RE: [PATCH] Remove __isinf uses that rely on signed return value

[Joseph Myers <joseph at codesourcery dot com>]

On Wed, 3 Jun 2015, Wilco Dijkstra wrote:

> > Joseph Myers wrote:
> > On Tue, 2 Jun 2015, Wilco Dijkstra wrote:
> > 
> > > The printf code contains a few uses of __isinf where the sign is used -
> > > replace these with separate isinf and signbit.
> > 
> > Why?  Isn't the point of glibc's isinf returning a signed value that you
> > can do such combined operations?
> 
> We discussed this a while back (and I thought it was agreed on going forward 
> like this) - to recap, it is not required by any standard (so no standard

I don't recall any such discussion or agreement.  The discussion in bug 
15367 is on the basis of using __builtin_isinf_sign to implement the 
header macro, not of changing the requirements and sometimes using 
__builtin_isinf.

> compliant software may rely on this GLIBC specific behaviour), and is
> actually incompatible with C++ which requires a boolean to be returned. 

C++ is the responsibility of libstdc++ (and defines overloaded functions, 
not type-generic macros; libstdc++ needs to override things done in the C 
header anyway).

> So the idea is to only support the old behaviour in GNUC mode for backwards 
> compatibility, but not in C99/C++.

glibc is built with _GNU_SOURCE and can freely use all GNU features 
(subject to link-time namespace issues).  I see no need to change the 
interface to this macro in glibc.  Presumably the aim should be to make 
the GCC __builtin_isinf / __builtin_isinf_sign at least as efficient as 
the out-of-line functions, and safe for all inputs, make the isinf macro 
use __builtin_isinf_sign, and make GCC optimize __builtin_isinf_sign calls 
so it's as efficient as __builtin_isinf if the sign isn't used, and as 
efficient as __builtin_isinf + __builtin_signbit if the sign is used.

-- 
Joseph S. Myers
joseph@codesourcery.com