cmd/compile: add ssa.Block.truncateValues

It is a common operation.

Passes toolstash-check.

Change-Id: Icc34600b0f79d0ecb19f257e3c7f23b6f01a26ab
Reviewed-on: https://go-review.googlesource.com/c/go/+/229599
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

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

@@ -256,6 +256,17 @@ func (b *Block) resetWithControl2(kind BlockKind, v, w *Value) {
 	w.Uses++
 }
 
+// truncateValues truncates b.Values at the ith element, zeroing subsequent elements.
+// The values in b.Values after i must already have had their args reset,
+// to maintain correct value uses counts.
+func (b *Block) truncateValues(i int) {
+	tail := b.Values[i:]
+	for j := range tail {
+		tail[j] = nil
+	}
+	b.Values = b.Values[:i]
+}
+
 // AddEdgeTo adds an edge from block b to block c. Used during building of the
 // SSA graph; do not use on an already-completed SSA graph.
 func (b *Block) AddEdgeTo(c *Block) {

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

@@ -296,12 +296,7 @@ func deadcode(f *Func) {
 				f.freeValue(v)
 			}
 		}
-		// aid GC
-		tail := b.Values[i:]
-		for j := range tail {
-			tail[j] = nil
-		}
-		b.Values = b.Values[:i]
+		b.truncateValues(i)
 	}
 
 	// Remove dead blocks from WBLoads list.

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

@@ -171,10 +171,7 @@ func nilcheckelim(f *Func) {
 				b.Pos = b.Pos.WithIsStmt()
 				pendingLines.remove(b.Pos)
 			}
-			for j := i; j < len(b.Values); j++ {
-				b.Values[j] = nil
-			}
-			b.Values = b.Values[:i]
+			b.truncateValues(i)
 
 			// Add all dominated blocks to the work list.
 			for w := sdom[node.block.ID].child; w != nil; w = sdom[w.ID].sibling {
@@ -331,10 +328,7 @@ func nilcheckelim2(f *Func) {
 			b.Pos = b.Pos.WithIsStmt()
 		}
 
-		for j := i; j < len(b.Values); j++ {
-			b.Values[j] = nil
-		}
-		b.Values = b.Values[:i]
+		b.truncateValues(i)
 
 		// TODO: if b.Kind == BlockPlain, start the analysis in the subsequent block to find
 		// more unnecessary nil checks.  Would fix test/nilptr3.go:159.

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

@@ -152,13 +152,7 @@ func applyRewrite(f *Func, rb blockRewriter, rv valueRewriter) {
 			b.Pos = b.Pos.WithIsStmt()
 			pendingLines.remove(b.Pos)
 		}
-		if j != len(b.Values) {
-			tail := b.Values[j:]
-			for j := range tail {
-				tail[j] = nil
-			}
-			b.Values = b.Values[:j]
-		}
+		b.truncateValues(j)
 	}
 }