diff --git a/src/cmd/go.mod b/src/cmd/go.mod
index 3d9b4a8d244..19496a3c671 100644
--- a/src/cmd/go.mod
+++ b/src/cmd/go.mod
@@ -5,7 +5,7 @@ go 1.12
 require (
 	github.com/google/pprof v0.0.0-20190515194954-54271f7e092f
 	github.com/ianlancetaylor/demangle v0.0.0-20180524225900-fc6590592b44 // indirect
-	golang.org/x/arch v0.0.0-20181203225421-5a4828bb7045
+	golang.org/x/arch v0.0.0-20190815191158-8a70ba74b3a1
 	golang.org/x/crypto v0.0.0-20190325154230-a5d413f7728c
 	golang.org/x/sys v0.0.0-20190502175342-a43fa875dd82 // indirect
 	golang.org/x/tools v0.0.0-20190611154301-25a4f137592f
diff --git a/src/cmd/go.sum b/src/cmd/go.sum
index da3123b9f03..9aa94eee7b3 100644
--- a/src/cmd/go.sum
+++ b/src/cmd/go.sum
@@ -2,8 +2,8 @@ github.com/google/pprof v0.0.0-20190515194954-54271f7e092f h1:Jnx61latede7zDD3Di
 github.com/google/pprof v0.0.0-20190515194954-54271f7e092f/go.mod h1:zfwlbNMJ+OItoe0UupaVj+oy1omPYYDuagoSzA8v9mc=
 github.com/ianlancetaylor/demangle v0.0.0-20180524225900-fc6590592b44 h1:pKqc8lAAA6rcwpvsephnRuZp4VHbfszZRClvqAE6Sq8=
 github.com/ianlancetaylor/demangle v0.0.0-20180524225900-fc6590592b44/go.mod h1:aSSvb/t6k1mPoxDqO4vJh6VOCGPwU4O0C2/Eqndh1Sc=
-golang.org/x/arch v0.0.0-20181203225421-5a4828bb7045 h1:Pn8fQdvx+z1avAi7fdM2kRYWQNxGlavNDSyzrQg2SsU=
-golang.org/x/arch v0.0.0-20181203225421-5a4828bb7045/go.mod h1:cYlCBUl1MsqxdiKgmc4uh7TxZfWSFLOGSRR090WDxt8=
+golang.org/x/arch v0.0.0-20190815191158-8a70ba74b3a1 h1:A71BZbKSu+DtCNry/x5JKn20C+64DirDHmePEA8k0FY=
+golang.org/x/arch v0.0.0-20190815191158-8a70ba74b3a1/go.mod h1:flIaEI6LNU6xOCD5PaJvn9wGP0agmIOqjrtsKGRguv4=
 golang.org/x/crypto v0.0.0-20190308221718-c2843e01d9a2/go.mod h1:djNgcEr1/C05ACkg1iLfiJU5Ep61QUkGW8qpdssI0+w=
 golang.org/x/crypto v0.0.0-20190325154230-a5d413f7728c h1:Vj5n4GlwjmQteupaxJ9+0FNOmBrHfq7vN4btdGoDZgI=
 golang.org/x/crypto v0.0.0-20190325154230-a5d413f7728c/go.mod h1:djNgcEr1/C05ACkg1iLfiJU5Ep61QUkGW8qpdssI0+w=
@@ -17,3 +17,4 @@ golang.org/x/text v0.3.0 h1:g61tztE5qeGQ89tm6NTjjM9VPIm088od1l6aSorWRWg=
 golang.org/x/text v0.3.0/go.mod h1:NqM8EUOU14njkJ3fqMW+pc6Ldnwhi/IjpwHt7yyuwOQ=
 golang.org/x/tools v0.0.0-20190611154301-25a4f137592f h1:6awn5JC4pwVI5HiBqs7MDtRxnwV9PpO5iSA9v6P09pA=
 golang.org/x/tools v0.0.0-20190611154301-25a4f137592f/go.mod h1:/rFqwRUd4F7ZHNgwSSTFct+R/Kf4OFW1sUzUTQQTgfc=
+rsc.io/pdf v0.1.1/go.mod h1:n8OzWcQ6Sp37PL01nO98y4iUCRdTGarVfzxY20ICaU4=
diff --git a/src/cmd/vendor/golang.org/x/arch/x86/x86asm/decode.go b/src/cmd/vendor/golang.org/x/arch/x86/x86asm/decode.go
index 148870b73dd..8c984970bd8 100644
--- a/src/cmd/vendor/golang.org/x/arch/x86/x86asm/decode.go
+++ b/src/cmd/vendor/golang.org/x/arch/x86/x86asm/decode.go
@@ -203,7 +203,9 @@ func instPrefix(b byte, mode int) (Inst, error) {
 // For now we use instPrefix but perhaps later we will return
 // a specific error here.
 func truncated(src []byte, mode int) (Inst, error) {
-	//	return Inst{}, len(src), ErrTruncated
+	if len(src) == 0 {
+		return Inst{}, ErrTruncated
+	}
 	return instPrefix(src[0], mode) // too long
 }
 
