cmd/compile: bug in string comparison

[dsymonds]

Please answer these questions before submitting your issue. Thanks!

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

go1.10.1 (playground). This also reproduces with go version devel +b1335037fa Wed Mar 7 22:03:43 2018 +0000 linux/amd64.

Does this issue reproduce with the latest release?

It does in the playground.

What did you do?

https://play.golang.org/p/qJc3MD8uB9j

package main

import (
	"fmt"
)

func main() {
	s := "123"
	const t = "123"
	fmt.Println("" < s)
	fmt.Println("" < t)
}

What did you expect to see?

The two string comparisons should be the same, yielding true and true.

What did you see instead?

false
true

[dsymonds]

It's still broken at head (go version devel +2dfb423e6e Wed Apr 11 23:46:30 2018 +0000 linux/amd64).

[dsymonds]

Oddly, it is not a commutative bug. If the empty string is on the RHS then it is fine (https://play.golang.org/p/TULS3BC_uuY).

[dsnet]

Interesting regression from Go1.8. Root cause: https://golang.org/cl/26758

\cc @josharian @bradfitz

[josharian]

Nice find!

On my phone, but I believe the offending line is:

r := nod(cmp, nod(OLEN, ncs, nil), nodintconst(int64(len(s))))

Unlike = and !=, the order of args here matters for > and <. Oops.

This only happens for the empty string.

The cleanest fix is probably to rewrite >, <, >=, and <= for “” into !=, false, true, and == respectively. I’ll send a CL first chance I get.

[bradfitz]

Ouch. This will need some backports too.

[gopherbot]

Change https://golang.org/cl/106655 mentions this issue: cmd/compile: fix evaluation of "" < s

[josharian]

For the record, the affected comparisons are:

"" < s
"" <= s
"" > s
"" >= s

[josharian]

Reopening for backport.

[andybons]

@josharian can you open another issue on the 1.9.6 milestone per https://golang.org/wiki/MinorReleases?

(This process is new and will be communicated more widely very soon)