Agner`s CPU blog

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Do we need instructions with two outputs?
Author: Agner Date: 2016-04-01 03:49
Thank you for your comments. It is nice to have sparring partners to discuss with. We are really getting somewhere.

I think there are many advantages to storing the vector length in the vector register itself. This makes it much cheaper to save callee-save registers: You only have to save the part of the register that is actually used. I plan to make special instructions for save/restore. These instructions will save or restore the length and as much of the data as is actually used. The save instruction can increment a pointer by the actual amount of space used. The restore instruction will have to be followed by an extra instruction to increment or decrement a pointer by the amount of space used. The same instruction can be used to adjust the stack pointer before saving. This will save a lot of data cache. If a very long vector register actually contains only a floating point scalar then we need not save any more than that. If the vector register is unused, which will probably happen more than 50% of the time, then we only need to save the length, which is zero. Also, we don't need a complicated lazy save mechanism for task switches. And we get one less input dependence because we don't need a separate register for vector length.

The disadvantage is, of course, that the compiler needs to distinguish between saving the data of a vector and saving everything for a later complete restore. Once we have this distinction, however, there is little extra cost to also saving a few carry flags. You want to keep the carry bits separate from the control flags. I agree. My idea is to use the carry bits for overflow detection (so that we can avoid interrupts in vector instructions), and use a flags register, as already present in my initial proposal, for masking and for a lot of option bits that we don't have room for in the instruction code. A few of these option bits will determine the use of the carry flag, i.e. whether it is unused or it detects signed or unsigned integer overflow, floating point overflow or invalid operations or both, and whether it is propagated from input vector operands to output operands. All of these features are only activated if the software actually has a try/catch clause to detect overflows. Otherwise, the compiler doesn't need to care. But you want to avoid instructions with two outputs, and this is the best solution to that problem I can think of :-) We can separate the carry/overflow bits from the control flags without generating any extra output dependence.

We can have an instruction to extract the carry bits to an integer bitfield. This will make it possible to do a fast carry-lookahead for a BIGNUM addition in a large vector register with just a few integer instructions.

I agree that integer registers should not have these extra bits.

The idea of having separate registers for system code only, sounds good. Would eight 64-bit registers be a suitable number?

I have fiddled a little with the instruction format. The coding gets simpler when we get rid of the vector length register and a few option bits. I think this makes it possible to make the instructions shorter so that we can avoid triple-size instructions. Of course, we will have to use two instructions for loading a 64-bit immediate value then, but I think we can live with that.

 
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