[release-branch.go1.9] cmd/compile: fix sign-extension merging rules

If we have

  y = <int16> (MOVBQSX x)
  z = <int32> (MOVWQSX y)

We used to use this rewrite rule:

(MOVWQSX x:(MOVBQSX _)) -> x

But that resulted in replacing z with a value whose type
is only int16.  Then if z is spilled and restored, it gets
zero extended instead of sign extended.

Instead use the rule

(MOVWQSX (MOVBQSX x)) -> (MOVBQSX x)

The result is has the correct type, so it can be spilled
and restored correctly.  It might mean that a few more extension
ops might not be eliminated, but that's the price for correctness.

Fixes #21963

Change-Id: I6ec82c3d2dbe43cc1fee6fb2bd6b3a72fca3af00
Reviewed-on: https://go-review.googlesource.com/65290
Reviewed-by: Cherry Zhang <cherryyz@google.com>
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-on: https://go-review.googlesource.com/70986
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>

3 files changed, 113 insertions, 84 deletions

diff --git a/src/cmd/compile/internal/ssa/gen/AMD64.rules b/src/cmd/compile/internal/ssa/gen/AMD64.rules
index f1d53bc51f..ff38be550e 100644
--- a/src/cmd/compile/internal/ssa/gen/AMD64.rules
+++ b/src/cmd/compile/internal/ssa/gen/AMD64.rules
@@ -2405,15 +2405,17 @@
 (BSFQ (ORQconst <t> [1<<16] (MOVWQZX x))) -> (BSFQ (ORQconst <t> [1<<16] x))
 
