cmd/compile: debug rewrite

If -d=ssa/PASS/debug=N is specified (N >= 2) for a rewrite pass
(e.g. lower), when a Value (or Block) is rewritten, print the
Value (or Block) before and after.

For #31915.
Updates #19013.

Change-Id: I80eadd44302ae736bc7daed0ef68529ab7a16776
Reviewed-on: https://go-review.googlesource.com/c/go/+/176718
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>

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

@@ -23,9 +23,19 @@ func applyRewrite(f *Func, rb blockRewriter, rv valueRewriter) {
 	// repeat rewrites until we find no more rewrites
 	pendingLines := f.cachedLineStarts // Holds statement boundaries that need to be moved to a new value/block
 	pendingLines.clear()
+	debug := f.pass.debug
+	if debug > 1 {
+		fmt.Printf("%s: rewriting for %s\n", f.pass.name, f.Name)
+	}
 	for {
 		change := false
 		for _, b := range f.Blocks {
+			var b0 *Block
+			if debug > 1 {
+				b0 = new(Block)
+				*b0 = *b
+				b0.Succs = append([]Edge{}, b.Succs...) // make a new copy, not aliasing
+			}
 			for i, c := range b.ControlValues() {
 				for c.Op == OpCopy {
 					c = c.Args[0]
@@ -34,9 +44,22 @@ func applyRewrite(f *Func, rb blockRewriter, rv valueRewriter) {
 			}
 			if rb(b) {
 				change = true
+				if debug > 1 {
+					fmt.Printf("rewriting %s  ->  %s\n", b0.LongString(), b.LongString())
+				}
 			}
 			for j, v := range b.Values {
-				change = phielimValue(v) || change
+				var v0 *Value
+				if debug > 1 {
+					v0 = new(Value)
+					*v0 = *v
+					v0.Args = append([]*Value{}, v.Args...) // make a new copy, not aliasing
+				}
+
+				vchange := phielimValue(v)
+				if vchange && debug > 1 {
+					fmt.Printf("rewriting %s  ->  %s\n", v0.LongString(), v.LongString())
+				}
 
 				// Eliminate copy inputs.
 				// If any copy input becomes unused, mark it
@@ -70,17 +93,20 @@ func applyRewrite(f *Func, rb blockRewriter, rv valueRewriter) {
 						}
 						a.Pos = a.Pos.WithNotStmt()
 					}
-					change = true
+					vchange = true
 					for a.Uses == 0 {
 						b := a.Args[0]
 						a.reset(OpInvalid)
 						a = b
 					}
 				}
+				if vchange && debug > 1 {
+					fmt.Printf("rewriting %s  ->  %s\n", v0.LongString(), v.LongString())
+				}
 
 				// apply rewrite function
 				if rv(v) {
-					change = true
+					vchange = true
 					// If value changed to a poor choice for a statement boundary, move the boundary
 					if v.Pos.IsStmt() == src.PosIsStmt {
 						if k := nextGoodStatementIndex(v, j, b); k != j {
@@ -89,6 +115,11 @@ func applyRewrite(f *Func, rb blockRewriter, rv valueRewriter) {
 						}
 					}
 				}
+
+				change = change || vchange
+				if vchange && debug > 1 {
+					fmt.Printf("rewriting %s  ->  %s\n", v0.LongString(), v.LongString())
+				}
 			}
 		}
 		if !change {

test/devirt.go

@@ -1,4 +1,4 @@
-// errorcheck -0 -d=ssa/opt/debug=3
+// errorcheck -0 -d=ssa/opt/debug=1
 
 package main