cmd/compile: math.Trunc produces incorrect result on riscv64 in Go 1.23rc1

[mathetake]

Go version

go version go1.23rc1 darwin/arm64

Output of go env in your module/workspace:

GO111MODULE='on'
GOARCH='arm64'
GOBIN=''
GOCACHE='/Users/mathetake/Library/Caches/go-build'
GOENV='/Users/mathetake/Library/Application Support/go/env'
GOEXE=''
GOEXPERIMENT=''
GOFLAGS=''
GOHOSTARCH='arm64'
GOHOSTOS='darwin'
GOINSECURE=''
GOMODCACHE='/Users/mathetake/go/pkg/mod'
GONOPROXY=''
GONOSUMDB=''
GOOS='darwin'
GOPATH='/Users/mathetake/go'
GOPRIVATE=''
GOPROXY='https://proxy.golang.org,direct'
GOROOT='/Users/mathetake/sdk/go1.23rc1'
GOSUMDB='sum.golang.org'
GOTMPDIR=''
GOTOOLCHAIN='auto'
GOTOOLDIR='/Users/mathetake/sdk/go1.23rc1/pkg/tool/darwin_arm64'
GOVCS=''
GOVERSION='go1.23rc1'
GODEBUG=''
GOTELEMETRY='local'
GOTELEMETRYDIR='/Users/mathetake/Library/Application Support/go/telemetry'
GCCGO='gccgo'
GOARM64='v8.0'
AR='ar'
CC='clang'
CXX='clang++'
CGO_ENABLED='1'
GOMOD='/Users/mathetake/wazero/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 -arch arm64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -ffile-prefix-map=/var/folders/zv/cj1015lx5bndpw3n_qsx9mh40000gn/T/go-build971855478=/tmp/go-build -gno-record-gcc-switches -fno-common'

What did you do?

I created the following reproducer

func TestRISCVTruncIncorrect(t *testing.T) {
	if math.Trunc(18446744073709549568.0) != 18446744073709549568.0 {
		t.Fatalf("math.Trunc(18446744073709549568.0) = %f, want 18446744073709549568.0", math.Trunc(18446744073709549568.0))
	}
}

What did you see happen?

I cross compiled the test with GOOS=linux GOARCH=riscv64, and run it in the qemu environment. Then the test passed with Go 1.22 but was failing with Go 1.23rc1.

What did you expect to see?

The test passes.

[gabyhelp]

Related Issues

• cmd/compile: inconsistent integer arithmetic result on Go 1.22+arm64 with/without -race #68227 (closed)
• Parsing float64 to uint64 yields different results on GOARCH="arm64" #52217 (closed)
• cmd/compile: miscompilation of simple arithmetic program targeting darwin/arm64 #62723 (closed)
• runtime: off-by-one-error in wasmTruncS and wasmTruncU causes a crash #38839 (closed)
• cmd/compile: unstable arithmetic #52293 (closed)
• runtime: floating point error on arm64 #44528 (closed)

_{(Emoji vote if this was helpful or unhelpful; more detailed feedback welcome in this discussion.)}

[randall77]

@golang/riscv64

[Jorropo]

Very probably ad377e9
cc @mengzhuo

[randall77]

Yes, that riscv64 assembly looks bad. It converts float64->int64->float64, which doesn't work if the input float is bigger than 2^63-1. (For your example it is 2^64-2^11, I think, but generally it is a problem for very large floats.) We could probably compare |input| to a large value and if it is bigger than 2^53 (or whatever the last fractional float is), return the input. (And maybe we could do that instead of the Inf comparison?)

[gopherbot]

Change https://go.dev/cl/596516 mentions this issue: math: remove riscv64 assembly implementations of rounding

[gopherbot]

Change https://go.dev/cl/600075 mentions this issue: math: add round assembly implementations on riscv64