[go] cmd/compile/internal/amd64: use X15 for 4- and 8-byte zero stores

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Gopher Robot (Gerrit)

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Aug 10, 2026, 1:59:56 PM (16 hours ago) Aug 10

to Egon Elbre, goph...@pubsubhelper.golang.org, golang-...@googlegroups.com, Keith Randall, Dmitri Shuralyov, Dmitri Shuralyov, golang...@luci-project-accounts.iam.gserviceaccount.com, Jorropo, Keith Randall, Martin Möhrmann, Michael Pratt, golang-co...@googlegroups.com

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Change information

Commit message:

cmd/compile/internal/amd64: use X15 for 4- and 8-byte zero stores

MOVQ $0, mem encodes two bytes longer than MOVQ X15, mem, and
MOVL $0, mem encodes one byte longer than MOVL X15, mem. In
ABIInternal functions on amd64, X15 is maintained as zero except on
Plan 9.

Use the XMM forms of MOVQ and MOVL when emitting plain and indexed
zero stores. Selecting the encoding during emission leaves the
existing SSA operations available to store-combining rules. Retain
immediate stores for ABI0 and Plan 9, where X15 is not known to
contain zero.

This replaces 5,598 stores when compiling the standard library,
reducing text by 11,004 bytes with no change in instruction count.

Change-Id: Ibccdce5fd17f81be8c2cf505c8144fa612e71101

Reviewed-on: https://go-review.googlesource.com/c/go/+/810120

Reviewed-by: Jorropo <jorro...@gmail.com>

Auto-Submit: Jorropo <jorro...@gmail.com>

Reviewed-by: Dmitri Shuralyov <dmit...@google.com>

LUCI-TryBot-Result: golang...@luci-project-accounts.iam.gserviceaccount.com <golang...@luci-project-accounts.iam.gserviceaccount.com>

Reviewed-by: Keith Randall <k...@google.com>

Files:

M src/cmd/compile/internal/amd64/ssa.go
M src/runtime/unsafepoint_test.go
M test/codegen/math.go
M test/codegen/memcombine.go
M test/codegen/memops.go
M test/codegen/structs.go
M test/codegen/zerosize.go

Change size: M

Delta: 7 files changed, 60 insertions(+), 16 deletions(-)

Branch: refs/heads/master

Submit Requirements:

Code-Review: +2 by Jorropo, +1 by Dmitri Shuralyov, +1 by Keith Randall
TryBots-Pass: LUCI-TryBot-Result+1 by golang...@luci-project-accounts.iam.gserviceaccount.com

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