From this PDF of performance tips by AMD:
http://amd-dev.wpengine.netdna-cdn.com/wordpress/media/2013/05/GCNPerformanceTweets.pdf
GCN Performance Tip 38: Passing interpolated screenpos can be better thandeclaring SV_POSITION in pixel shader especially if PS is short.Notes: Declaring SV_POSITION in the pixel shader will not be as efficient as passingscreen coordinates from the previous shader stage because its use is hard-coded tofixed-function hardware that is also used for other purposes.If a pixel shader needs access to fragment position it is recommended to pass it viatexture coordinates instead.
This statement isn't making much sense to me based on testing this in a VX/PS combo.
Take this VS output.
struct VS_OUTPUT_SIMPLE{
float4 Position : SV_POSITION;
float3 Normal : NORMAL;
float4 Color : TEXCOORD0;
};
If I write these in the VS, and create a PS like so
Output PixelShader(VS_OUTPUT_SIMPLE In){}
The AMD statement would lead me to believe I can create an alternate struct that lacks SV_Position, and use that as input to the PS.
PS_INPUT_SIMPLE{
float3 Normal : NORMAL;
float4 Color : TEXCOORD0;
};
Output PixelShader(PS_INPUT_SIMPLE In){}
But this does not compile
It fails with this error.
Stage linkage warning: Semantic NORMAL has been placed in different registers in the two stages.
(which is what I'd expect, aside from this mysterious AMD statement).
Anyone grok what AMD is saying here?
