Go version

1.21.7

Output of go env in your module/workspace:

GOARCH=wasm
GOOS=js
GOHOSTARCH=amd64

(I'll update this with the values I used when experimenting with a fix when I can)

What did you do?

It seems that bytes.IndexByte when compiled to js/wasm can return values that are below -1. The code snippet that triggered this issue is this:

result := []byte(strconv.FormatFloat(absValue, 'g', -1, 64))
// ...
dot := bytes.IndexByte(result, '.')

(extracted from https://github.com/evanw/esbuild/blob/2af5ccf478812d2d7226ad4435d46fbbb3419a8c/internal/js_printer/js_printer.go#L3464)

Original issue: stackblitz/webcontainer-core#1310

I haven't been able to create a minimal example exhibiting the same behaviour just yet. I'm still working on that. 👈

However the bug randomly appears when memory usage gets near 2^31. I have also verified that the issue is present both in Chrome and Firefox (so this does not appear to be a WASM vm bug)

What did you see happen?

bytes.IndexByte returns -4294967295 on the following snippet:

dot := bytes.IndexByte([]byte(strconv.FormatFloat(0.5, 'g', -1, 64)), '.')

I've also seen similar values such as -4294967293 for other input.

What did you expect to see?

I expect to see the correct, positive values and no other negative values than -1.

Note that I did a simple patch modifying:

https://github.com/golang/go/blob/master/src/bytes/bytes.go#L95-L97

to be:

func IndexByte(b []byte, c byte) int {
	res := bytealg.IndexByte(b, c)
	if res < -1 {
		return res + 1 << 32
	}

	return res
}

And it did fix the issue as it can be seen there:

https://stackblitz.com/edit/github-9i57jp-wrcype

Again, apologies for the lack of a good and simple reproducible example. I'll try to update this issue as soon as I have found one.