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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 17:07:51 +0100 |
User-agent: |
Mutt/1.5.13-vl-r14963 (2006-12-20) |
On 2006-12-31 10:19:59 -0500, Richard Kenner wrote:
> > 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.
>
> I don't follow. The purpose of this hand-optimization (whch is also done
> by GCC) is to replace two comparisons with one subtraction and one
> comparison, which is faster on most machines. It's not CORRECT to do this
> transformation in signed unless you know there won't be overflow.
OK, I was assuming that the code were correct in signed arithmetic
(meaning no overflow) and that the "a - 10" were done for another
purpose (see my example in another message).
--
Vincent Lefèvre <address@hidden> - Web: <http://www.vinc17.org/>
100% accessible validated (X)HTML - Blog: <http://www.vinc17.org/blog/>
Work: CR INRIA - computer arithmetic / Arenaire project (LIP, ENS-Lyon)
- Re: changing "configure" to default to "gcc -g -O2 -fwrapv ...", (continued)
- Re: changing "configure" to default to "gcc -g -O2 -fwrapv ...", Richard Kenner, 2006/12/31
- Re: changing "configure" to default to "gcc -g -O2 -fwrapv ...", Vincent Lefevre, 2006/12/31
- Re: changing "configure" to default to "gcc -g -O2 -fwrapv ...", Robert Dewar, 2006/12/31
- Re: changing "configure" to default to "gcc -g -O2 -fwrapv ...", Vincent Lefevre, 2006/12/31
- Re: changing "configure" to default to "gcc -g -O2 -fwrapv ...", Robert Dewar, 2006/12/31
- Re: changing "configure" to default to "gcc -g -O2 -fwrapv ...", Vincent Lefevre, 2006/12/31
- Re: changing "configure" to default to "gcc -g -O2 -fwrapv ...", Gerald Pfeifer, 2006/12/31
- Re: changing "configure" to default to "gcc -g -O2 -fwrapv ...", Robert Dewar, 2006/12/31
- Re: changing "configure" to default to "gcc -g -O2 -fwrapv ...", Gabriel Dos Reis, 2006/12/31
- Re: changing "configure" to default to "gcc -g -O2 -fwrapv ...", Richard Kenner, 2006/12/31
- Re: changing "configure" to default to "gcc -g -O2 -fwrapv ...",
Vincent Lefevre <=
- Re: changing "configure" to default to "gcc -g -O2 -fwrapv ...", Duncan Sands, 2006/12/31
- Re: changing "configure" to default to "gcc -g -O2 -fwrapv ...", Richard Kenner, 2006/12/31
- Re: changing "configure" to default to "gcc -g -O2 -fwrapv ...", Robert Dewar, 2006/12/31
- Re: changing "configure" to default to "gcc -g -O2 -fwrapv ...", Vincent Lefevre, 2006/12/31
- Re: changing "configure" to default to "gcc -g -O2 -fwrapv ...", Robert Dewar, 2006/12/31
- Re: changing "configure" to default to "gcc -g -O2 -fwrapv ...", Vincent Lefevre, 2006/12/31
- Re: changing "configure" to default to "gcc -g -O2 -fwrapv ...", Richard Kenner, 2006/12/31
- Re: changing "configure" to default to "gcc -g -O2 -fwrapv ...", Michael Veksler, 2006/12/31
- Re: changing "configure" to default to "gcc -g -O2 -fwrapv ...", Richard Kenner, 2006/12/31
- Re: changing "configure" to default to "gcc -g -O2 -fwrapv ...", Michael Veksler, 2006/12/31