cmd/compile: check domination of loop return in both controls

Fixes #74473

Change-Id: I72ff6b95955ae9407271508aa80f230dcf1b6c74
Reviewed-on: https://go-review.googlesource.com/c/go/+/685816
Reviewed-by: Mark Freeman <mark@golang.org>
Auto-Submit: Jorropo <jorropo.pgm@gmail.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: Keith Randall <khr@golang.org>

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

@@ -37,19 +37,20 @@ type indVar struct {
 //   - the minimum bound
 //   - the increment value
 //   - the "next" value (SSA value that is Phi'd into the induction variable every loop)
+//   - the header's edge returning from the body
 //
 // Currently, we detect induction variables that match (Phi min nxt),
 // with nxt being (Add inc ind).
 // If it can't parse the induction variable correctly, it returns (nil, nil, nil).
-func parseIndVar(ind *Value) (min, inc, nxt *Value) {
+func parseIndVar(ind *Value) (min, inc, nxt *Value, loopReturn Edge) {
 	if ind.Op != OpPhi {
 		return
 	}
 
 	if n := ind.Args[0]; (n.Op == OpAdd64 || n.Op == OpAdd32 || n.Op == OpAdd16 || n.Op == OpAdd8) && (n.Args[0] == ind || n.Args[1] == ind) {
-		min, nxt = ind.Args[1], n
+		min, nxt, loopReturn = ind.Args[1], n, ind.Block.Preds[0]
 	} else if n := ind.Args[1]; (n.Op == OpAdd64 || n.Op == OpAdd32 || n.Op == OpAdd16 || n.Op == OpAdd8) && (n.Args[0] == ind || n.Args[1] == ind) {
-		min, nxt = ind.Args[0], n
+		min, nxt, loopReturn = ind.Args[0], n, ind.Block.Preds[1]
 	} else {
 		// Not a recognized induction variable.
 		return
@@ -111,13 +112,13 @@ func findIndVar(f *Func) []indVar {
 
 		// See if this is really an induction variable
 		less := true
-		init, inc, nxt := parseIndVar(ind)
+		init, inc, nxt, loopReturn := parseIndVar(ind)
 		if init == nil {
 			// We failed to parse the induction variable. Before punting, we want to check
 			// whether the control op was written with the induction variable on the RHS
 			// instead of the LHS. This happens for the downwards case, like:
 			//     for i := len(n)-1; i >= 0; i--
-			init, inc, nxt = parseIndVar(limit)
+			init, inc, nxt, loopReturn = parseIndVar(limit)
 			if init == nil {
 				// No recognized induction variable on either operand
 				continue
@@ -145,6 +146,20 @@ func findIndVar(f *Func) []indVar {
 			continue
 		}
 
+		// startBody is the edge that eventually returns to the loop header.
+		var startBody Edge
+		switch {
+		case sdom.IsAncestorEq(b.Succs[0].b, loopReturn.b):
+			startBody = b.Succs[0]
+		case sdom.IsAncestorEq(b.Succs[1].b, loopReturn.b):
+			// if x { goto exit } else { goto entry } is identical to if !x { goto entry } else { goto exit }
+			startBody = b.Succs[1]
+			less = !less
+			inclusive = !inclusive
+		default:
+			continue
+		}
+
 		// Increment sign must match comparison direction.
 		// When incrementing, the termination comparison must be ind </<= limit.
 		// When decrementing, the termination comparison must be ind >/>= limit.
@@ -172,14 +187,14 @@ func findIndVar(f *Func) []indVar {
 		// First condition: loop entry has a single predecessor, which
 		// is the header block.  This implies that b.Succs[0] is
 		// reached iff ind < limit.
-		if len(b.Succs[0].b.Preds) != 1 {
+		if len(startBody.b.Preds) != 1 {
-			// b.Succs[1] must exit the loop.
+			// the other successor must exit the loop.
 			continue
 		}
 
-		// Second condition: b.Succs[0] dominates nxt so that
+		// Second condition: startBody.b dominates nxt so that
 		// nxt is computed when inc < limit.
-		if !sdom.IsAncestorEq(b.Succs[0].b, nxt.Block) {
+		if !sdom.IsAncestorEq(startBody.b, nxt.Block) {
 			// inc+ind can only be reached through the branch that enters the loop.
 			continue
 		}
@@ -298,7 +313,7 @@ func findIndVar(f *Func) []indVar {
 				nxt:   nxt,
 				min:   min,
 				max:   max,
-				entry: b.Succs[0].b,
+				entry: startBody.b,
 				flags: flags,
 			})
 			b.Logf("found induction variable %v (inc = %v, min = %v, max = %v)\n", ind, inc, min, max)

test/prove.go

@@ -2319,6 +2319,17 @@ func resliceBytes(b []byte) byte {
 	return 0
 }
 
+func issue74473(s []uint) {
+	i := 0
+	for {
+		if i >= len(s) { // ERROR "Induction variable: limits \[0,\?\), increment 1$"
+			break
+		}
+		_ = s[i] // ERROR "Proved IsInBounds$"
+		i++
+	}
+}
+
 //go:noinline
 func useInt(a int) {
 }