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From: preston@ariel.rice.edu (Preston Briggs)
Newsgroups: comp.arch
Subject: Re: Compilers, architecture, and efficiency
Message-ID: <1991Apr30.025904.13028@rice.edu>
Date: 30 Apr 91 02:59:04 GMT
References: <1991Apr28.154603.8003@rice.edu> <1991Apr29.155945.29907@rice.edu> <11588@mentor.cc.purdue.edu>
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hrubin@pop.stat.purdue.edu (Herman Rubin) writes:

>Is it the compiler writer's job to recognize combinations of expressions in
>a language which really represent an operation not contained in that
>language?

It's the compiler writer's job to write code that recognizes
combinations of expressions that are supported by machine instructions.
It's one of the significant tasks of instruction selection.

I don't know how to handle everything (e.g., recognizing a
routine that computes square root), but many of the combinations
you (and others) give as examples _are_ easily handled.

>What is needed for the user to be able to communicate what operations are
>wanted.

I agree that the user should be able to instruction the computer.
However, I don't think I want to specify the assembly operations.
Instead, I want to manipulate numbers, sets, sequences, and objects in a
somewhat high-level fashion.  Then I want the compiler to make very good
code out of my specification.

>> If you change the machine, you need to change compilers.
> 
>This newsgroup is comp.arch.  How are the computer architects to recognize
>these situations and others?  

I expect they learn from reading and by experience.
It seems that one of the major accomplishments of the RISC evolution
was a recognition that architects and compiler people should talk
more often than in the past.

>What you have shown is that a compiler can, in effect, extend a language
>by looking for certain code patterns.

I disagree.  The language (Fortran 77) didn't change when I added my
optimization.  The compiler translates exactly the same set of
programs; some programs simply ran slightly faster.

>  I fail to see that this procedure,
>which will always be limited by the information given to the compiler
>writers, and which can also sometimes lead to difficulties, is better
>than allowing the user to extend the language and pass the information
>to the compiler in such a way that it can use more of the 
>machine capabilities.

The difference is that we (computer science community in general)
know how to do optimizing compilers.  You pose a much harder problem.

Preston Briggs