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Re: changing "configure" to default to "gcc -g -O2 -fwrapv ..."

From: Vincent Lefevre
Subject: Re: changing "configure" to default to "gcc -g -O2 -fwrapv ..."
Date: Sun, 31 Dec 2006 15:48:21 +0100
User-agent: Mutt/1.5.13-vl-r14963 (2006-12-20)

On 2006-12-31 09:08:21 -0500, Richard Kenner wrote:
> > In fact the wrap around range test is a standard idiom for "hand
> > optimization" of range tests.
> It's also one that GCC uses internally, but you do it in *unsigned*
> to avoid the undefined overflow.

If done in unsigned, this won't lead to any optimization, as unsigned
arithmetic doesn't have overflows. So, if you write "a - 10" where a
is unsigned, the compiler can't assume anything, whereas is a is
signed, the compiler can assume that a >= INT_MIN + 10, reducing
the range for a, and possibly allowing some optimizations.

Vincent Lefèvre <address@hidden> - Web: <>
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Work: CR INRIA - computer arithmetic / Arenaire project (LIP, ENS-Lyon)

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