cmd/compile: teach prove about min/max phi operations

If there is a phi that is computing the minimum of its two inputs,
then we know the result of the phi is smaller than or equal to both
of its inputs. Similarly for maxiumum (although max seems less useful).

This pattern happens for the case

  n := copy(a, b)

n is the minimum of len(a) and len(b), so with this optimization we
know both n <= len(a) and n <= len(b). That extra information is
helpful for subsequent slicing of a or b.

Fixes #16833

Change-Id: Ib4238fd1edae0f2940f62a5516a6b363bbe7928c
Reviewed-on: https://go-review.googlesource.com/c/go/+/622240
Reviewed-by: Carlos Amedee <carlos@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: David Chase <drchase@google.com>

1 files changed, 42 insertions, 0 deletions

diff --git a/test/prove.go b/test/prove.go
index 2265b637ba..edfd8908a2 100644
--- a/test/prove.go
+++ b/test/prove.go
@@ -1670,6 +1670,48 @@ func neg64mightOverflowDuringNeg(a uint64, ensureAllBranchesCouldHappen func() b
 	return z
 }
 
+func phiMin(a, b []byte) {
+	_ = a[:min(len(a), len(b))] // ERROR "Proved IsSliceInBounds"
+	_ = b[:min(len(a), len(b))] // ERROR "Proved IsSliceInBounds"
+	_ = a[:max(len(a), len(b))]
+	_ = b[:max(len(a), len(b))]
+	x := len(a)
+	if x > len(b) {
+		x = len(b)
+		useInt(0)
+	}
+	_ = a[:x] // ERROR "Proved IsSliceInBounds"
+	y := len(a)
+	if y > len(b) {
+		y = len(b)
+		useInt(0)
+	} else {
+		useInt(1)
+	}
+	_ = b[:y] // ERROR "Proved IsSliceInBounds"
+}
+
+func issue16833(a, b []byte) {
+	n := copy(a, b)
+	_ = a[n:] // ERROR "Proved IsSliceInBounds"
+	_ = b[n:] // ERROR "Proved IsSliceInBounds"
+	_ = a[:n] // ERROR "Proved IsSliceInBounds"
+	_ = b[:n] // ERROR "Proved IsSliceInBounds"
+}
+
+func clampedIdx1(x []int, i int) int {
+	if len(x) == 0 {
+		return 0
+	}
+	return x[min(max(0, i), len(x)-1)] // ERROR "Proved IsInBounds"
+}
+func clampedIdx2(x []int, i int) int {
+	if len(x) == 0 {
+		return 0
+	}
+	return x[max(min(i, len(x)-1), 0)] // TODO: can't get rid of this bounds check yet
+}
+
 //go:noinline
 func useInt(a int) {
 }