From: Gabriel Ravier <gabravier@gmail.com>
To: Segher Boessenkool <segher@kernel.crashing.org>,
David Brown <david.brown@hesbynett.no>
Cc: gcc-help@gcc.gnu.org
Subject: Re: pragma GCC optimize prevents inlining
Date: Sun, 7 Jan 2024 18:36:07 +0000 [thread overview]
Message-ID: <94e4b171-17e5-45e4-9222-14dfe1e88540@gmail.com> (raw)
In-Reply-To: <20240107175144.GF19790@gate.crashing.org>
On 1/7/24 17:51, Segher Boessenkool wrote:
> On Sat, Jan 06, 2024 at 06:02:45PM +0100, David Brown wrote:
>> On 05/01/2024 19:19, Segher Boessenkool wrote:
>>> That's not the point. A program can be perfectly fine, with bounded
>>> errors and all, and then -ffast-math will typically completely destroy
>>> all that, and replace all arithmetic by the equivalent of a dice roll.
>> The only difference between IEEE calculations and -ffast-math
>> calculations is that with IEEE, the ordering and rounding is controlled
>> and consistent.
> No, that is not the only difference.
>
> '-ffast-math'
> Sets the options '-fno-math-errno', '-funsafe-math-optimizations',
> '-ffinite-math-only', '-fno-rounding-math', '-fno-signaling-nans',
> '-fcx-limited-range' and '-fexcess-precision=fast'.
>
> Many of those do much more than what you say, can result in the compiler
> generating completely different code.
>
>> For any given /single/ arithmetic operation that is
>> performed, each can have the same amount of rounding error or error due
>> to the limited length of the mantissa. Agreed?
> I don't understand what you mean to say even.
>
>>>> The rounding errors in -ffast-math will be very similar to those in IEEE
>>>> mode, for normal numbers.
>>> No, not at all. Look at what -fassociative-math does, for example.
>>> This can **and does** cause the loss of **all** bits of precision in
>>> certain programs. This is not theoretical. This is real.
>> a = 1e120;
>> b = 2;
>>
>> x = (a + b) - a;
>>
>> IEEE rules will give "x" equal to 1e120 - mathematically /completely/
>> wrong. -ffast-math will give "x" equal to 2, which is mathematically
>> precisely correct.
> The IEEE result is 0. Which is the **exactly correct** result. This is
> a computer program, not some formulas that you can manipulate at will.
That seems to be where the disagreement lies. Those that use -ffast-math
with full knowledge of what it does are presumably acting with the
intent that their program should indeed be treated as "some formulas you
can manipulate at will".
>
>>> The -ffast-math flag can only reasonably be used with programs that did
>>> not want any specific results anyway. It would be even faster (and just
>>> as correct!) to always return 0.
>> That is simply wrong.
> It is an exaggeration for dramatic effect, but it is fundamentally
> correct.
>
>
> Segher
next prev parent reply other threads:[~2024-01-07 18:36 UTC|newest]
Thread overview: 17+ messages / expand[flat|nested] mbox.gz Atom feed top
2024-01-04 9:01 Hashan Gayasri
2024-01-04 9:27 ` LIU Hao
2024-01-05 0:56 ` Hashan Gayasri
2024-01-04 14:51 ` David Brown
2024-01-04 15:03 ` Segher Boessenkool
2024-01-04 15:24 ` David Brown
2024-01-04 16:37 ` Richard Earnshaw
2024-01-09 13:38 ` Florian Weimer
2024-01-04 16:55 ` Segher Boessenkool
2024-01-05 14:24 ` David Brown
2024-01-05 15:00 ` Segher Boessenkool
2024-01-05 15:53 ` David Brown
2024-01-05 18:19 ` Segher Boessenkool
2024-01-06 17:02 ` David Brown
2024-01-07 17:51 ` Segher Boessenkool
2024-01-07 18:36 ` Gabriel Ravier [this message]
2024-01-08 15:53 ` David Brown
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