 // Redundant sign/zero extensions
-(MOVLQSX x:(MOVLQSX _)) -> x
-(MOVLQSX x:(MOVWQSX _)) -> x
-(MOVLQSX x:(MOVBQSX _)) -> x
-(MOVWQSX x:(MOVWQSX _)) -> x
-(MOVWQSX x:(MOVBQSX _)) -> x
-(MOVBQSX x:(MOVBQSX _)) -> x
-(MOVLQZX x:(MOVLQZX _)) -> x
-(MOVLQZX x:(MOVWQZX _)) -> x
-(MOVLQZX x:(MOVBQZX _)) -> x
-(MOVWQZX x:(MOVWQZX _)) -> x
-(MOVWQZX x:(MOVBQZX _)) -> x
-(MOVBQZX x:(MOVBQZX _)) -> x
+// Note: see issue 21963. We have to make sure we use the right type on
+// the resulting extension (the outer type, not the inner type).
+(MOVLQSX (MOVLQSX x)) -> (MOVLQSX x)
+(MOVLQSX (MOVWQSX x)) -> (MOVWQSX x)
+(MOVLQSX (MOVBQSX x)) -> (MOVBQSX x)
+(MOVWQSX (MOVWQSX x)) -> (MOVWQSX x)
+(MOVWQSX (MOVBQSX x)) -> (MOVBQSX x)
+(MOVBQSX (MOVBQSX x)) -> (MOVBQSX x)
+(MOVLQZX (MOVLQZX x)) -> (MOVLQZX x)
+(MOVLQZX (MOVWQZX x)) -> (MOVWQZX x)
+(MOVLQZX (MOVBQZX x)) -> (MOVBQZX x)
+(MOVWQZX (MOVWQZX x)) -> (MOVWQZX x)
+(MOVWQZX (MOVBQZX x)) -> (MOVBQZX x)
+(MOVBQZX (MOVBQZX x)) -> (MOVBQZX x)
diff --git a/src/cmd/compile/internal/ssa/rewriteAMD64.go b/src/cmd/compile/internal/ssa/rewriteAMD64.go
index f2f4896410..9213616f83 100644
--- a/src/cmd/compile/internal/ssa/rewriteAMD64.go
+++ b/src/cmd/compile/internal/ssa/rewriteAMD64.go
@@ -4319,16 +4319,16 @@ func rewriteValueAMD64_OpAMD64MOVBQSX_0(v *Value) bool {
 		v.AddArg(x)
 		return true
 	}
-	// match: (MOVBQSX x:(MOVBQSX _))
+	// match: (MOVBQSX (MOVBQSX x))
 	// cond:
-	// result: x
+	// result: (MOVBQSX x)
 	for {
-		x := v.Args[0]
-		if x.Op != OpAMD64MOVBQSX {
+		v_0 := v.Args[0]
+		if v_0.Op != OpAMD64MOVBQSX {
 			break
 		}
-		v.reset(OpCopy)
-		v.Type = x.Type
+		x := v_0.Args[0]
+		v.reset(OpAMD64MOVBQSX)
 		v.AddArg(x)
 		return true
 	}
@@ -4536,16 +4536,16 @@ func rewriteValueAMD64_OpAMD64MOVBQZX_0(v *Value) bool {
 		v.AddArg(x)
 		return true
 	}
-	// match: (MOVBQZX x:(MOVBQZX _))
+	// match: (MOVBQZX (MOVBQZX x))
 	// cond:
-	// result: x
+	// result: (MOVBQZX x)
 	for {
-		x := v.Args[0]
-		if x.Op != OpAMD64MOVBQZX {
+		v_0 := v.Args[0]
+		if v_0.Op != OpAMD64MOVBQZX {
 			break
 		}
-		v.reset(OpCopy)
-		v.Type = x.Type
+		x := v_0.Args[0]
+		v.reset(OpAMD64MOVBQZX)
 		v.AddArg(x)
 		return true
 	}
@@ -6392,42 +6392,42 @@ func rewriteValueAMD64_OpAMD64MOVLQSX_0(v *Value) bool {
 		v.AddArg(x)
 		return true
 	}
-	// match: (MOVLQSX x:(MOVLQSX _))
+	// match: (MOVLQSX (MOVLQSX x))
 	// cond:
-	// result: x
+	// result: (MOVLQSX x)
 	for {
-		x := v.Args[0]
-		if x.Op != OpAMD64MOVLQSX {
+		v_0 := v.Args[0]
+		if v_0.Op != OpAMD64MOVLQSX {
 			break
 		}
-		v.reset(OpCopy)
-		v.Type = x.Type
+		x := v_0.Args[0]
+		v.reset(OpAMD64MOVLQSX)
 		v.AddArg(x)
 		return true
 	}
-	// match: (MOVLQSX x:(MOVWQSX _))
+	// match: (MOVLQSX (MOVWQSX x))
 	// cond:
-	// result: x
+	// result: (MOVWQSX x)
 	for {
-		x := v.Args[0]
-		if x.Op != OpAMD64MOVWQSX {
+		v_0 := v.Args[0]
+		if v_0.Op != OpAMD64MOVWQSX {
 			break
 		}
-		v.reset(OpCopy)
-		v.Type = x.Type
+		x := v_0.Args[0]
+		v.reset(OpAMD64MOVWQSX)
 		v.AddArg(x)
 		return true
 	}
-	// match: (MOVLQSX x:(MOVBQSX _))
+	// match: (MOVLQSX (MOVBQSX x))
 	// cond:
-	// result: x
+	// result: (MOVBQSX x)
 	for {
-		x := v.Args[0]
-		if x.Op != OpAMD64MOVBQSX {
+		v_0 := v.Args[0]
+		if v_0.Op != OpAMD64MOVBQSX {
 			break
 		}
-		v.reset(OpCopy)
-		v.Type = x.Type
+		x := v_0.Args[0]
+		v.reset(OpAMD64MOVBQSX)
 		v.AddArg(x)
 		return true
 	}
@@ -6611,42 +6611,42 @@ func rewriteValueAMD64_OpAMD64MOVLQZX_0(v *Value) bool {
