From 8f1fdf33ded3511f9d150c5b5e931ceccbf81762 Mon Sep 17 00:00:00 2001
From: Alan Donovan <adonovan@google.com>
Date: Thu, 21 Nov 2013 14:19:38 -0500
Subject: [PATCH] =?UTF-8?q?go.tools/ssa:=20opt:=20improve=20the=20"unused?=
 =?UTF-8?q?=20=CF=86-node"=20check=20to=20eliminate=20self-refs,=20debug-r?=
 =?UTF-8?q?efs.?=
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This removes about 5% of φ-nodes in one large program
and eliminates many zero-value constants.

(This does cause some Idents to no longer map to an ssa.Value.
This is observable in the oracle, whose tests are here updated.)

R=gri, gri
CC=golang-dev
https://golang.org/cl/26980043
---
 oracle/testdata/src/main/describe-json.go     |  2 +-
 oracle/testdata/src/main/describe-json.golden |  2 +-
 oracle/testdata/src/main/describe.go          |  2 +-
 ssa/lift.go                                   | 17 +++++++++++++++--
 4 files changed, 18 insertions(+), 5 deletions(-)

diff --git a/oracle/testdata/src/main/describe-json.go b/oracle/testdata/src/main/describe-json.go
index 56c29321a95..906bfaf8f0f 100644
--- a/oracle/testdata/src/main/describe-json.go
+++ b/oracle/testdata/src/main/describe-json.go
@@ -15,7 +15,7 @@ func main() { //
 	if i == nil {
 		i = new(D)
 	}
-	_ = i // @describe desc-val-i "i"
+	print(i) // @describe desc-val-i "\\bi\\b"
 
 	go main() // @describe desc-stmt "go"
 }
diff --git a/oracle/testdata/src/main/describe-json.golden b/oracle/testdata/src/main/describe-json.golden
index 48b85f0b687..db917ee90a4 100644
--- a/oracle/testdata/src/main/describe-json.golden
+++ b/oracle/testdata/src/main/describe-json.golden
@@ -98,7 +98,7 @@
 	"mode": "describe",
 	"describe": {
 		"desc": "identifier",
-		"pos": "testdata/src/main/describe-json.go:18:6",
+		"pos": "testdata/src/main/describe-json.go:18:8",
 		"detail": "value",
 		"value": {
 			"type": "I",
diff --git a/oracle/testdata/src/main/describe.go b/oracle/testdata/src/main/describe.go
index ed4eea6b1fe..20a04b16d53 100644
--- a/oracle/testdata/src/main/describe.go
+++ b/oracle/testdata/src/main/describe.go
@@ -45,7 +45,7 @@ func main() { // @describe func-def-main "main"
 	if i != nil {
 		i = D{} // @describe var-ref-i-D "i"
 	}
-	_ = i // @describe var-ref-i "i"
+	print(i) // @describe var-ref-i "\\bi\\b"
 
 	// const objects
 	const localpi = 3.141     // @describe const-local-pi "localpi"
diff --git a/ssa/lift.go b/ssa/lift.go
index 3a68c129bd4..3cf1e041962 100644
--- a/ssa/lift.go
+++ b/ssa/lift.go
@@ -209,8 +209,8 @@ func lift(fn *Function) {
 		nps := newPhis[b]
 		j := 0
 		for _, np := range nps {
-			if len(*np.phi.Referrers()) == 0 {
-				continue // unreferenced phi
+			if !phiIsLive(np.phi) {
+				continue // discard it
 			}
 			nps[j] = np
 			j++
@@ -266,6 +266,19 @@ func lift(fn *Function) {
 	fn.Locals = fn.Locals[:j]
 }
 
+func phiIsLive(phi *Phi) bool {
+	for _, instr := range *phi.Referrers() {
+		if instr == phi {
+			continue // self-refs don't count
+		}
+		if _, ok := instr.(*DebugRef); ok {
+			continue // debug refs don't count
+		}
+		return true
+	}
+	return false
+}
+
 type blockSet struct{ big.Int } // (inherit methods from Int)
 
 // add adds b to the set and returns true if the set changed.