Re: What is the meaning of an expression?

Christopher F Clark <christopher.f.clark@compiler-resources.com> schrieb:


> The second one was from my own experience working on the Alpha
> optimizer at DEC. The C++ compiler was one of the last pieces of
> software to use the one based upon Fred Chow's work. That was my
> responsibility at the time (the optimizer, not the C++ compiler). In
> any case, the front end writers were very aggressive in inlining
> subroutines and doing whole-program optimization by that trick.


When to inline and when not to inline is still a major issue today.


> The result was very massive files of the intermediate representation. The
> result, for certain large (and important) C++ programs the compiler
> would work for days before it had filled up all the paging space on
> the engineering cluster at DEC, which was multiple disks of paging and
> as a result crashed not only the compiler, but in some cases the
> entire cluster. This made for some very unhappy users, because they
> had waited days and they still didn't get a successful compilation.


https://xkcd.com/303/ and https://dilbert.com/strip/1998-06-04
come to mind :-)




> Fortunately, a small amount of analysis on my part allowed me to
> realize that most of the optimizers data structures while N-squared in
> size, were actually filled with mostly zero values.


Quadratic algorithms (if in space or in time) are a progressively
bad idea. Programs are getting bigger, and anybody who thinkis
a quadratic algrorithm is OK, will be hit sooner or later with a
(real-life) test case which brings it out into the open.


Just a random example: Some time ago, a test case for gcc was
submitted which used up hours and Gigabytes for compiliation.
It consisted of a single basic block in a subroutine, with thousands
of variables and thousands of assignments (getting translated into
ten thousands of SSA statements).


This code was not written by a human, but by a computer algrebra
system expanding some complicated formula.

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