cmd/compile: unnecessary bounds check when indexing slice from range

[dsnet]

Go version

go1.23

Output of go env in your module/workspace:

GOARCH=amd64
GOOS=linux

What did you do?

Compile the following:

package main

import "archive/tar"

var src []tar.Header
var dst *tar.Header

func main() {
	for i := range src {
		dst = &src[i] // line 10
	}
}

What did you see happen?

I see this compiled out:

...
0x0069 00105 (main.go:10)	PCDATA	$1, $0
0x0069 00105 (main.go:10)	CALL	runtime.panicIndex(SB)
0x006e 00110 (main.go:10)	XCHGL	AX, AX
...

What did you expect to see?

No such call to runtime.panicIndex. The slice is indexed from an iteration integer that is provably bounded by the length of src itself. The only possible way a panic occurs is if src is mutated during the iteration (or asynchronously in another goroutine, in which case there must be synchronization primitives).

[dsnet]

Oh interesting, the following does not emit a call to runtime.panicIndex:

for i := 0; i < len(src); i++ {
	dst = &src[i]
}

which I assume is because i < len(src) is evaluated on every iteration.

[randall77]

The compiler generally gives up on analyzing globals. Load from the global to a local before the loop and it should be fine.

I think the difference between your two examples is in the first, src is read once outside the loop and once inside the loop. Those two reads are not CSEd. In the second example, the two loads of src are both inside the loop so they get CSEd. That lets the bounds check removal logic see that the bounds check is unnecessary.

[gabyhelp]

Related Issues

• cmd/compile: avoid unnecessary bounds check in range loop #52019 (closed)
• cmd/compile: unnecessary bounds check with slice offset #66691
• cmd/compile: unnecessary bounds check #64902 (closed)
• cmd/compile: impossible slice access is compiled in spite of always panicking #27250 (closed)
• cmd/compile: bounds check elimination for len(x) - 1 #23354 (closed)
• Incorrect slice indexing doesn't panic #27466 (closed)
• cmd/compile: unnecessary bounds check when indexing relative to the end #51622 (closed)
• cmd/compile: The bounds check can be removed in u%uint(x) for all x != 0 && x <= len(slice). #15079 (closed)
• cmd/compile: bounds check not needed #61333
• cmd/gc: Golang bug or compiler bug ??? #7674 (closed)

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

[zigo101]

#47736

[mknyszek]

In triage, @randall77 notes there isn't really much to be done here, so closing for now. Thanks.