cmd/compile: checkptr false positive from (*[Big]T)(ptr)[:n:n] pattern

[mdempsky]

checkptr has special case logic for recognizing the pattern (*[Big]T)(ptr)[:n:n]. In particular, instead of diagnosing whether (*[Big]T)(ptr) is an invalid conversion, it diagnoses (*[n]T)(ptr).

However, currently the checkptr instrumentation is inserted before the slice operation has validated n <= Big, so it's possible to have false positive throws in cases that should just be panics.

For example:

package main

import "unsafe"

func main() {
	s := make([]int64, 100)
	p := unsafe.Pointer(&s[0])
	n := 1000

	_ = (*[10]int64)(p)[:n:n] // throws; should just panic
}

This issue has existed since checkptr's inclusion in Go 1.14, and I'm not aware of any user reports about it. It's also easy to workaround.

Marking for Go 1.18.

[gopherbot]

Change https://golang.org/cl/343972 mentions this issue: cmd/compile: fix checkptr false positive for (*[Big]T)(ptr)[:n:n] pattern