PTX "bit bucket" registers

Question

...are just mentioned in the PTX manual. There is no hint about what they are good for or how to use them.

Does anyone know more? Am I just missing a common concept?

Answer 1

Bart's comment is basically right. In more detail, as stated in the PTX ISA 3.1 manual,

For some instructions the destination operand is optional. A “bit bucket” operand denoted with an underscore (_) may be used in place of a destination register.

There is actually only one class of instruction listed in the 3.1 PTX spec for which _ is a valid destination: atom. Here are the semantics of atom:

Atomically loads the original value at location a into destination register d, performs a reduction operation with operand b and the value in location a, and stores the result of the specified operation at location a, overwriting the original value.

And there is a note for atom:

Simple reductions may be specified by using the “bit bucket” destination operand ‘_’.

So, we can construct an example:

atom.global.add.s32 _, [a], 4

This would add 4 to the signed integer at memory location a, and not return the previous value of location a in a register. So if you don't need the previous value, you can use this. I assume that the compiler would generate this for this code

atomicAdd(&a, 4);

since the return value of atomicAdd is not stored to a variable.