cmd/compile: eliminate bound check for slices of the same length
If two slices start out with the same length and decrease in length by
the same amount on each round of the loop (or in the if block), then
we think their length are always equal.
For example:
if len(a) != len(b) {
return
}
for len(a) >= 4 {
a = a[4:]
b = b[4:] // proved here, omit boundary check
}
if len(a) == len(b) { // proved here
//...
}
Or, change 'for' to 'if':
if len(a) != len(b) {
return
}
if len(a) >= 4 {
a = a[4:]
b = b[4:]
}
if len(a) == len(b) { // proved here
//...
}
Fixes #75144
Change-Id: I4e5902a02b5cf8fdc122715a7dbd2fb5e9a8f5dc
Reviewed-on: https://go-review.googlesource.com/c/go/+/699155 Reviewed-by: Michael Pratt <mpratt@google.com> Reviewed-by: Keith Randall <khr@golang.org> Reviewed-by: Keith Randall <khr@google.com>
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