Re: C Compilers which use full 486 capabilities

preston@ariel.rice.edu (Preston Briggs)
Fri, 15 Mar 91 17:35:51 GMT

          From comp.compilers

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C Compilers which use full 486 capabilities mcarrick@gara.une.oz.au (Mick Carrick) (1991-03-13)
Re: C Compilers which use full 486 capabilities mason+@transarc.com (1991-03-15)
Re: C Compilers which use full 486 capabilities preston@ariel.rice.edu (1991-03-15)
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Newsgroups: comp.compilers
From: preston@ariel.rice.edu (Preston Briggs)
Keywords: 486, optimize
Organization: Rice University, Houston
References: <9103130754.AA19293@gara.une.oz.au> <wbsAAQr0BwwMR4Tu8E@transarc.com>
Date: Fri, 15 Mar 91 17:35:51 GMT

mason+@transarc.com (Tony Mason) writes:
>[quoting from Dr. Dobbs]
>"The 486 represents a fundamental break with 8088-style optimization.
>Virtually all the old rules fail on the 486, where, incredibly, a move
>to or from memory often takes just one cycle, but exchanging two
>registers takes three cycles."


Does anyone's compiler generate an exchange instruction? I can visualize
uses, but I'm not sure how I'd recognize it. I can imagine peephole
optimizers that could catch it, but generally, I dislike instructions with
more than 1 result.


How would it integrate with register allocation?


Preston Briggs
[As far as I can tell, most actual XCHG instructions are really test-and-sets
that set locks between processes or processors. -John]
--


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