[cfe-dev] library builtins and exceptions

[Hal Finkel]

----- Original Message -----
> 
> On 06.09.2013, at 11:19, Hal Finkel <hfinkel at anl.gov> wrote:
> 
> > Ben, Chad, et al.,
> > 
> > Looking at the list of libm library builtins in Builtins.def, only
> > cimag and creal are marked with 'n' (nothrow). Is this because the
> > other functions can issue floating-point exceptions? It seems that
> > this prevents the functions from being marked as 'nounwind' in the
> > IR (but only in C++ mode). A couple of questions:
> > 
> > 1. Is this really necessary? First, if it is necessary, then it
> > seems wrong for C (because we only don't emit nounwind for C++
> > codes). Also, if the function is already marked as 'readonly', is
> > that not sufficient? A 'readonly' function can still trap (by
> > dereferencing a null pointer, for example).
> > 
> > 2. If I'm right about the 'n' -> floating-point exception
> > relationship, then it should be turned off when -fno-trapping-math
> > is provided (but its not). If the answer to question 1 is, 'yes,
> > it is needed", then is the best way to implement this to do the
> > same thing here as is done for the 'e' (const when -fmath-errno=0)
> > flag?
> 
> Floating point exceptions do not unwind the stack, so marking all
> floating point functions as nounwind should be safe. This was
> probably just an oversight, note that "const" implies nounwind so
> most of the functions were getting the attribute already.

Thanks! I'll post a patch for review soon that should clean this up.

 -Hal

> 
> - Ben
> 
> 

-- 
Hal Finkel
Assistant Computational Scientist
Leadership Computing Facility
Argonne National Laboratory