diff --git a/src/ast.fs b/src/ast.fs
index 56aaf990..3b8a2920 100644
--- a/src/ast.fs
+++ b/src/ast.fs
@@ -91,6 +91,10 @@ and Type = {
         not (Set.intersect (set this.typeQ) (set ["out"; "inout"])).IsEmpty
     member this.IsExternal =
         List.exists (fun s -> Set.contains s Builtin.externalQualifiers) this.typeQ
+    member this.isScalar =
+        match this.name with
+            | TypeName n -> Builtin.builtinScalarTypes.Contains n
+            | _ -> false
     override t.ToString() =
         let name = match t.name with
                    | TypeName n -> n
diff --git a/src/builtin.fs b/src/builtin.fs
index 704a209f..688486b1 100644
--- a/src/builtin.fs
+++ b/src/builtin.fs
@@ -10,18 +10,24 @@ let keywords = System.Collections.Generic.HashSet<_>([
   "const"; "uniform"; "buffer"; "shared"; "attribute"; "varying"
 ])
 
-let builtinTypes = set([
-    yield! [ "void"; "bool"; "int"; "uint"; "float"; "double" ]
+let builtinScalarTypes = set [
+    "bool"; "int"; "uint"; "float"; "double"
+]
+let builtinVectorTypes = set([
     for p in [""; "d"; "b"; "i"; "u"] do
         for n in ["2"; "3"; "4"] do
             yield p+"vec"+n
+])
+let builtinMatrixTypes = set([
     for p in [""; "d"] do
         for n in ["2"; "3"; "4"] do
             yield p+"mat"+n
         for c in ["2"; "3"; "4"] do
             for r in ["2"; "3"; "4"] do
                 yield p+"mat"+c+"x"+r
-    ])
+])
+
+let builtinTypes = set [ "void" ] + builtinScalarTypes + builtinVectorTypes + builtinMatrixTypes;
 
 let implicitConversions = // (from, to)
     [
diff --git a/src/rewriter.fs b/src/rewriter.fs
index f97e791a..67290717 100644
--- a/src/rewriter.fs
+++ b/src/rewriter.fs
@@ -218,8 +218,13 @@ module private RewriterImpl =
 
         // x=...+x  ->  x+=...
         // Works only if the operator is commutative. * is not commutative with vectors and matrices.
-        | FunCall(Op "=", [Var x; FunCall(Op ("+"|"&"|"^"|"|" as op), [e; Var y])])
-                when x.Name = y.Name ->
+        | FunCall(Op "=", [Var x; FunCall(Op ("+"|"*"|"&"|"^"|"|" as op), [e; Var y])])
+                when x.Name = y.Name
+                && match x.VarDecl with
+                    // * is commutative when at least one operand is scalar
+                    | Some d -> op <> "*" || d.ty.isScalar
+                    | _ -> false
+                ->
             FunCall(Op (op + "="), [Var x; e])
 
         // Unsafe when x contains NaN or Inf values.
diff --git a/tests/real/audio-flight-v2.frag.expected b/tests/real/audio-flight-v2.frag.expected
index a957f514..b31e758a 100644
--- a/tests/real/audio-flight-v2.frag.expected
+++ b/tests/real/audio-flight-v2.frag.expected
@@ -188,7 +188,7 @@ vec2 marcher(vec3 ro,vec3 rd)
 }
 vec3 normal(vec3 p,float t)
 {
-  t=MINDIST*t;
+  t*=MINDIST;
   vec2 h=vec2(1,-1)*.5773;
   return normalize(h.xyy*map(p+h.xyy*t,0.).x+h.yyx*map(p+h.yyx*t,0.).x+h.yxy*map(p+h.yxy*t,0.).x+h.xxx*map(p+h.xxx*t,0.).x);
 }
diff --git a/tests/unit/inline.no.expected b/tests/unit/inline.no.expected
index 64dfc981..6400c5ce 100644
--- a/tests/unit/inline.no.expected
+++ b/tests/unit/inline.no.expected
@@ -4,7 +4,7 @@ float result;
 void main()
 {
   float x=.5;
-  x=.6*x*x;
+  x*=.6*x;
   result=x;
 }
 int arithmetic()