cmd/compile: add x.Uses==1 test to load combiners

We need to make sure that when we combine loads, we only do
so if there are no other uses of the load.  We can't split
one load into two because that can then lead to inconsistent
loaded values in the presence of races.

Add some aggressive copy removal code so that phantom
"dead copy" uses of values are cleaned up promptly.  This lets
us use x.Uses==1 conditions reliably.

Change-Id: I9037311db85665f3868dbeb3adb3de5c20728b38
Reviewed-on: https://go-review.googlesource.com/21853Reviewed-by: Todd Neal <todd@tneal.org>

src/cmd/compile/internal/gc/ssa_test.go

@@ -101,3 +101,5 @@ func TestPhi(t *testing.T) { runTest(t, "phi_ssa.go") }
 func TestSlice(t *testing.T) { runTest(t, "slice.go") }
 
 func TestNamedReturn(t *testing.T) { runTest(t, "namedReturn.go") }
+
+func TestDuplicateLoad(t *testing.T) { runTest(t, "dupLoad.go") }

src/cmd/compile/internal/gc/testdata/dupLoad.go

+// run
+
+// Copyright 2016 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.
+
+// This test makes sure that we don't split a single
+// load up into two separate loads.
+
+package main
+
+import "fmt"
+
+//go:noinline
+func read(b []byte) (uint16, uint16) {
+	// There is only a single read of b[0].  The two
+	// returned values must have the same low byte.
+	v := b[0]
+	return uint16(v), uint16(v) | uint16(b[1])<<8
+}
+
+const N = 100000
+
+func main() {
+	done := make(chan struct{})
+	b := make([]byte, 2)
+	go func() {
+		for i := 0; i < N; i++ {
+			b[0] = byte(i)
+			b[1] = byte(i)
+		}
+		done <- struct{}{}
+	}()
+	go func() {
+		for i := 0; i < N; i++ {
+			x, y := read(b)
+			if byte(x) != byte(y) {
+				fmt.Printf("x=%x y=%x\n", x, y)
+				panic("bad")
+			}
+		}
+		done <- struct{}{}
+	}()
+	<-done
+	<-done
+}

src/cmd/compile/internal/gc/testdata/namedReturn.go

 // run
 
+// Copyright 2016 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.
+
 // This test makes sure that naming named
 // return variables in a return statement works.
 // See issue #14904.

src/cmd/compile/internal/ssa/gen/AMD64.rules

@@ -1369,13 +1369,13 @@
 // There are many ways these combinations could occur.  This is
 // designed to match the way encoding/binary.LittleEndian does it.
 (ORW               x0:(MOVBload [i]   {s} p mem)
-    (SHLWconst [8] x1:(MOVBload [i+1] {s} p mem))) && mergePoint(b,x0,x1) != nil -> @mergePoint(b,x0,x1) (MOVWload [i] {s} p mem)
+    (SHLWconst [8] x1:(MOVBload [i+1] {s} p mem))) && x0.Uses == 1 && x1.Uses == 1 && mergePoint(b,x0,x1) != nil -> @mergePoint(b,x0,x1) (MOVWload [i] {s} p mem)
 
 (ORL (ORL (ORL
                     x0:(MOVBload [i]   {s} p mem)
     (SHLLconst [8]  x1:(MOVBload [i+1] {s} p mem)))
     (SHLLconst [16] x2:(MOVBload [i+2] {s} p mem)))
-    (SHLLconst [24] x3:(MOVBload [i+3] {s} p mem))) && mergePoint(b,x0,x1,x2,x3) != nil -> @mergePoint(b,x0,x1,x2,x3) (MOVLload [i] {s} p mem)
+    (SHLLconst [24] x3:(MOVBload [i+3] {s} p mem))) && x0.Uses == 1 && x1.Uses == 1 && x2.Uses == 1 && x3.Uses == 1 && mergePoint(b,x0,x1,x2,x3) != nil -> @mergePoint(b,x0,x1,x2,x3) (MOVLload [i] {s} p mem)
 
