Agner`s CPU blog

Since it's a program's exclusive virtual memory space,
a universe of our own, why can't we use arbitrary and
much, much smaller addresses?

My priority is the performance of big systems. That's
why I have 64-bit address space. All addresses are
relative to some pointer (instruction pointer, data
section pointer, stack pointer, or an arbitrary
pointer) with a signed offset of 8 bits or 32 bits.
The instruction size will not be reduced by having a
smaller address space, but of course we could save
some stack space by having a 32-bit mode. I don't like
having two different modes, though. Then we would have
problems with stack alignment for doubles, etc. Byte
code languages can have their own smaller address
space of course.

I just don't like all the waste with 64-bit types and pointers. It becomes less of an issue over time, but it still bothers me. Relatedly, I like what these Berkeley people did with Precimonious, a tool that tunes floating point precision and eliminates some of the waste: www.eecs.berkeley.edu/~rubio/includes/sc13.pdf

Question: Does an ISA really need to specify the number of architectural registers? What would the implications be of not doing so, and having an infinite number of architectural registers like LLVM's IR? It seems like the number of registers is a fiction anyway (I was stunned to discover that x86-64 processors from Intel and AMD have nearly 200 physical registers.) This would make the number of registers implementation-dependent, rather than part of the ISA specification. See Vikram Adve's recent talk at Microsoft: research.microsoft.com/apps/video/default.aspx?id=249344 (The Microsoft Research people must have been having a bad day or something – their questions and comments reveal that they thoroughly misunderstood his ideas.)

Question 2: Let's assume that we have registers R0 - R31. Might it be useful to also have an unspecified register – call it Rx – that basically tells the CPU "give me whatever register you have – I don't care which one". I can imagine some scenarios where this might be useful for a compiler. And it seems to fit with the reality of register renaming anyway.