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From: addison@pollux.usc.edu (Richard Addison)
Newsgroups: comp.sys.amiga.tech
Subject: Re: Another Manx v3.6 bug
Summary: Stupid sign extensions
Keywords: code generation,C,stupid,corn flakes
Message-ID: <17762@usc.edu>
Date: 11 Jun 89 23:15:13 GMT
References: <14328@watdragon.waterloo.edu> <14395@watdragon.waterloo.edu>
Sender: news@usc.edu
Reply-To: addison@pollux.usc.edu (Richard Addison)
Organization: University of Southern California, Los Angeles, CA
Lines: 24

In article <14395@watdragon.waterloo.edu> ehoogerbeets@rose.waterloo.edu (Edwin Hoogerbeets) writes:
>In article <14328@watdragon.waterloo.edu> ehoogerbeets@rose.waterloo.edu (Edwin Hoogerbeets) writes:
>% I know it is pretty gross
>% C code, but it occurs in GnuGrep (fish 204), and probably other Gnu
>% code.
> ...
>	ext.w	d0              ; sign extend it to a word  [?????]
> ...
>	ext.w	d0              ; sign extend it to a word  [?????]
>	ext.l	d0              ; sign extend it to a long  [2*?????]

This brings up something that I've always found horribly stupid.  I've seen
C compilers use ext.w (followed by ext.l if ints are long) to test to see if
a char is equal to 0, or to compare two chars, or to add, subtract two chars
when the result will end up in a char.  These are very clearly instances of
unnecessary promotion:  the additional high-order bits will never effect the
lower-order bits.

Caveat:  I haven't been looking as closely at the code generation of the most
current versions of Manx and Lattice because I have the source level debuggers.
I don't know if they are still as weird about this unnecesarry code generation.

Richard Addison
"Jim:  Life is not a dream." -- Spock, Star Trek V:  The Final (?) Frontier