 (ORQ (ORQ (ORQ (ORQ (ORQ (ORQ (ORQ
                     x0:(MOVBload [i]   {s} p mem)
@@ -1385,16 +1385,16 @@
     (SHLQconst [32] x4:(MOVBload [i+4] {s} p mem)))
     (SHLQconst [40] x5:(MOVBload [i+5] {s} p mem)))
     (SHLQconst [48] x6:(MOVBload [i+6] {s} p mem)))
-    (SHLQconst [56] x7:(MOVBload [i+7] {s} p mem))) && mergePoint(b,x0,x1,x2,x3,x4,x5,x6,x7) != nil -> @mergePoint(b,x0,x1,x2,x3,x4,x5,x6,x7) (MOVQload [i] {s} p mem)
+    (SHLQconst [56] x7:(MOVBload [i+7] {s} p mem))) && x0.Uses == 1 && x1.Uses == 1 && x2.Uses == 1 && x3.Uses == 1 && x4.Uses == 1 && x5.Uses == 1 && x6.Uses == 1 && x7.Uses == 1 && mergePoint(b,x0,x1,x2,x3,x4,x5,x6,x7) != nil -> @mergePoint(b,x0,x1,x2,x3,x4,x5,x6,x7) (MOVQload [i] {s} p mem)
 
 (ORW               x0:(MOVBloadidx1 [i]   {s} p idx mem)
-    (SHLWconst [8] x1:(MOVBloadidx1 [i+1] {s} p idx mem))) && mergePoint(b,x0,x1) != nil -> @mergePoint(b,x0,x1) (MOVWloadidx1 <v.Type> [i] {s} p idx mem)
+    (SHLWconst [8] x1:(MOVBloadidx1 [i+1] {s} p idx mem))) && x0.Uses == 1 && x1.Uses == 1 && mergePoint(b,x0,x1) != nil -> @mergePoint(b,x0,x1) (MOVWloadidx1 <v.Type> [i] {s} p idx mem)
 
 (ORL (ORL (ORL
                     x0:(MOVBloadidx1 [i]   {s} p idx mem)
     (SHLLconst [8]  x1:(MOVBloadidx1 [i+1] {s} p idx mem)))
     (SHLLconst [16] x2:(MOVBloadidx1 [i+2] {s} p idx mem)))
-    (SHLLconst [24] x3:(MOVBloadidx1 [i+3] {s} p idx mem))) && mergePoint(b,x0,x1,x2,x3) != nil -> @mergePoint(b,x0,x1,x2,x3) (MOVLloadidx1 <v.Type> [i] {s} p idx mem)
+    (SHLLconst [24] x3:(MOVBloadidx1 [i+3] {s} p idx mem))) && x0.Uses == 1 && x1.Uses == 1 && x2.Uses == 1 && x3.Uses == 1 && mergePoint(b,x0,x1,x2,x3) != nil -> @mergePoint(b,x0,x1,x2,x3) (MOVLloadidx1 <v.Type> [i] {s} p idx mem)
 
 (ORQ (ORQ (ORQ (ORQ (ORQ (ORQ (ORQ
                     x0:(MOVBloadidx1 [i]   {s} p idx mem)
@@ -1404,4 +1404,4 @@
     (SHLQconst [32] x4:(MOVBloadidx1 [i+4] {s} p idx mem)))
     (SHLQconst [40] x5:(MOVBloadidx1 [i+5] {s} p idx mem)))
     (SHLQconst [48] x6:(MOVBloadidx1 [i+6] {s} p idx mem)))
-    (SHLQconst [56] x7:(MOVBloadidx1 [i+7] {s} p idx mem))) && mergePoint(b,x0,x1,x2,x3,x4,x5,x6,x7) != nil -> @mergePoint(b,x0,x1,x2,x3,x4,x5,x6,x7) (MOVQloadidx1 <v.Type> [i] {s} p idx mem)
+    (SHLQconst [56] x7:(MOVBloadidx1 [i+7] {s} p idx mem))) && x0.Uses == 1 && x1.Uses == 1 && x2.Uses == 1 && x3.Uses == 1 && x4.Uses == 1 && x5.Uses == 1 && x6.Uses == 1 && x7.Uses == 1 && mergePoint(b,x0,x1,x2,x3,x4,x5,x6,x7) != nil -> @mergePoint(b,x0,x1,x2,x3,x4,x5,x6,x7) (MOVQloadidx1 <v.Type> [i] {s} p idx mem)

