cmd/compile: add testcase for #24876

This is still not fixed, the testcase reflects that there are still
a few boundchecks. Let's fix the good alternative with an explicit
test though.

Updates #24876

Change-Id: I4da35eb353e19052bd7b69ea6190a69ced8b9b3d
Reviewed-on: https://go-review.googlesource.com/107355Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Giovanni Bajo <rasky@develer.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

test/checkbce.go

@@ -10,6 +10,8 @@
 
 package main
 
+import "encoding/binary"
+
 func f0(a []int) {
 	a[0] = 1 // ERROR "Found IsInBounds$"
 	a[0] = 1
@@ -142,6 +144,33 @@ func g4(a [100]int) {
 	}
 }
 
+func decode1(data []byte) (x uint64) {
+	for len(data) >= 32 {
+		x += binary.BigEndian.Uint64(data[:8])
+		x += binary.BigEndian.Uint64(data[8:16])
+		x += binary.BigEndian.Uint64(data[16:24])
+		x += binary.BigEndian.Uint64(data[24:32])
+		data = data[32:]
+	}
+	return x
+}
+
+func decode2(data []byte) (x uint64) {
+	// TODO(rasky): this should behave like decode1 and compile to no
+	// boundchecks. We're currently not able to remove all of them.
+	for len(data) >= 32 {
+		x += binary.BigEndian.Uint64(data)
+		data = data[8:]
+		x += binary.BigEndian.Uint64(data) // ERROR "Found IsInBounds$"
+		data = data[8:]
+		x += binary.BigEndian.Uint64(data) // ERROR "Found IsInBounds$"
+		data = data[8:]
+		x += binary.BigEndian.Uint64(data) // ERROR "Found IsInBounds$"
+		data = data[8:]
+	}
+	return x
+}
+
 //go:noinline
 func useInt(a int) {
 }