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Re: flag_complex_method = 2 in C++?


Hi Mark,

Paolo Carlini wrote:

Therefore, I'm not sure... I would certainly like to see method 2
becoming the default everywhere, on the other hand, I'm not sure if
method 2 isn't too slow as default for C89 (outside fast-math, I
repeat). Shall we carry out performance tests perhaps, or peak
performance outside fast-math aren't really an issue here? I don't know.


Performance tests would certainly be useful.


I totally agree. I would ask Richard to help here...

In case of doubt, I would still switch to method 2 as default together
with C++0x, in that case the reference C language is C99 and things are
very neat...


The problem is that this isn't how users think about it.

If a user does:

 gcc test.c
 g++ test.c

and gets different performance between the two, I think we're going to
have a "C++ is slow" bug report. In a certain sense, the user may be
right, in that C99/C++ mandate a slower approach than is used by GNU
C89. But, because GNU C89 is the default, we still have a problem.


I see your point. As you clearly see in your second sentence, I see things slightly differently, I would not be much on the side of users comparing the performance of C++0x to C89... But anyway this is just an academic issue, for now...

Since C99 has defined semantics for this, and C++ is following, it seems
reasonable to me to change C89 to always use these semantics.  Then,
some GNU C89 programs may go a bit slower, but we don't have weird
differences between the languages.  And, users can use -ffast-math to
get the performance back -- in all languages, uniformly.

... assuming we can enable method everywhere, that would be great.

Ah, one final remark wrt C++0x: note that currently, irrespective of the switches, there is *no way* for the user to choose method 2 in C++. I think this is very bad in C++0x mode: essentially, as regards this feature, it cannot track C99 as it *must*. I really hope we can enable method 2 one way or another...

Paolo.


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