cmd/compile: in prove.go:addLocalFacts ft.update propagates limits depends on value ordering and can't propagate ft.update → ft.flowLimit dependencies

[Jorropo]

Go version

go1.24-793b14b455

Output of go env in your module/workspace:

GO111MODULE=''
GOARCH='amd64'
GOBIN=''
GOCACHE='/tmp/go-build'
GOENV='/home/hugo/.config/go/env'
GOEXE=''
GOEXPERIMENT='rangefunc'
GOFLAGS=''
GOHOSTARCH='amd64'
GOHOSTOS='linux'
GOINSECURE=''
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'
GOTMPDIR=''
GOTOOLCHAIN='local'
GOTOOLDIR='/home/hugo/k/go/pkg/tool/linux_amd64'
GOVCS=''
GOVERSION='devel go1.24-793b14b455 Tue Aug 13 17:23:56 2024 +0200 X:rangefunc'
GODEBUG=''
GOTELEMETRY='local'
GOTELEMETRYDIR='/home/hugo/.config/go/telemetry'
GCCGO='/usr/bin/gccgo'
GOAMD64='v3'
AR='ar'
CC='gcc'
CXX='g++'
CGO_ENABLED='1'
GOMOD='/home/hugo/k/go/src/go.mod'
GOWORK=''
CGO_CFLAGS='-O2 -g'
CGO_CPPFLAGS=''
CGO_CXXFLAGS='-O2 -g'
CGO_FFLAGS='-O2 -g'
CGO_LDFLAGS='-O2 -g'
PKG_CONFIG='pkg-config'
GOGCCFLAGS='-fPIC -m64 -pthread -Wl,--no-gc-sections -fmessage-length=0 -ffile-prefix-map=/tmp/go-build127661744=/tmp/go-build -gno-record-gcc-switches'

What did you do?

Add theses in test/prove.go and run:

func mod64uWithSmallerDividendMax(a, b uint64, ensureBothBranchesCouldHappen bool) int {
	a &= 0xff
	b &= 0xfff

	z := bits.Len64(a % b)

	if ensureBothBranchesCouldHappen {
		if z > bits.Len64(0xff) { // ERROR "Disproved Less64$"
			return 42
		}
	} else {
		if z <= bits.Len64(0xff) { // ERROR "Proved Leq64$"
			return 1337
		}
	}
	return z
}
func mod64uWithSmallerDivisorMax(a, b uint64, ensureBothBranchesCouldHappen bool) int {
	a &= 0xfff
	b &= 0x10 // we need bits.Len64(b.umax) != bits.Len64(b.umax-1)

	z := bits.Len64(a % b)

	if ensureBothBranchesCouldHappen {
		if z > bits.Len64(0x10-1) { // ERROR "Disproved Less64$"
			return 42
		}
	} else {
		if z <= bits.Len64(0x10-1) { // ERROR "Proved Leq64$"
			return 1337
		}
	}
	return z
}
func mod64uWithIdenticalMax(a, b uint64, ensureBothBranchesCouldHappen bool) int {
	a &= 0x10
	b &= 0x10 // we need bits.Len64(b.umax) != bits.Len64(b.umax-1)

	z := bits.Len64(a % b)

	if ensureBothBranchesCouldHappen {
		if z > bits.Len64(0x10-1) { // ERROR "Disproved Less64$"
			return 42
		}
	} else {
		if z <= bits.Len64(0x10-1) { // ERROR "Proved Leq64$"
			return 1337
		}
	}
	return z
}

What did you see happen?

--- FAIL: Test (0.02s)
    --- FAIL: Test/prove.go (0.31s)
        testdir_test.go:145: 
            prove.go:1436: missing error "Disproved Less64$"
            prove.go:1440: missing error "Proved Leq64$"
            prove.go:1453: missing error "Disproved Less64$"
            prove.go:1457: missing error "Proved Leq64$"
            prove.go:1470: missing error "Disproved Less64$"
            prove.go:1474: missing error "Proved Leq64$"
            
FAIL
FAIL	cmd/internal/testdir	0.336s
FAIL

What did you expect to see?

ok  	cmd/internal/testdir	0.350s

[Jorropo]

cc @randall77

[gabyhelp]

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[gopherbot]

Change https://go.dev/cl/605156 mentions this issue: cmd/compile: compute Modu's maximum limits from argument's limits