[go] cmd/compile/internal/ssa: constant fold math/bits functions

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Gopher Robot (Gerrit)

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Aug 6, 2026, 9:53:01 PM (3 days ago) Aug 6

to Gilbert Morgan, goph...@pubsubhelper.golang.org, golang-...@googlegroups.com, golang...@luci-project-accounts.iam.gserviceaccount.com, Keith Randall, Keith Randall, Martin Möhrmann, golang-co...@googlegroups.com

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Change information

Commit message:

cmd/compile/internal/ssa: constant fold math/bits functions

Add constant folding rewrite rules for RotateLeft64/32/16/8,
Sub64borrow, and Div128u generic SSA operations when their inputs
are compile-time constants.

Currently, these operations are evaluated at runtime by target-specific
instruction sequences, even when all their arguments are constant.
This change leverages compiler-level optimization to evaluate these
operations statically during the generic SSA optimization pass, replacing
them with simple constant loads.

Add corresponding codegen assertions to test/codegen/mathbits.go
to verify instruction elimination across major CPU architectures
(amd64, arm64, ppc64x, loong64, riscv64, s390x, and WebAssembly).

Fixes #21872.

Change-Id: Ia3e6f6f93f2255ff4271f75c8636923e62488f18

Reviewed-on: https://go-review.googlesource.com/c/go/+/810700

Reviewed-by: Keith Randall <k...@golang.org>

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

Auto-Submit: Keith Randall <k...@golang.org>

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

Files:

M src/cmd/compile/internal/ssa/_gen/generic.rules
M src/cmd/compile/internal/ssa/rewrite.go
M src/cmd/compile/internal/ssa/rewritegeneric.go
M test/codegen/mathbits.go

Change size: L

Delta: 4 files changed, 264 insertions(+), 0 deletions(-)

Branch: refs/heads/master

Submit Requirements:

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

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