runtime: wrong float result in Apple Silicon

[breezewish]

Go version

go version go1.22.0 darwin/arm64

Output of go env in your module/workspace:

GO111MODULE=''
GOARCH='arm64'
GOBIN=''
GOCACHE='/Users/breezewish/Library/Caches/go-build'
GOENV='/Users/breezewish/Library/Application Support/go/env'
GOEXE=''
GOEXPERIMENT=''
GOFLAGS=''
GOHOSTARCH='arm64'
GOHOSTOS='darwin'
GOINSECURE=''
GOMODCACHE='/Users/breezewish/go/pkg/mod'
GONOPROXY=''
GONOSUMDB=''
GOOS='darwin'
GOPATH='/Users/breezewish/go'
GOPRIVATE=''
GOPROXY='https://proxy.golang.org,direct'
GOROOT='/opt/homebrew/Cellar/go/1.22.0/libexec'
GOSUMDB='sum.golang.org'
GOTMPDIR=''
GOTOOLCHAIN='auto'
GOTOOLDIR='/opt/homebrew/Cellar/go/1.22.0/libexec/pkg/tool/darwin_arm64'
GOVCS=''
GOVERSION='go1.22.0'
GCCGO='gccgo'
AR='ar'
CC='cc'
CXX='c++'
CGO_ENABLED='1'
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/6m/phy2_frd0vd1hb7g1py32qz40000gn/T/go-build3863423000=/tmp/go-build -gno-record-gcc-switches -fno-common'

What did you do?

package main

import (
	"fmt"
)

func main() {
	vb := []float32{4.7, 4.9, 2.6, 5.2, 7.4}

	var nb float32
	for _, v := range vb {
		nb += v * v
	}

	fmt.Println(nb)
}

What did you see happen?

In M1 Pro, the output is:

134.65999

In Go Playground (which is Go 1.22 as well), the output is:

134.66

What did you expect to see?

The output in two architectures should be the same.

Additionally my Clang outputs 134.66 for the C++ version:

#include <iostream>

int main() {
  float vb[] = {4.7, 4.9, 2.6, 5.2, 7.4};
  float nb = 0;

  for (int i = 0; i < 5; i++) {
    nb += vb[i] * vb[i];
  }

  std::cout << nb << std::endl;
}

❯ g++ --version
Apple clang version 15.0.0 (clang-1500.1.0.2.5)
Target: arm64-apple-darwin23.1.0
Thread model: posix
InstalledDir: /Library/Developer/CommandLineTools/usr/bin

[ianlancetaylor]

This is most likely due to different use of floating-point multiply-add instructions. See the discussion in the "Floating-point operators" section of https://go.dev/ref/spec#Arithmetic_operators.