cmd/compile: prove only notices one of a pair of induction variables (missed optimization) #76269
Description
Go version
go1.26-devel_b0f75c6bd1
Output of go env in your module/workspace:
AR='ar' CC='clang' CGO_CFLAGS='-O2 -g' CGO_CPPFLAGS='' CGO_CXXFLAGS='-O2 -g' CGO_ENABLED='1' CGO_FFLAGS='-O2 -g' CGO_LDFLAGS='-O2 -g' CXX='clang++' GCCGO='gccgo' GO111MODULE='' GOARCH='arm64' GOARM64='v8.0' GOAUTH='netrc' GOBIN='' GOCACHE='/Users/drchase/Library/Caches/go-build' GOCACHEPROG='' GODEBUG='' GOENV='/Users/drchase/Library/Application Support/go/env' GOEXE='' GOEXPERIMENT='' GOFIPS140='off' GOFLAGS='' GOGCCFLAGS='-fPIC -arch arm64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -ffile-prefix-map=/var/folders/vl/jd15hgb540x9qrp01g6896pc0095tn/T/go-build198174780=/tmp/go-build -gno-record-gcc-switches -fno-common' GOHOSTARCH='arm64' GOHOSTOS='darwin' GOINSECURE='' GOMOD='/dev/null' GOMODCACHE='/Users/drchase/go/pkg/mod' GONOPROXY='' GONOSUMDB='' GOOS='darwin' GOPATH='/Users/drchase/go' GOPRIVATE='' GOPROXY='https://proxy.golang.org,direct' GOROOT='/Users/drchase/work/go' GOSUMDB='sum.golang.org' GOTELEMETRY='local' GOTELEMETRYDIR='/Users/drchase/Library/Application Support/go/telemetry' GOTMPDIR='' GOTOOLCHAIN='auto' GOTOOLDIR='/Users/drchase/work/go/pkg/tool/darwin_arm64' GOVCS='' GOVERSION='go1.26-devel_b0f75c6bd1 Tue Nov 11 16:09:30 2025 -0500' GOWORK='' PKG_CONFIG='pkg-config'What did you do?
foo.go:
package foo func f(a, b []byte) byte { lenA := len(a) lenB := len(b) idxA := lenA - 1 idxB := lenB - 1 c := byte(0) for idxA >= 0 && idxB >= 0 { c ^= a[idxA] // random op c ^= b[idxB] // random op idxA-- idxB-- } return c } and commands plus output:
GOSSAFUNC=f go build -gcflags=-d=ssa/prove/debug=1 foo.go # command-line-arguments dumped SSA for f,1 to ./ssa.html ./foo.go:11:16: Induction variable: limits [0,?], increment 1 ./foo.go:12:9: Proved IsInBounds What did you see happen?
I notice one bounds check removal, and examining the generated code in ssa.html, saw that there was still a bounds check remaining (i.e., there were two, one was removed, the other remained).
What did you expect to see?
Two bounds checks removed, none remaining.
This seemed "obvious" but of course I know it is not, but it is a deficiency and if we/someone can figure out what went wrong here and fix it, then number go up.
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