Skip useless vec constructors

vec3(v.xxz)  =>  v.xxz

README.md

@@ -484,12 +484,14 @@ Calls to `vec2`, `vec3`, and `vec4` can be simplified using swizzles.
 
 Input:
 ```glsl
-vec4(v1.x, v1.z, v2.r, v2.t)
+vec4(v1.x, v1.z, v2.r, v2.t);
+vec2(v1.xx);
 ```
 
 Output:
 ```glsl
-vec4(v1.xz,v2.xy)
+vec4(v1.xz,v2.xy);
+v1.xx;
 ```
 
 <!--

src/rewriter.fs

@@ -178,14 +178,22 @@ let private simplifyOperator env = function
 
 // Simplify calls to the vec constructor.
 let private simplifyVec constr args =
+    // Combine swizzles, e.g.
+    //    vec4(v1.x, v1.z, v2.r, v2.t)  =>  vec4(v1.xz, v2.xy)
     let rec combineSwizzles = function
         | [] -> []
-        | Dot (Var v1, fields1) :: Dot (Var v2, fields2) :: args
-            when isFieldSwizzle fields1 && isFieldSwizzle fields2 && v1.Name = v2.Name ->
-                combineSwizzles (Dot (Var v1, fields1 + fields2) :: args)
+        | Dot (Var v1, field1) :: Dot (Var v2, field2) :: args
+            when isFieldSwizzle field1 && isFieldSwizzle field2 && v1.Name = v2.Name ->
+                combineSwizzles (Dot (Var v1, field1 + field2) :: args)
 
         | e::l -> e :: combineSwizzles l
-    FunCall (Var constr, combineSwizzles args)
+    let args = combineSwizzles args
+    match args with
+    | [Dot (_, field) as arg] when field.Length > 1 && isFieldSwizzle field ->
+        // vec3(v.xxy)  =>  v.xxy
+        // However, vec3(v.x) should be preserved.
+        arg
+    | _ -> FunCall (Var constr, args)
 
 let private simplifyExpr (didInline: bool ref) env = function
     | FunCall(Var v, passedArgs) as e when v.ToBeInlined ->

tests/real/ed-209.frag.expected

@@ -53,7 +53,7 @@ float sdOctogon(vec2 p,float r)
 {
   const vec3 k=vec3(-.9238795325,.3826834323,.4142135623);
   p=abs(p);
-  p-=2.*min(dot(vec2(k.xy),p),0.)*vec2(k.xy);
+  p-=2.*min(dot(k.xy,p),0.)*k.xy;
   p-=2.*min(dot(vec2(-k.x,k.y),p),0.)*vec2(-k.x,k.y);
   p-=vec2(clamp(p.x,-k.z*r,k.z*r),r);
   return length(p)*sign(p.y);

tests/unit/vectors.frag

@@ -4,7 +4,7 @@ float swizzles() {
   vec3 v3 = vec3(v1.x, v2.x, v2.r);
   vec4 v4 = vec4(v1.xx, v1.y, v1.x);
   vec4 v5 = vec4(1., v2.z, v2.g, 2.);
-  vec4 v6 = vec4(v1.x, v1.z, v2.r, v2.t);
+  vec4 v6 = vec4(v1.x, v1.y, v2.r, v2.t);
   return v1.x + v2.x + v3.x + v4.x + v5.x + v6.x;
 }
 

tests/unit/vectors.frag.expected

@@ -1,8 +1,8 @@
 float swizzles()
 {
   vec2 v1=vec2(1.,1.);
-  vec3 v2=vec3(v1.xyx),v3=vec3(v1.x,v2.xx);
-  vec4 v4=vec4(v1.xxyx),v5=vec4(1.,v2.zy,2.),v6=vec4(v1.xz,v2.xy);
+  vec3 v2=v1.xyx,v3=vec3(v1.x,v2.xx);
+  vec4 v4=v1.xxyx,v5=vec4(1.,v2.zy,2.),v6=vec4(v1.xy,v2.xy);
   return v1.x+v2.x+v3.x+v4.x+v5.x+v6.x;
 }
 void main()