Keep operand order when removing parens around + and - operators (#406)

Followup to #151 

This transformation preserves the order of the operands (x, y, z), but
it changes the order of evaluation of the operators. This should be fine
for + and -.

For the multiplication: we preserve the evaluation order of the
multiplications (because of float precision), but we swap the operands
only when it's pure.

---------

Co-authored-by: Eldritch Conundrum <eldritch.conundrum@gmail.com>

src/rewriter.fs

@@ -202,12 +202,12 @@ module private RewriterImpl =
 
         // Swap operands to get rid of parentheses
         // x*(y*z) -> y*z*x
-        | FunCall(Op "*", [NoParen x; FunCall(Op "*", [y; z])]) ->
+        | FunCall(Op "*", [NoParen x; FunCall(Op "*", [y; z])]) when isPure x && isPure y && isPure z ->
             FunCall(Op "*", [FunCall(Op "*", [y; z]); x]) |> env.fExpr env
-        // x+(y+z) -> y+z+x
-        // x+(y-z) -> y-z+a
+        // x+(y+z) -> x+y+z
+        // x+(y-z) -> x+y-z
         | FunCall(Op "+", [NoParen x; FunCall(Op ("+"|"-") as op, [y; z])]) ->
-            FunCall(Op "+", [FunCall(op, [y; z]); x]) |> env.fExpr env
+            FunCall(op, [FunCall(Op "+", [x; y]); z]) |> env.fExpr env
         // x-(y+z) -> x-y-z
         | FunCall(Op "-", [x; FunCall(Op "+", [y; z])]) ->
             FunCall(Op "-", [FunCall(Op "-", [x; y]); z]) |> env.fExpr env

tests/compression_results.log

@@ -7,12 +7,12 @@ ed-209.frag                               7672 => 1338.529
 elevated.hlsl                             3401 =>  602.542
 endeavour.frag                            2568 =>  529.992
 from-the-seas-to-the-stars.frag          14211 => 2282.423
-frozen-wasteland.frag                     4513 =>  804.591
+frozen-wasteland.frag                     4515 =>  804.953
 kinder_painter.frag                       2832 =>  442.132
 leizex.frag                               2252 =>  506.309
 lunaquatic.frag                           5222 => 1043.256
 mandelbulb.frag                           2317 =>  532.741
-ohanami.frag                              3246 =>  711.753
+ohanami.frag                              3246 =>  712.723
 orchard.frag                              5394 => 1003.067
 oscars_chair.frag                         4648 =>  986.069
 robin.frag                                6196 => 1039.087
@@ -21,4 +21,4 @@ terrarium.frag                            3575 =>  747.116
 the_real_party_is_in_your_pocket.frag    11986 => 1774.550
 valley_ball.glsl                          4307 =>  881.820
 yx_long_way_from_home.frag                2936 =>  598.845
-Total: 133422 => 23299.280
+Total: 133424 => 23300.612

tests/real/frozen-wasteland.frag.expected

@@ -148,7 +148,7 @@ float map(vec3 p)
 {
   p.y+=height(p.zx);
   float d=smin(smin(p.y+.5,vine(p+vec3(.8,0,0),30.,3.3)),vine(p.zyx+vec3(0,0,17),33.,1.4));
-  d+=p.y*1.2*Noise3d(p*.05);
+  d+=Noise3d(p*.05)*(p.y*1.2);
   p.xz*=.3;
   return d+Noise3d(p*.3);
 }

tests/real/monjori.frag.expected

@@ -18,7 +18,7 @@ void main()
   i=cos(h+r*p.x/1.3)*(e+e+a)+cos(q*.025*6.)*(r+h/3.);
   h=sin(f*.025)*144.-sin(e*.025)*212.*p.x;
   h=(h+(f-e)*q+sin(r-(a+h)/7.)*10.+i/4.)*.025;
-  i=mod((cos(h*2.3*sin(a/350.-q))*184.*sin(q-(r*4.3+a/12.)*.025)+tan(r*.025+h)*184.*cos(r*.025+h)+i)/5.6,256.)/64.;
+  i=mod((i+cos(h*2.3*sin(a/350.-q))*184.*sin(q-(r*4.3+a/12.)*.025)+tan(r*.025+h)*184.*cos(r*.025+h))/5.6,256.)/64.;
   if(i<0.)
     i+=4.;
   if(i>=2.)

tests/real/ohanami.frag.expected

@@ -46,7 +46,7 @@ void main()
   if(gl_TexCoord[0].w>.5)
     {
       ti=max(0,time)/3.;
-      ti=floor(ti)+clamp(fract(ti)*2,0,1)+noise(floor(gl_FragCoord.xy));
+      ti=noise(floor(gl_FragCoord.xy))+floor(ti)+clamp(fract(ti)*2,0,1);
       ti=floor(ti);
       float zoom=1.5;
       vec2 filmoffset=vec2(0);

tests/unit/arg-inlining.expected

@@ -76,7 +76,7 @@ float f()
   s+=s+noinline_canInlineWhenArgIsInlinable5();
   s+=s+noinline_canInlineWhenArgIsInlinable6();
   s+=s+noinline_cannotInlineWhenArgIsNotInlinable(s);
-  return s+noinline_cannotInlineWhenArgIsNotInlinable(acos(s))+s;
+  return s+s+noinline_cannotInlineWhenArgIsNotInlinable(acos(s));
 }
 void main()
 {

tests/unit/operators.expected

@@ -16,7 +16,7 @@
  "}"
  "int no_parens(int a,int b,int c)"
  "{"
-   "return b+c+a+a+b*c;"
+   "return a+b+c+a+b*c;"
  "}"
  "int no_parens2(int a,int b,int c)"
  "{"
@@ -47,7 +47,12 @@
    "g/=-g+g;"
    "g-=-g--- --g;"
    "g*=g+g;"
-   "return--g-++g+g;"
+   "return g+--g-++g;"
+ "}"
+ "float swap_op_order(float g)"
+ "{"
+   "float a=g*(g--*g--),b=(g+2.)*(g+3.)*g;"
+   "return a*b*g;"
  "}",
 
 #endif

tests/unit/operators.frag

@@ -61,3 +61,10 @@ float cool_ops(float g)
 	f += --f + (- +(++f));
 	return f;
 }
+
+float swap_op_order(float g)
+{
+	float a = g*((g--)*(g--));
+	float b = g*((g+2.)*(g+3.));
+	return a*b*g;
+}

tests/unit/shadowing.frag.expected

@@ -1,7 +1,7 @@
 float f(float g)
 {
   g=0.;
-  return g+++ ++g+g;
+  return g+g+++ ++g;
 }
 float g(float f)
 {

tests/unit/simplify.expected

@@ -12,7 +12,7 @@ float baz(float a)
 {
   a=a+4.+sin(a);
   a=a+5.+sin(a);
-  return-a-a+a;
+  return a-a-a;
 }
 out vec3 outputvar;
 void notMain(float x)