diff --git a/src/cmd/asm/internal/lex/input.go b/src/cmd/asm/internal/lex/input.go
index 8768b4f6484..737b12e134c 100644
--- a/src/cmd/asm/internal/lex/input.go
+++ b/src/cmd/asm/internal/lex/input.go
@@ -203,12 +203,21 @@ func (in *Input) defineMacro(name string, args []string, tokens []Token) {
 // The argument list is nil for no parens on the definition; otherwise a list of
 // formal argument names.
 func (in *Input) macroDefinition(name string) ([]string, []Token) {
+	prevCol := in.Stack.Col()
 	tok := in.Stack.Next()
 	if tok == '\n' || tok == scanner.EOF {
 		in.Error("no definition for macro:", name)
 	}
 	var args []string
-	if tok == '(' {
+	// The C preprocessor treats
+	//	#define A(x)
+	// and
+	//	#define A (x)
+	// distinctly: the first is a macro with arguments, the second without.
+	// Distinguish these cases using the column number, since we don't
+	// see the space itself. Note that text/scanner reports the position at the
+	// end of the token. It's where you are now, and you just read this token.
+	if tok == '(' && in.Stack.Col() == prevCol+1 {
 		// Macro has arguments. Scan list of formals.
 		acceptArg := true
 		args = []string{} // Zero length but not nil.
diff --git a/src/cmd/asm/internal/lex/lex.go b/src/cmd/asm/internal/lex/lex.go
index 3bd832db86c..c48b74a101c 100644
--- a/src/cmd/asm/internal/lex/lex.go
+++ b/src/cmd/asm/internal/lex/lex.go
@@ -29,6 +29,11 @@ const (
 	macroName                          // name of macro that should not be expanded
 )
 
+// IsRegisterShift reports whether the token is one of the ARM register shift operators.
+func IsRegisterShift(r ScanToken) bool {
+	return ROT <= r && r <= LSH // Order looks backwards because these are negative.
+}
+
 func (t ScanToken) String() string {
 	switch t {
 	case scanner.EOF:
@@ -94,6 +99,8 @@ type TokenReader interface {
 	File() string
 	// Line reports the source line number of the token.
 	Line() int
+	// Col reports the source column number of the token.
+	Col() int
 	// SetPos sets the file and line number.
 	SetPos(line int, file string)
 	// Close does any teardown required.
diff --git a/src/cmd/asm/internal/lex/lex_test.go b/src/cmd/asm/internal/lex/lex_test.go
index 64f6784495b..59e01c6699e 100644
--- a/src/cmd/asm/internal/lex/lex_test.go
+++ b/src/cmd/asm/internal/lex/lex_test.go
@@ -41,6 +41,16 @@ var lexTests = []lexTest{
 		"#define A() 1234\n" + "A()\n",
 		"1234.\n",
 	},
+	{
+		"macro with just parens as body",
+		"#define A () \n" + "A\n",
+		"(.).\n",
+	},
+	{
+		"macro with parens but no arguments",
+		"#define A (x) \n" + "A\n",
+		"(.x.).\n",
+	},
 	{
 		"macro with arguments",
 		"#define A(x, y, z) x+z+y\n" + "A(1, 2, 3)\n",
diff --git a/src/cmd/asm/internal/lex/slice.go b/src/cmd/asm/internal/lex/slice.go
index 6ac72f469e4..e94106b0af5 100644
--- a/src/cmd/asm/internal/lex/slice.go
+++ b/src/cmd/asm/internal/lex/slice.go
@@ -43,6 +43,11 @@ func (s *Slice) Line() int {
 	return s.line
 }
 
+func (s *Slice) Col() int {
+	// Col is only called when defining a macro, which can't reach here.
+	panic("cannot happen: slice col")
+}
+
 func (s *Slice) SetPos(line int, file string) {
 	// Cannot happen because we only have slices of already-scanned
 	// text, but be prepared.
diff --git a/src/cmd/asm/internal/lex/stack.go b/src/cmd/asm/internal/lex/stack.go
index 9766af51de3..72d7f8a165f 100644
--- a/src/cmd/asm/internal/lex/stack.go
+++ b/src/cmd/asm/internal/lex/stack.go
@@ -41,6 +41,10 @@ func (s *Stack) Line() int {
 	return s.tr[len(s.tr)-1].Line()
 }
 
+func (s *Stack) Col() int {
+	return s.tr[len(s.tr)-1].Col()
+}
+
 func (s *Stack) SetPos(line int, file string) {
 	s.tr[len(s.tr)-1].SetPos(line, file)
 }
diff --git a/src/cmd/asm/internal/lex/tokenizer.go b/src/cmd/asm/internal/lex/tokenizer.go
index 24a72479db5..28a4b852534 100644
--- a/src/cmd/asm/internal/lex/tokenizer.go
+++ b/src/cmd/asm/internal/lex/tokenizer.go
@@ -89,6 +89,10 @@ func (t *Tokenizer) Line() int {
 	return t.line
 }
 
+func (t *Tokenizer) Col() int {
+	return t.s.Pos().Column
+}
+
 func (t *Tokenizer) SetPos(line int, file string) {
 	t.line = line
 	t.fileName = file