Re: Sythetic registers: modrm/gas question.

[dewar at gnat dot com (Robert Dewar)]

> Synthregs improve locality at a slight cost in code size.  I agree
> with ja_walker that the worthiness of that trade-off is an emprical
> question worth measuring.  I'd go beyond him by saying that in the 
> medium term future, the trade-off almost certainly wins often.

I disagree. In practice references to the local stack frame are nearly
always in cache. So there is just nothing much to improve. Actually it
is instruction cache that causes problems more often than data cache,
and making code larger is almost always a loss (that's way for example
-O3 frequently slows things down compared with -O2, and in the case of
Ada, where it is more normal to explicitly control inlining, -O3 is
almost always worse).

> x86 is a "rapidly" exploding range of physical architectures.

True, but the basic observation above holds for all of them, from the
486 onwards.

> "Yeah, well, until you've done the same you've no
> business talking about the SR proposal"

Gosh, you took the words out of my mouth :-) :-)

Seriously, I do think this needs to be discussed at a detailed level, which
is why I asked for an example. If we look at a specific example, then we
can fill in any details that are needed for the example, and not rely on
everyone having detailed knowledge of the architecture.

By the way, to repeat an idea that I think is definitely worth following up
on, register renaming plays a very important part in these architectures.
The Pentium-4 has something like 40 registers (can't remember exact number
and can't be bothered to look it up :-) Only 8 of these are directly 
nameable, but in a real program, many of these can get used.

I think it would be really interesting to study the issue of taking the
renaming into account when allocating registers. Consider

   mov ax, mem1
   add ax, 1
   mov mem1, ax
   mov ax, mem2
   add ax, 2
   mov mem2, ax

Classical optimization suggests

   mov ax, mem1
   mov bx, mem2
   add ax, 1
   add bx, 1
   mov mem1, ax
   mov mem2, bx

so that the two operations can be done in parallel in separate pipelines,
but in fact the first sequence is better, since register renaming will
allow the use of two separate pipelines, and you don't use up another
nameable register, which are what are in very short supply. 

The above is just an illustrative example of the kind of thing I am
talking about here. The devil is in the details (as with the SR proposal
itself), and you really have to know EXACTLY how the renaming works to
make sure you generate code that cooperates with it.

I am trying to interest a PhD student to do research in this area :-)

There is some work done, but not that much.