@@ -216,7 +218,6 @@ var (
 
 // decoderCover records coverage information for which parts
 // of the byte code have been executed.
-// TODO(rsc): This is for testing. Only use this if a flag is given.
 var decoderCover []bool
 
 // Decode decodes the leading bytes in src as a single instruction.
@@ -406,7 +407,7 @@ ReadPrefixes:
 
 		//Group 5 - Vex encoding
 		case 0xC5:
-			if pos == 0 && (mode == 64 || (mode == 32 && pos+1 < len(src) && src[pos+1]&0xc0 == 0xc0)) {
+			if pos == 0 && pos+1 < len(src) && (mode == 64 || (mode == 32 && src[pos+1]&0xc0 == 0xc0)) {
 				vex = p
 				vexIndex = pos
 				inst.Prefix[pos] = p
@@ -418,7 +419,7 @@ ReadPrefixes:
 				break ReadPrefixes
 			}
 		case 0xC4:
-			if pos == 0 && (mode == 64 || (mode == 32 && pos+2 < len(src) && src[pos+1]&0xc0 == 0xc0)) {
+			if pos == 0 && pos+2 < len(src) && (mode == 64 || (mode == 32 && src[pos+1]&0xc0 == 0xc0)) {
 				vex = p
 				vexIndex = pos
 				inst.Prefix[pos] = p
@@ -460,9 +461,6 @@ ReadPrefixes:
 	// opshift gives the shift to use when saving the next
 	// opcode byte into inst.Opcode.
 	opshift = 24
-	if decoderCover == nil {
-		decoderCover = make([]bool, len(decoder))
-	}
 
 	// Decode loop, executing decoder program.
 	var oldPC, prevPC int
@@ -474,7 +472,9 @@ Decode:
 			println("run", pc)
 		}
 		x := decoder[pc]
-		decoderCover[pc] = true
+		if decoderCover != nil {
+			decoderCover[pc] = true
+		}
 		pc++
 
 		// Read and decode ModR/M if needed by opcode.
diff --git a/src/cmd/vendor/modules.txt b/src/cmd/vendor/modules.txt
index caf340a7528..e873ad44555 100644
--- a/src/cmd/vendor/modules.txt
+++ b/src/cmd/vendor/modules.txt
@@ -16,7 +16,7 @@ github.com/google/pprof/third_party/d3flamegraph
 github.com/google/pprof/third_party/svgpan
 # github.com/ianlancetaylor/demangle v0.0.0-20180524225900-fc6590592b44
 github.com/ianlancetaylor/demangle
-# golang.org/x/arch v0.0.0-20181203225421-5a4828bb7045
+# golang.org/x/arch v0.0.0-20190815191158-8a70ba74b3a1
 golang.org/x/arch/arm/armasm
 golang.org/x/arch/arm64/arm64asm
 golang.org/x/arch/ppc64/ppc64asm