Say I have a shader that uses 4 textures as input. For debugging purposes, I just want to check what's in the 4th texture, so I comment out the majority of the shader and just sample that one. I get all black, because apparently the shader has optimized that texture to slot t0 instead of t3 where i declared it.

I'm compiling with D3DCOMPILE_DEBUG | D3DCOMPILE_SKIP_OPTIMIZATION, and yet this optimization still happens. Is there a way to disable it, for debug builds? It's not a problem I can't work around, but it's a nuisance.

it does not. I'm still assigning 4 textures on the CPU side, and regardless of if I put t0 or t3, it samples from the first-assigned cubemap, not the 4th.

and regardless of if I put t0 or t3, it samples from the first-assigned cubemap

I get all black

This is... odd. Is the first bound cubemap actually black, or is there some calculation going on that would turn black if you enter a texture with different content than what it would expect? Because you shouldn't really get black in any other case if it really samples just from the first cubemap. So try to, instead of executing the rest of the shader, just output the actual content of the t3 declared cubemap and see if it matches your t0-bound texture first.

Say I have a shader that uses 4 textures as input. For debugging purposes, I just want to check what's in the 4th texture, so I comment out the majority of the shader and just sample that one. I get all black, because apparently the shader has optimized that texture to slot t0 instead of t3 where i declared it.

I'm compiling with D3DCOMPILE_DEBUG | D3DCOMPILE_SKIP_OPTIMIZATION, and yet this optimization still happens. Is there a way to disable it, for debug builds? It's not a problem I can't work around, but it's a nuisance.

What dx are you targeting?