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Do we need instructions with two outputs?
Author: Agner Date: 2016-03-31 01:55
An alternative to two register outputs for add-with-carry is to have extra carry bits in the register. We could, for example, have one extra flag bit for each 32 bits of vector registers. The extra bit can be used for carry, overflow, masks, etc. In this way we can implement add-with-carry with just two inputs and one output. Nobody needs add-with-carry for 8-bit and 16-bit integers. Maybe it is worth the cost of having 3% more bits of vector storage if we can avoid instructions with two outputs? But now we have a problem with saving the vector to memory. We will need two different save instructions - with and without the extra flag bits. This will cause complications in the compiler. And, do we need an extra bit on integer registers as well for the sake of orthogonality?

We can live without pop instructions. If we can handle the problems of an extra flag bit, then the only need for instructions with two outputs is read-and-increment-pointer. Without this, how do we save all registers in a task switch in an efficient and compact way? The extra bits will make it still more complicated to save registers for task switch. Any suggestions?

BTW, if we can accept extra bits in a vector register, we can also have extra bits for vector length. This will remove the input dependency on a vector-length register.

 
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