Eliminate if's in shaders

[lukaspj]

This isn't exactly an issue, just a reminder for myself, and others, to look at all the shaders in T3D and eliminate if's where possible.

A small test where I tried eliminating all if's from my new terrain blending shader gave a 0.5 MSPF reduction. (155FPS -> 165FPS)

E.g:

if( lerpBlend <= 0 ) 
{ 
   invBlend = 1-detailBlend0;
   ma = max(bumpNormal.a + detailBlend0, currentAlpha + invBlend) - blendDepth0;
   b1 = max(bumpNormal.a + detailBlend0 - ma, 0);
   b2 = max(currentAlpha + invBlend - ma, 0);
   currentAlpha = max(currentAlpha,bumpNormal.a);
}
IN.detCoord0.xy += parallaxOffset( normalMap0, IN.detCoord0.xy, negViewTS, detailIdStrengthParallax0.z * detailBlend0 );
if ( detailBlend0 > 0.0f )
{
   detailColor *= detailIdStrengthParallax0.y * IN.detCoord0.w;
   if( lerpBlend <= 0 ) 
      OUT.col.rgb = ((baseColor + detailColor).rgb * b1 + OUT.col.rgb * b2) / (b1 + b2);
   else
      OUT.col = lerp( OUT.col, baseColor + detailColor, detailBlend0 );
}
if( lerpBlend <= 0 ) 
   currentAlpha = (bumpNormal.a * b1 + currentAlpha * b2) / (b1 + b2);

Became:

invBlend = 1-detailBlend0;
ma = max(bumpNormal.a + detailBlend0, currentAlpha + invBlend) - blendDepth0;
b1 = max(bumpNormal.a + detailBlend0 - ma, 0);
b2 = max(currentAlpha + invBlend - ma, 0);
currentAlpha = max(currentAlpha,bumpNormal.a);
IN.detCoord0.xy += parallaxOffset( normalMap0, IN.detCoord0.xy, negViewTS, detailIdStrengthParallax0.z * detailBlend0 );
detailColor *= detailIdStrengthParallax0.y * IN.detCoord0.w;
OUT.col.rgb = ((baseColor + detailColor).rgb * b1 + OUT.col.rgb * b2) / (b1 + b2);
currentAlpha = (bumpNormal.a * b1 + currentAlpha * b2) / (b1 + b2);

This is because the GPU branches out on if-statements, and because of current GPU architecture, branching means it runs each branch for each pixel and discards the branches not used by the different pixels.

Meaning that if you have 1 if-statement, your code is run twice for each pixel.. Once for each possible outcome of the if-statement. It's best to try to avoid if's where possible to avoid this branching, even if it means doing some extra computations in the main body.

[crabmusket]

Great idea.

[Azaezel]

http://msdn.microsoft.com/en-us/library/windows/desktop/bb509610 additional note for fallbacks where folks just can't figure out an alternate. Will need to ask @LuisAntonRebollo if there is an equivalent problem and solution for OpenGL.

[lukaspj]

Okay, made some benchmarks with some better tools (the metrics(fps) is near useless in its current state) And here is my results for performance.

The benchmarking happened using a "camera bookmark" so it's recording the exact same place completely static. Numbers are the average FPS of 20 samples:

Blending Method	[branch]	default	no ifs
Lerp	167	162	164
Heightmap	155	154	157

So the initial "0.5 mspf" gain was a little over-estimated (a result of the metrics(fps) stuff... Too damn unstable FPS if you don't have an average value) but it's still noticeable for such a small change.

([branch] method is the one described in @Azaezel 's link.

[LuisAntonRebollo]

For OpenGL3+ and DX10+ branching are a recomended method for improve performance.

DX9 has some problems, not sure if are a API limitation or a shader model 2/3 hardware restriction.

I prefer no remove if from code, we can add [branch] to help compiler.

@lukaspj, thx for benchmark :)