel=... Hello`:
>
>
> # Before
> tier1: nmethod total size: 430104 bytes
> tier2: nmethod total size: 467336 bytes
> tier3: nmethod total size: 923384 bytes
>
> # After
> tier1: nmethod total size: 427584 bytes (-0.59%)
> tier2: nmethod total size: 464352 bytes (-0.64%)
> tier3: nmethod total size: 918328 bytes (-0.55%)
>
>
> Additional testing:
> - [x] Linux x86_64 `tier1 tier2 tier3` x (C1 level 1, 2, 3) x (Parallel, G1, Shenandoah)
This pull request has now been integrated.
Changeset: bd477810
Author: Aleksey Shipilev <shade at openjdk.org>
URL: https://git.openjdk.org/jdk/commit/bd477810b176696e0fd043f5594663ebcf9884cf
Stats: 15 lines in 1 file changed: 5 ins; 0 del; 10 mod
8315554: C1: Replace "cmp reg, 0" with "test reg, reg" on x86
Reviewed-by: iveresov, chagedorn, thartmann
-------------
PR: https://git.openjdk.org/jdk/pull/15543
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