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From: chased@rbbb.Eng.Sun.COM (David Chase)
Newsgroups: comp.lang.misc
Subject: Re: Re: Aggressive optimization
Summary: Define, please "low level language", and why that should inhibit optimization
Message-ID: <1849@exodus.Eng.Sun.COM>
Date: 30 Oct 90 21:22:38 GMT
References: <2301@wn1.sci.kun.nl> <8960018@hpfcso.HP.COM> <1990Oct30.105241.9733@tkou02.enet.dec.com>
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diamond@tkou02.enet.dec.com (diamond@tkovoa) writes:
>C was invented in order to be that better assembler.  If you want machine
>level, you're supposed to program in C.  That is exactly the purpose for
>which C was created.  If you want an HLL, use one; don't cripple C.

This may be true for (one dialect of) pre-ANSI C, but the
specification of ANSI C allows (if you don't say "volatile") a large
number of entertaining optimizing transformations, despite assertions
by you and other posters here that C is a "low level language" and
"low level languages ought not be aggressively optimized".  C *is* a
low level language, but the optimizations are 100% legal, often tend
to make the code go faster, and thus they are applied.  The
compiler-writer's claim is "I can prove that these optimizations
conform to the standard"; can you *prove* that they should not be
done?  How will it sell more computers/compilers to NOT do the
transformations?  (Lest you forget, dollars and cents is what it all
comes to in the end.)

Two things to note:

1) several optimizations could be applied much more frequently to C if
it weren't for possible aliasing in procedure pointer/array
parameters.  If you don't understand what I'm talking about, go read a
book on dependence analysis -- Wolfe's _Optimizing Supercompilers ..._
will give you a feel for what can be done to Fortran.

2) it's senseless to argue with the optimizer authors; they'll do
everything that time, the standard, and the customers permit.  If ANSI
has changed Your Favorite Language and you had a substantial
investment in software that was harmed by this, take it up with ANSI.
You might look into the history of ANSI Cobol.

David Chase
Sun Microsystems, Inc.