cmd/compile: enhance the chunked indexing case to include reslicing

this helps SIMD, but also helps plain old Go

Cherry-picked from the dev.simd branch. This CL is not
necessarily SIMD specific. Apply early to reduce risk.

Change-Id: Idcdacd54b6776f5c32b497bc94485052611cfa8d
Reviewed-on: https://go-review.googlesource.com/c/go/+/704756
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Keith Randall <khr@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-on: https://go-review.googlesource.com/c/go/+/708862
Reviewed-by: David Chase <drchase@google.com>

Author: dr2chase

src/cmd/compile/internal/ssa/prove.go

@@ -2177,10 +2177,18 @@ func unsignedSubUnderflows(a, b uint64) bool {
 // checkForChunkedIndexBounds looks for index expressions of the form
 // A[i+delta] where delta < K and i <= len(A)-K.  That is, this is a chunked
 // iteration where the index is not directly compared to the length.
-func checkForChunkedIndexBounds(ft *factsTable, b *Block, index, bound *Value) bool {
-	if bound.Op != OpSliceLen {
+// if isReslice, then delta can be equal to K.
+func checkForChunkedIndexBounds(ft *factsTable, b *Block, index, bound *Value, isReslice bool) bool {
+	if bound.Op != OpSliceLen && bound.Op != OpSliceCap {
 		return false
 	}
+
+	// this is a slice bounds check against len or capacity,
+	// and refers back to a prior check against length, which
+	// will also work for the cap since that is not smaller
+	// than the length.
+
+	slice := bound.Args[0]
 	lim := ft.limits[index.ID]
 	if lim.min < 0 {
 		return false
@@ -2206,22 +2214,25 @@ func checkForChunkedIndexBounds(ft *factsTable, b *Block, index, bound *Value) b
 		}
 		if ow := o.w; ow.Op == OpAdd64 {
 			var lenOffset *Value
-			if ow.Args[0] == bound {
+			if bound := ow.Args[0]; bound.Op == OpSliceLen && bound.Args[0] == slice {
 				lenOffset = ow.Args[1]
-			} else if ow.Args[1] == bound {
+			} else if bound := ow.Args[1]; bound.Op == OpSliceLen && bound.Args[0] == slice {
 				lenOffset = ow.Args[0]
 			}
 			if lenOffset == nil || lenOffset.Op != OpConst64 {
 				continue
 			}
 			if K := -lenOffset.AuxInt; K >= 0 {
 				or := o.r
+				if isReslice {
+					K++
+				}
 				if or == lt {
 					or = lt | eq
 					K++
-					if K < 0 {
-						continue
-					}
+				}
+				if K < 0 { // We hate thinking about overflow
+					continue
 				}
 
 				if delta < K && or == lt|eq {
@@ -2345,12 +2356,19 @@ func addLocalFacts(ft *factsTable, b *Block) {
 				ft.update(b, v, v.Args[0].Args[2], signed, eq)
 			}
 		case OpIsInBounds:
-			if checkForChunkedIndexBounds(ft, b, v.Args[0], v.Args[1]) {
+			if checkForChunkedIndexBounds(ft, b, v.Args[0], v.Args[1], false) {
 				if b.Func.pass.debug > 0 {
 					b.Func.Warnl(v.Pos, "Proved %s for blocked indexing", v.Op)
 				}
 				ft.booleanTrue(v)
 			}
+		case OpIsSliceInBounds:
+			if checkForChunkedIndexBounds(ft, b, v.Args[0], v.Args[1], true) {
+				if b.Func.pass.debug > 0 {
+					b.Func.Warnl(v.Pos, "Proved %s for blocked reslicing", v.Op)
+				}
+				ft.booleanTrue(v)
+			}
 		case OpPhi:
 			addLocalFactsPhi(ft, v)
 		}