From: me22 <me22.ca@gmail.com>
To: NightStrike <nightstrike@gmail.com>
Cc: gcc-help@gcc.gnu.org
Subject: Re: Floating point optimizations
Date: Sat, 05 Apr 2008 21:26:00 -0000 [thread overview]
Message-ID: <fa28b9250804051204u14c309bbpdae44db02a481d23@mail.gmail.com> (raw)
In-Reply-To: <b609cb3b0804050606q2035bbb5ye18b2c16ffaed9c9@mail.gmail.com>
On Sat, Apr 5, 2008 at 9:06 AM, NightStrike <nightstrike@gmail.com> wrote:
> Wait, so are things like "c - (c - a)" optimized down to "a" or not? I
> use a lot of that in very time-intensive c++ code.
>
I think only if you tell it to be loose with requirements for applying
floating-point math optimizations (-ffast-math and related), since
DBL_MAX - (DBL_MAX-1) is, to the best of my knowledge, 0 --- not 1 ---
in IEEE math.
On the other hand, I managed to get >1 by adding DBL_EPSILON/10 to 1.0
repeatedly (though only on 1 of 4 computers on which I tried it) so my
intuitions could all be wrong.
prev parent reply other threads:[~2008-04-05 19:05 UTC|newest]
Thread overview: 10+ messages / expand[flat|nested] mbox.gz Atom feed top
2008-04-03 17:00 Christian Keil
2008-04-03 17:27 ` John Fine
2008-04-04 12:04 ` Christian Keil
2008-04-03 18:24 ` Alexander Monakov
2008-04-04 15:48 ` Christian Keil
2008-04-05 1:15 ` Tim Prince
2008-04-05 13:07 ` Andrew Haley
2008-04-05 13:32 ` NightStrike
2008-04-05 19:05 ` Tim Prince
2008-04-05 21:26 ` me22 [this message]
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