runtime: floating point error on arm64

[owulveryck]

What version of Go are you using (go version)?

$ go version
go version go1.16 linux/arm64

Does this issue reproduce with the latest release?

yes

What operating system and processor architecture are you using (go env)?

go env Output

$ go env
GO111MODULE=""
GOARCH="arm64"
GOBIN=""
GOCACHE="/home/ubuntu/.cache/go-build"
GOENV="/home/ubuntu/.config/go/env"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="arm64"
GOHOSTOS="linux"
GOINSECURE=""
GOMODCACHE="/home/ubuntu/go/pkg/mod"
GONOPROXY=""
GONOSUMDB=""
GOOS="linux"
GOPATH="/home/ubuntu/go"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/opt/go"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/opt/go/pkg/tool/linux_arm64"
GOVCS=""
GOVERSION="go1.16"
GCCGO="gccgo"
AR="ar"
CC="gcc"
CXX="g++"
CGO_ENABLED="1"
GOMOD="/dev/null"
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"
GOGCCFLAGS="-fPIC -pthread -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build864276053=/tmp/go-build -gno-record-gcc-switches"

What did you do?

While debugging some tests for Gorgonia, we found this strange behavior:

func TestMathTanh(t *testing.T) {
	v := 0.2228940815087735
	tanh := math.Tanh(v)
	a := 1 - tanh*tanh + 1 - 1
	b := 1 - tanh*tanh - 1 + 1
	if a != b {
		t.Fail()
	}
}

This test is passing on amd64 and is failing on arm64.

What did you expect to see?

The test should pass

What did you see instead?

The test is failing

[randall77]

This may be multiply-add optimizations. Try

a := 1 - float64(tanh*tanh) + 1 - 1
b := 1 - float64(tanh*tanh) - 1 + 1

and see if that changes anything.

From the spec:

An implementation may combine multiple floating-point operations into a single fused operation, possibly across statements, and produce a result that differs from the value obtained by executing and rounding the instructions individually. An explicit floating-point type conversion rounds to the precision of the target type, preventing fusion that would discard that rounding.

[owulveryck]

@randall77 you are right, this test passes:

func TestMathTanh(t *testing.T) {
	v := 0.2228940815087735
	tanh := math.Tanh(v)
	a := 1 - float64(tanh*tanh) + 1 - 1
	b := 1 - float64(tanh*tanh) - 1 + 1
	if a != b {
		t.Fail()
	}
}

[owulveryck]

Anyway, I guess that the behavior should be the same on arm64 and amd64

[randall77]

We do not guarantee identical floating-point behavior across architectures, as the spec describes.
If you need that, you would need to add casts in the appropriate places.