 		v.AddArg(x)
 		return true
 	}
-	// match: (MOVLQZX x:(MOVLQZX _))
+	// match: (MOVLQZX (MOVLQZX x))
 	// cond:
-	// result: x
+	// result: (MOVLQZX x)
 	for {
-		x := v.Args[0]
-		if x.Op != OpAMD64MOVLQZX {
+		v_0 := v.Args[0]
+		if v_0.Op != OpAMD64MOVLQZX {
 			break
 		}
-		v.reset(OpCopy)
-		v.Type = x.Type
+		x := v_0.Args[0]
+		v.reset(OpAMD64MOVLQZX)
 		v.AddArg(x)
 		return true
 	}
-	// match: (MOVLQZX x:(MOVWQZX _))
+	// match: (MOVLQZX (MOVWQZX x))
 	// cond:
-	// result: x
+	// result: (MOVWQZX x)
 	for {
-		x := v.Args[0]
-		if x.Op != OpAMD64MOVWQZX {
+		v_0 := v.Args[0]
+		if v_0.Op != OpAMD64MOVWQZX {
 			break
 		}
-		v.reset(OpCopy)
-		v.Type = x.Type
+		x := v_0.Args[0]
+		v.reset(OpAMD64MOVWQZX)
 		v.AddArg(x)
 		return true
 	}
-	// match: (MOVLQZX x:(MOVBQZX _))
+	// match: (MOVLQZX (MOVBQZX x))
 	// cond:
-	// result: x
+	// result: (MOVBQZX x)
 	for {
-		x := v.Args[0]
-		if x.Op != OpAMD64MOVBQZX {
+		v_0 := v.Args[0]
+		if v_0.Op != OpAMD64MOVBQZX {
 			break
 		}
-		v.reset(OpCopy)
-		v.Type = x.Type
+		x := v_0.Args[0]
+		v.reset(OpAMD64MOVBQZX)
 		v.AddArg(x)
 		return true
 	}
@@ -10767,29 +10767,29 @@ func rewriteValueAMD64_OpAMD64MOVWQSX_0(v *Value) bool {
 		v.AddArg(x)
 		return true
 	}
-	// match: (MOVWQSX x:(MOVWQSX _))
+	// match: (MOVWQSX (MOVWQSX x))
 	// cond:
-	// result: x
+	// result: (MOVWQSX x)
 	for {
-		x := v.Args[0]
-		if x.Op != OpAMD64MOVWQSX {
+		v_0 := v.Args[0]
+		if v_0.Op != OpAMD64MOVWQSX {
 			break
 		}
-		v.reset(OpCopy)
-		v.Type = x.Type
+		x := v_0.Args[0]
+		v.reset(OpAMD64MOVWQSX)
 		v.AddArg(x)
 		return true
 	}
-	// match: (MOVWQSX x:(MOVBQSX _))
+	// match: (MOVWQSX (MOVBQSX x))
 	// cond:
-	// result: x
+	// result: (MOVBQSX x)
 	for {
-		x := v.Args[0]
-		if x.Op != OpAMD64MOVBQSX {
+		v_0 := v.Args[0]
+		if v_0.Op != OpAMD64MOVBQSX {
 			break
 		}
-		v.reset(OpCopy)
-		v.Type = x.Type
+		x := v_0.Args[0]
+		v.reset(OpAMD64MOVBQSX)
 		v.AddArg(x)
 		return true
 	}
@@ -10999,29 +10999,29 @@ func rewriteValueAMD64_OpAMD64MOVWQZX_0(v *Value) bool {
 		v.AddArg(x)
 		return true
 	}
-	// match: (MOVWQZX x:(MOVWQZX _))
+	// match: (MOVWQZX (MOVWQZX x))
 	// cond:
-	// result: x
+	// result: (MOVWQZX x)
 	for {
-		x := v.Args[0]
-		if x.Op != OpAMD64MOVWQZX {
+		v_0 := v.Args[0]
+		if v_0.Op != OpAMD64MOVWQZX {
 			break
 		}
-		v.reset(OpCopy)
-		v.Type = x.Type
+		x := v_0.Args[0]
+		v.reset(OpAMD64MOVWQZX)
 		v.AddArg(x)
 		return true
 	}
-	// match: (MOVWQZX x:(MOVBQZX _))
+	// match: (MOVWQZX (MOVBQZX x))
 	// cond:
-	// result: x
+	// result: (MOVBQZX x)
 	for {
-		x := v.Args[0]
-		if x.Op != OpAMD64MOVBQZX {
+		v_0 := v.Args[0]
+		if v_0.Op != OpAMD64MOVBQZX {
 			break
 		}
-		v.reset(OpCopy)
-		v.Type = x.Type
+		x := v_0.Args[0]
+		v.reset(OpAMD64MOVBQZX)
 		v.AddArg(x)
 		return true
 	}
diff --git a/test/fixedbugs/issue21963.go b/test/fixedbugs/issue21963.go
new file mode 100644
index 0000000000..996bd63d09
--- /dev/null
+++ b/test/fixedbugs/issue21963.go
@@ -0,0 +1,27 @@
+// run
+
+// Copyright 2017 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package main
+
+import (
+	"fmt"
+	"runtime"
+)
+
+//go:noinline
+func f(x []int32, y *int8) int32 {
+	c := int32(int16(*y))
+	runtime.GC()
+	return x[0] * c
+}
+
+func main() {
+	var x = [1]int32{5}
+	var y int8 = -1
+	if got, want := f(x[:], &y), int32(-5); got != want {
+		panic(fmt.Sprintf("wanted %d, got %d", want, got))
+	}
+}