Problem testing DTid.x Direct3D ComputeShader HLSL - hlsl

Question

I’m attempting to write a slightly simple compute shader that does a simple moving average.
It is my first shader where I had to test DTid.x for certain conditions related to logic.
The shader works, the moving average is calculated as expected, except (ugh), for the case of DTid.x = 0 where I get a bad result.
It seems my testing of value DTid.x is somehow corrupted or not possible for case DTid.x = 0
I may be missing some fundamental understanding how compute shaders work as this piece of code seems super simple but it doesn't work as I'd expect it to.
Hopefully someone can tell me why this code doesn't work for case DTid.x = 0
For example, I simplified the shader to...
[numthreads(1024, 1, 1)]
void CSSimpleMovingAvgDX(uint3 DTid : SV_DispatchThreadID)
{
// I added below trying to limit the logic?
// I initially had it check for a range like >50 and <100 and this did work as expected.
// But I saw that my value at DTid.x = 0 was corrupted and I started to work on solving why. But no luck.
// It is just the case of DTid.x = 0 where this shader does not work.
if (DTid.x > 0)
{
return;
}
nAvgCnt = 1;
ft0 = asfloat(BufferY0.Load(DTid.x * 4)); // load data at actual DTid.x location
if (DTid.x > 0) // to avoid loading a second value for averaging
{
// somehow this code is still being called for case DTid.x = 0 ?
nAvgCnt = nAvgCnt + 1;
ft1 = asfloat(BufferY0.Load((DTid.x - 1) * 4)); // load data value at previous DTid.x location
}
if (nAvgCnt > 1) // If DTid.X was larger than 0, then we should have loaded ft1 and we can avereage ft0 and ft1
{
result = ((ft0 + ft1) / ((float)nAvgCnt));
}
else
{
result = ft0;
}
// And when I add code below, which should override above code, the result is still corrupted? //
if (DTid.x < 2)
result = ft0;
llByteOffsetLS = ((DTid.x) * dwStrideSomeBuffer);
BufferOut0.Store(llByteOffsetLS, asuint(result)); // store result, where all good except for case DTid.x = 0
}

Answer 1

I am compiling the shader with FXC. My shader was slightly more involved than above, I added the /Od option and the code behaved as expected. Without the /Od option I tried to refactor the code over and over with no luck but eventually I changed variable names for every possible section to make sure the compiler would treat them separately and eventually success. So, the lesson I learned is never reuse a variable in any way. Another solution, worse case, would be to decompile the compiled shader to understand how it was optimized. If attempting a large shader with several conditions/branches, I'd start with /Od and then eventually remove, and do not reuse variables, else you may start chasing problems that are not truly problems.