Re: left-to-right (was In-Out Parameters for functions)

["Robert I. Eachus" <rieachus@comcast.net>]

Stephen Leake wrote:

> But you haven't answered my question. Is there, in actual fact, any
> compiler that actually does these optimizations?

Sure, lots of them, but that is not really the interesting question. 
The interesting experiment is to find a way to turn off just the 
optimizations that take advantage of the reordering freedoms, then see 
what effect it has on the performance of the compiled code on some set 
of benchmarks.

It might take hacking source to be sure of doing exactly that in GNAT, 
but you can probably bound the benefit using current optimization switches.

Of course, the real interesting part of the project would be to run the 
suite on different hardware architectures and see how much of an effect 
it has on modern CPU architectures compared to say a Pentium or 80486 
without superscalar and OoO support in the CPU.

It would probably make a good senior project for some undergraduate, and 
if the results showed that the effect of such optimizations on 
superscalar out-of-order CPUs it might make a Master's or PhD thesis. 
In any case it should result in a couple of papers.

Why do I suggest this as an academic project?  Well years go the same 
sort of thing was done by hardware and language architects both in 
academia and in industry.  The conclusion AT THAT TIME, was that the 
freedom to reorder, and the compiler code to take advantage of it, was 
very worthwhile.  With OoO superscaler machines, it is not clear that 
there would be any advantage to requiring "more predictable" semantics 
in programming languages given what happens during execution.  But it 
could be that the "optimization freedoms" don't buy much in the way of 
performance anymore.

-- 
                                           Robert I. Eachus


"The only thing necessary for the triumph of evil is for good men to do 
nothing." --Edmund Burke