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

@@ -418,7 +418,7 @@ func TestNilcheckBug(t *testing.T) {
 			Goto("exit")),
 		Bloc("exit",
 			Valu("phi", OpPhi, TypeMem, 0, nil, "mem", "store"),
-			Exit("mem")))
+			Exit("phi")))
 
 	CheckFunc(fun.f)
 	// we need the opt here to rewrite the user nilcheck

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

@@ -40,9 +40,44 @@ func applyRewrite(f *Func, rb func(*Block) bool, rv func(*Value, *Config) bool)
 			}
 			curb = nil
 			for _, v := range b.Values {
-				change = copyelimValue(v) || change
 				change = phielimValue(v) || change
 
+				// Eliminate copy inputs.
+				// If any copy input becomes unused, mark it
+				// as invalid and discard its argument. Repeat
+				// recursively on the discarded argument.
+				// This phase helps remove phantom "dead copy" uses
+				// of a value so that a x.Uses==1 rule condition
+				// fires reliably.
+				for i, a := range v.Args {
+					if a.Op != OpCopy {
+						continue
+					}
+					x := a.Args[0]
+					// Rewriting can generate OpCopy loops.
+					// They are harmless (see removePredecessor),
+					// but take care to stop if we find a cycle.
+					slow := x // advances every other iteration
+					var advance bool
+					for x.Op == OpCopy {
+						x = x.Args[0]
+						if slow == x {
+							break
+						}
+						if advance {
+							slow = slow.Args[0]
+						}
+						advance = !advance
+					}
+					v.SetArg(i, x)
+					change = true
+					for a.Uses == 0 {
+						b := a.Args[0]
+						a.reset(OpInvalid)
+						a = b
+					}
+				}
+
 				// apply rewrite function
 				curv = v
 				if rv(v, config) {
@@ -52,7 +87,28 @@ func applyRewrite(f *Func, rb func(*Block) bool, rv func(*Value, *Config) bool)
 			}
 		}
 		if !change {
-			return
+			break
+		}
+	}
+	// remove clobbered copies
+	for _, b := range f.Blocks {
+		j := 0
+		for i, v := range b.Values {
+			if v.Op == OpInvalid {
+				f.freeValue(v)
+				continue
+			}
+			if i != j {
+				b.Values[j] = v
+			}
+			j++
+		}
+		if j != len(b.Values) {
+			tail := b.Values[j:]
+			for j := range tail {
+				tail[j] = nil
+			}
+			b.Values = b.Values[:j]
 		}
 	}
 }

