Re: Questions about the ATTRS field of builtins.def

["Kaveh R. Ghazi" <ghazi at caip dot rutgers dot edu>]

 > From: Roger Sayle <roger@eyesopen.com>
 > 
 > Hi Kaveh,
 > > I'm trying to make sense of the ATTRS field in builtins.def.  For the
 > > math functions, I see several variants:
 > >
 > > 1.  ATTR_CONST_NOTHROW_LIST
 > >
 > > 2.  flag_unsafe_math_optimizations ? ATTR_CONST_NOTHROW_LIST
 > >  				: ATTR_PURE_NOTHROW_LIST
 > >
 > > 3.  flag_errno_math ? ATTR_NOTHROW_LIST
 > >                                 : (flag_unsafe_math_optimizations
 > >                                    ? ATTR_CONST_NOTHROW_LIST
 > >                                    : ATTR_PURE_NOTHROW_LIST)
 > >
 > > I understand 1 and I understand 3's errno check.  What I don't
 > > understand is number 2, or why 3 contains 2 in one of its clauses.
 > > When is 2 necessary and why?
 > 
 > I've been using the 2 form to model the effects of changing the
 > rounding modes or other FPU control bits between calls.  Hence,
 > if -ffast-math, two calls to cos with the same argument are
 > considered equivalent independent of mysterious function calls
 > between them.  When being "safe", however, we only consider cos
 > to be const if there can't be a call to _setfpucw between them,
 > treated as a mysterious write to memory,  i.e. cos(x)*cos(x) is
 > always safe.  You can see how FPU state can be emulated by a
 > hidden global variable, using the "pure" attribute.

Ah, ok got it, thanks.  But it seems a bit obscure, IMHO this fact
could sure use a comment somewhere in the code.  I'm going to try
defining macros for the different ATTR cases above to make the
builtins.def easier to read.  Maybe these macros are a good place to
add some verbiage explaining this.  (Care to help name the macros? :-)


 > > I also see many discrepancies.  E.g. "cos" uses 2, but "round" and
 > > others use case 1.  What makes "cos" need case 2?  What's odd is that
 > > the man page on x86-linux says "round" may set errno to EDOM so
 > > shouldn't it (and perhaps others) use number 3?
 > 
 > Yes, anthing that sets errno should use version 3.  If FP rounding
 > isn't a factor, there could even be an argument for a fourth version
 > which is flag_errno_math ? NOTHROW : CONST_NOTHROW (i.e. a hybrid of
 > of 1 and 3 rather than 2 and 3).

Yes, I'd guess that the place where this fourth ATTR case would be
appropriate is for rounding functions that ignore the mode and always
round a particular way. (?)  Anywhere else?


 > I hope this explains my understanding of the distinctions.

Yes, thank you.

 > Thanks for the renaming of LONG_DOUBLE, btw.
 > Roger

You're welcome!

		--Kaveh
--
Kaveh R. Ghazi			ghazi@caip.rutgers.edu