Re: Interpreters and caller-saved registers

Thomas Koenig <tkoenig@netcologne.de> writes:
>It is well known that compilers in general and gcc specfically often
>generate superflous register moves
...
>However, if this got worse between releases, this is a regression.
>Those get higher priority for fixing. So, if it is reasonable
>to generate a reduced test case (for which cvise, for example,
>is an excellent tool) so filing a bug report would be a good thing.


If you want to file such a bug report, I can give you the commit of
Gforth before we added all the workarounds, where all the problems
are visible without ado.


>This sort of code is an example of the contradictions in today's
>compiler technology. On the one hand, they do amazing optimizations
>on large amounts of code which no programmer could hope to reach
>while staying productive.


There are certainly cases where compilers loop-unroll (plus
ramp-up/down and compensation code) code into huge swaths of code that
makes it hard to see where the action is going on, but amazing?


>So far, nobody has found an algorithm for "just remove the
>silliness" from compiled programs. Maybe it would be feasible to
>run some peephole optimization as last passes which could improve
>code like the one above, but that might also be difficult in the
>more general case where registers are reused in other basic blocks
>(which would mean just to redo the register allocation).


Yes, I doubt that it can be solved with peephole optimization, or it
would have been done already.


- anton
--
M. Anton Ertl
anton@mips.complang.tuwien.ac.at
http://www.complang.tuwien.ac.at/anton/

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