src/cmd/compile/internal/ssa/rewriteAMD64.go

@@ -12666,7 +12666,7 @@ func rewriteValueAMD64_OpAMD64ORL(v *Value, config *Config) bool {
 		return true
 	}
 	// match: (ORL (ORL (ORL                     x0:(MOVBload [i]   {s} p mem)     (SHLLconst [8]  x1:(MOVBload [i+1] {s} p mem)))     (SHLLconst [16] x2:(MOVBload [i+2] {s} p mem)))     (SHLLconst [24] x3:(MOVBload [i+3] {s} p mem)))
-	// cond: mergePoint(b,x0,x1,x2,x3) != nil
+	// cond: x0.Uses == 1 && x1.Uses == 1 && x2.Uses == 1 && x3.Uses == 1 && mergePoint(b,x0,x1,x2,x3) != nil
 	// result: @mergePoint(b,x0,x1,x2,x3) (MOVLload [i] {s} p mem)
 	for {
 		v_0 := v.Args[0]
@@ -12754,7 +12754,7 @@ func rewriteValueAMD64_OpAMD64ORL(v *Value, config *Config) bool {
 		if mem != x3.Args[1] {
 			break
 		}
-		if !(mergePoint(b, x0, x1, x2, x3) != nil) {
+		if !(x0.Uses == 1 && x1.Uses == 1 && x2.Uses == 1 && x3.Uses == 1 && mergePoint(b, x0, x1, x2, x3) != nil) {
 			break
 		}
 		b = mergePoint(b, x0, x1, x2, x3)
@@ -12768,7 +12768,7 @@ func rewriteValueAMD64_OpAMD64ORL(v *Value, config *Config) bool {
 		return true
 	}
 	// match: (ORL (ORL (ORL                     x0:(MOVBloadidx1 [i]   {s} p idx mem)     (SHLLconst [8]  x1:(MOVBloadidx1 [i+1] {s} p idx mem)))     (SHLLconst [16] x2:(MOVBloadidx1 [i+2] {s} p idx mem)))     (SHLLconst [24] x3:(MOVBloadidx1 [i+3] {s} p idx mem)))
-	// cond: mergePoint(b,x0,x1,x2,x3) != nil
+	// cond: x0.Uses == 1 && x1.Uses == 1 && x2.Uses == 1 && x3.Uses == 1 && mergePoint(b,x0,x1,x2,x3) != nil
 	// result: @mergePoint(b,x0,x1,x2,x3) (MOVLloadidx1 <v.Type> [i] {s} p idx mem)
 	for {
 		v_0 := v.Args[0]
@@ -12866,7 +12866,7 @@ func rewriteValueAMD64_OpAMD64ORL(v *Value, config *Config) bool {
 		if mem != x3.Args[2] {
 			break
 		}
-		if !(mergePoint(b, x0, x1, x2, x3) != nil) {
+		if !(x0.Uses == 1 && x1.Uses == 1 && x2.Uses == 1 && x3.Uses == 1 && mergePoint(b, x0, x1, x2, x3) != nil) {
 			break
 		}
 		b = mergePoint(b, x0, x1, x2, x3)
@@ -12980,7 +12980,7 @@ func rewriteValueAMD64_OpAMD64ORQ(v *Value, config *Config) bool {
 		return true
 	}
 	// match: (ORQ (ORQ (ORQ (ORQ (ORQ (ORQ (ORQ                     x0:(MOVBload [i]   {s} p mem)     (SHLQconst [8]  x1:(MOVBload [i+1] {s} p mem)))     (SHLQconst [16] x2:(MOVBload [i+2] {s} p mem)))     (SHLQconst [24] x3:(MOVBload [i+3] {s} p mem)))     (SHLQconst [32] x4:(MOVBload [i+4] {s} p mem)))     (SHLQconst [40] x5:(MOVBload [i+5] {s} p mem)))     (SHLQconst [48] x6:(MOVBload [i+6] {s} p mem)))     (SHLQconst [56] x7:(MOVBload [i+7] {s} p mem)))
-	// cond: mergePoint(b,x0,x1,x2,x3,x4,x5,x6,x7) != nil
+	// cond: x0.Uses == 1 && x1.Uses == 1 && x2.Uses == 1 && x3.Uses == 1 && x4.Uses == 1 && x5.Uses == 1 && x6.Uses == 1 && x7.Uses == 1 && mergePoint(b,x0,x1,x2,x3,x4,x5,x6,x7) != nil
 	// result: @mergePoint(b,x0,x1,x2,x3,x4,x5,x6,x7) (MOVQload [i] {s} p mem)
 	for {
 		v_0 := v.Args[0]
@@ -13176,7 +13176,7 @@ func rewriteValueAMD64_OpAMD64ORQ(v *Value, config *Config) bool {
 		if mem != x7.Args[1] {
 			break
 		}
-		if !(mergePoint(b, x0, x1, x2, x3, x4, x5, x6, x7) != nil) {
