cmd/compile: factor out code to remove phi argument

CL 358117 fixed a bug that Phi's argument wasn't updated correctly after
removing a predecessor of Block. This CL factor out the code that
updates phi argument into a Block's method, so it's easier to use,
maintain and hopefully prevent that kind of bug in the future.

Change-Id: Ie9741e19ea28f56860425089b6093a381aa10f5b
Reviewed-on: https://go-review.googlesource.com/c/go/+/357964
Trust: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>

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

@@ -279,7 +279,8 @@ func (b *Block) AddEdgeTo(c *Block) {
 
 // removePred removes the ith input edge from b.
 // It is the responsibility of the caller to remove
-// the corresponding successor edge.
+// the corresponding successor edge, and adjust any
+// phi values by calling b.removePhiArg(v, i).
 func (b *Block) removePred(i int) {
 	n := len(b.Preds) - 1
 	if i != n {
@@ -322,6 +323,28 @@ func (b *Block) swapSuccessors() {
 	b.Likely *= -1
 }
 
+// removePhiArg removes the ith arg from phi.
+// It must be called after calling b.removePred(i) to
+// adjust the corresponding phi value of the block:
+//
+// b.removePred(i)
+// for _, v := range b.Values {
+//     if v.Op != OpPhi {
+//         continue
+//     }
+//     b.removeArg(v, i)
+// }
+func (b *Block) removePhiArg(phi *Value, i int) {
+	n := len(b.Preds)
+	if numPhiArgs := len(phi.Args); numPhiArgs-1 != n {
+		b.Fatalf("inconsistent state, num predecessors: %d, num phi args: %d", n, numPhiArgs)
+	}
+	phi.Args[i].Uses--
+	phi.Args[i] = phi.Args[n]
+	phi.Args[n] = nil
+	phi.Args = phi.Args[:n]
+}
+
 // LackingPos indicates whether b is a block whose position should be inherited
 // from its successors.  This is true if all the values within it have unreliable positions
 // and if it is "plain", meaning that there is no control flow that is also very likely

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

@@ -91,14 +91,13 @@ func critical(f *Func) {
 				b.removePred(i)
 
 				// Update corresponding phi args
-				n := len(b.Preds)
-				phi.Args[i].Uses--
-				phi.Args[i] = phi.Args[n]
-				phi.Args[n] = nil
-				phi.Args = phi.Args[:n]
+				b.removePhiArg(phi, i)
+
 				// splitting occasionally leads to a phi having
 				// a single argument (occurs with -N)
-				if n == 1 {
+				// TODO(cuonglm,khr): replace this with phielimValue, and
+				//                    make removePhiArg incorporates that.
+				if len(b.Preds) == 1 {
 					phi.Op = OpCopy
 				}
 				// Don't increment i in this case because we moved

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

@@ -348,15 +348,11 @@ func (b *Block) removeEdge(i int) {
 	c.removePred(j)
 
 	// Remove phi args from c's phis.
-	n := len(c.Preds)
 	for _, v := range c.Values {
 		if v.Op != OpPhi {
 			continue
 		}
-		v.Args[j].Uses--
-		v.Args[j] = v.Args[n]
-		v.Args[n] = nil
-		v.Args = v.Args[:n]
+		c.removePhiArg(v, j)
 		phielimValue(v)
 		// Note: this is trickier than it looks. Replacing
 		// a Phi with a Copy can in general cause problems because

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

@@ -78,11 +78,7 @@ func fuseBranchRedirect(f *Func) bool {
 					if v.Op != OpPhi {
 						continue
 					}
-					n := len(v.Args)
-					v.Args[k].Uses--
-					v.Args[k] = v.Args[n-1]
-					v.Args[n-1] = nil
-					v.Args = v.Args[:n-1]
+					b.removePhiArg(v, k)
 					phielimValue(v)
 				}
 				// Fix up child to have one more predecessor.

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

@@ -196,11 +196,7 @@ func shortcircuitBlock(b *Block) bool {
 
 	// Remove b's incoming edge from p.
 	b.removePred(cidx)
-	n := len(b.Preds)
-	ctl.Args[cidx].Uses--
-	ctl.Args[cidx] = ctl.Args[n]
-	ctl.Args[n] = nil
-	ctl.Args = ctl.Args[:n]
+	b.removePhiArg(ctl, cidx)
 
 	// Redirect p's outgoing edge to t.
 	p.Succs[pi] = Edge{t, len(t.Preds)}