cmd/compile: prove uses signed `isPowerOfTwo` for testing unsigned constants

[Jorropo]

Go version

go version go1.26-devel_f22d37d574 Mon Dec 1 14:59:40 2025 -0800 linux/amd64

Output of go env in your module/workspace:

AR='ar'
CC='gcc'
CGO_CFLAGS='-O2 -g'
CGO_CPPFLAGS=''
CGO_CXXFLAGS='-O2 -g'
CGO_ENABLED='1'
CGO_FFLAGS='-O2 -g'
CGO_LDFLAGS='-O2 -g'
CXX='g++'
GCCGO='gccgo'
GO111MODULE=''
GOAMD64='v3'
GOARCH='amd64'
GOAUTH='netrc'
GOBIN=''
GOCACHE='/tmp/go-build'
GOCACHEPROG=''
GODEBUG=''
GOENV='/home/hugo/.config/go/env'
GOEXE=''
GOEXPERIMENT=''
GOFIPS140='off'
GOFLAGS=''
GOGCCFLAGS='-fPIC -m64 -pthread -Wl,--no-gc-sections -fmessage-length=0 -ffile-prefix-map=/tmp/go-build310673283=/tmp/go-build -gno-record-gcc-switches'
GOHOSTARCH='amd64'
GOHOSTOS='linux'
GOINSECURE=''
GOMOD='/home/hugo/k/go/src/go.mod'
GOMODCACHE='/home/hugo/go/pkg/mod'
GONOPROXY=''
GONOSUMDB=''
GOOS='linux'
GOPATH='/home/hugo/go'
GOPRIVATE=''
GOPROXY='https://proxy.golang.org,direct'
GOROOT='/home/hugo/k/go'
GOSUMDB='sum.golang.org'
GOTELEMETRY='local'
GOTELEMETRYDIR='/home/hugo/.config/go/telemetry'
GOTMPDIR=''
GOTOOLCHAIN='local'
GOTOOLDIR='/home/hugo/k/go/pkg/tool/linux_amd64'
GOVCS=''
GOVERSION='go1.26-devel_f22d37d574 Mon Dec 1 14:59:40 2025 -0800'
GOWORK=''
PKG_CONFIG='pkg-config'

What did you do?

Compile this code

package a

func f(x, y uint64) uint64 {
	if x > 1 || y != 1<<63 {
		return 42
	}
	return x * y
}

with go build -gcflags=-d=ssa/prove/debug=1 a.go

What did you see happen?

Rewrote Mul v21 into CondSelect; v7 is bool

What did you expect to see?

Nothing, theses lines of code:

go/src/cmd/compile/internal/ssa/prove.go

Lines 2888 to 2894 in 2b62144

	if xl.umin == xl.umax && isPowerOfTwo(int64(xl.umin)) ||
	xl.min == xl.max && isPowerOfTwo(xl.min) ||
	yl.umin == yl.umax && isPowerOfTwo(int64(yl.umin)) ||
	yl.min == yl.max && isPowerOfTwo(yl.min) {
	// 0,1 * a power of two is better done as a shift
	break
	}

are supposed to prevent rewriting multiplies to condselect for constant powers of two since opt should translate them to a left shift after prove does constant argument folding from limits.

However it uses a signed test for unsigned numbers which gives an incorrect result for 1 << (bitsize-1).

[gopherbot]

Change https://go.dev/cl/726680 mentions this issue: cmd/compile: use isUnsignedPowerOfTwo rather than isPowerOfTwo for unsigneds

[gabyhelp]

Related Issues

• cmd/compile: missed prove inferences with uint conversions #16653 (closed)
• cmd/compile: prove pass doesn't draw signed int conclusions from range checks #25115
• cmd/compile: incorrect math reduction #70481 (closed)
• cmd/compile: teach prove about range of results of math/bits.*Zeros #40084 (closed)
• cmd/compile: right shifting may cause extra bounds checks compared to division #76332 (closed)
• cmd/compile: teach prove that x ∈ [0, c) ⇒ c - x ∈ [0, c) #29477 (closed)
• cmd/compile: magic strength reduction cause flashing SSA optimizations #63110 (closed)
• cmd/compile: incorrectly reports `x - minimumValue` overflows for integers #68628 (closed)

Related Code Changes

• cmd/compile: in prove, zero right shifts of positive int by #bits - 1
• cmd/compile: add unsigned power-of-two detector

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