+		if !(x0.Uses == 1 && x1.Uses == 1 && x2.Uses == 1 && x3.Uses == 1 && x4.Uses == 1 && x5.Uses == 1 && x6.Uses == 1 && x7.Uses == 1 && mergePoint(b, x0, x1, x2, x3, x4, x5, x6, x7) != nil) {
 			break
 		}
 		b = mergePoint(b, x0, x1, x2, x3, x4, x5, x6, x7)
@@ -13190,7 +13190,7 @@ func rewriteValueAMD64_OpAMD64ORQ(v *Value, config *Config) bool {
 		return true
 	}
 	// match: (ORQ (ORQ (ORQ (ORQ (ORQ (ORQ (ORQ                     x0:(MOVBloadidx1 [i]   {s} p idx mem)     (SHLQconst [8]  x1:(MOVBloadidx1 [i+1] {s} p idx mem)))     (SHLQconst [16] x2:(MOVBloadidx1 [i+2] {s} p idx mem)))     (SHLQconst [24] x3:(MOVBloadidx1 [i+3] {s} p idx mem)))     (SHLQconst [32] x4:(MOVBloadidx1 [i+4] {s} p idx mem)))     (SHLQconst [40] x5:(MOVBloadidx1 [i+5] {s} p idx mem)))     (SHLQconst [48] x6:(MOVBloadidx1 [i+6] {s} p idx mem)))     (SHLQconst [56] x7:(MOVBloadidx1 [i+7] {s} p idx mem)))
-	// cond: mergePoint(b,x0,x1,x2,x3,x4,x5,x6,x7) != nil
+	// cond: x0.Uses == 1 && x1.Uses == 1 && x2.Uses == 1 && x3.Uses == 1 && x4.Uses == 1 && x5.Uses == 1 && x6.Uses == 1 && x7.Uses == 1 && mergePoint(b,x0,x1,x2,x3,x4,x5,x6,x7) != nil
 	// result: @mergePoint(b,x0,x1,x2,x3,x4,x5,x6,x7) (MOVQloadidx1 <v.Type> [i] {s} p idx mem)
 	for {
 		v_0 := v.Args[0]
@@ -13408,7 +13408,7 @@ func rewriteValueAMD64_OpAMD64ORQ(v *Value, config *Config) bool {
 		if mem != x7.Args[2] {
 			break
 		}
-		if !(mergePoint(b, x0, x1, x2, x3, x4, x5, x6, x7) != nil) {
+		if !(x0.Uses == 1 && x1.Uses == 1 && x2.Uses == 1 && x3.Uses == 1 && x4.Uses == 1 && x5.Uses == 1 && x6.Uses == 1 && x7.Uses == 1 && mergePoint(b, x0, x1, x2, x3, x4, x5, x6, x7) != nil) {
 			break
 		}
 		b = mergePoint(b, x0, x1, x2, x3, x4, x5, x6, x7)
@@ -13514,7 +13514,7 @@ func rewriteValueAMD64_OpAMD64ORW(v *Value, config *Config) bool {
 		return true
 	}
 	// match: (ORW               x0:(MOVBload [i]   {s} p mem)     (SHLWconst [8] x1:(MOVBload [i+1] {s} p mem)))
-	// cond: mergePoint(b,x0,x1) != nil
+	// cond: x0.Uses == 1 && x1.Uses == 1 && mergePoint(b,x0,x1) != nil
 	// result: @mergePoint(b,x0,x1) (MOVWload [i] {s} p mem)
 	for {
 		x0 := v.Args[0]
@@ -13548,7 +13548,7 @@ func rewriteValueAMD64_OpAMD64ORW(v *Value, config *Config) bool {
 		if mem != x1.Args[1] {
 			break
 		}
-		if !(mergePoint(b, x0, x1) != nil) {
+		if !(x0.Uses == 1 && x1.Uses == 1 && mergePoint(b, x0, x1) != nil) {
 			break
 		}
 		b = mergePoint(b, x0, x1)
@@ -13562,7 +13562,7 @@ func rewriteValueAMD64_OpAMD64ORW(v *Value, config *Config) bool {
 		return true
 	}
 	// match: (ORW               x0:(MOVBloadidx1 [i]   {s} p idx mem)     (SHLWconst [8] x1:(MOVBloadidx1 [i+1] {s} p idx mem)))
-	// cond: mergePoint(b,x0,x1) != nil
+	// cond: x0.Uses == 1 && x1.Uses == 1 && mergePoint(b,x0,x1) != nil
 	// result: @mergePoint(b,x0,x1) (MOVWloadidx1 <v.Type> [i] {s} p idx mem)
 	for {
 		x0 := v.Args[0]
@@ -13600,7 +13600,7 @@ func rewriteValueAMD64_OpAMD64ORW(v *Value, config *Config) bool {
 		if mem != x1.Args[2] {
 			break
 		}
-		if !(mergePoint(b, x0, x1) != nil) {
+		if !(x0.Uses == 1 && x1.Uses == 1 && mergePoint(b, x0, x1) != nil) {
 			break
 		}
 		b = mergePoint(b, x0, x1)