[Cfp-interest] draft of syntax discussion for C committee

[Rajan Bhakta]

Not to derail the discussion, but Fred's point about interaction with the 
existing C support for signal handlers leads to the thoughts of the 
existing C support for (floating point) exceptions in 7.6.2.

I can't recall (nor can I find in my notes) any discussion about extending 
what is already in the C standard for floating point exceptions.

Although arguably not as convenient as what we are discussing right now, 
can we not add in new macros as we are already discussing for the floating 
point exceptions, and use the existing feclearexcept, fegetexceptflag, 
feraiseexcept, fesetexceptflag, and fetestexcept functions? 

The example code given in the standard shows how to do pretty much 
everything we are discussing, though more verbosely.

The advantages are that it is building on something that is already in the 
C standard and hence more easily accepted by both the C committee, 
implementers and users due to its familiarity, and avoids "ugly" pragmas. 
Also, the signal handling issue falls under the current C standard model 
so we do not need to specify anything extra here (whereas we would for the 
try-catch/pragma style model).

The disadvantages are that it is not as "clean" as the C++ style exception 
handling, it may make it harder for optimizing compilers to recognize the 
exception pattern (though I would argue it is actually easier on the 
compiler and hardware with the current C standard way), and in a 
non-optimizing case, may be much slower on some hardware.

Now there may be the issue of lack of bits (given exceptions are stored in 
int's from the interface) on some implementations, but that can be worked 
around (ex. version check, or even a new set of functions created for 
these new sub-exceptions).

Regards,

Rajan Bhakta
z/OS XL C/C++ Compiler Technical Architect
ISO C Standards Representative for Canada
C Compiler Development
Contact: rbhakta at us.ibm.com, Rajan Bhakta/Houston/IBM



From:   "Fred J. Tydeman" <tydeman at tybor.com>
To:     "cfp-interest at ucbtest.org" <cfp-interest at ucbtest.org>, 
Date:   06/06/2014 10:37 PM
Subject:        Re: [Cfp-interest] draft of syntax discussion for C 
committee
Sent by:        cfp-interest-bounces at oakapple.net



On Fri, 6 Jun 2014 16:25:15 -0700 (PDT) David Hough CFP wrote:
>
> #pragma STDC catch_fe fe_exception1 label1
> #pragma STDC catch_fe fe_exception2 label2

What is the interaction between this idea and
a signal handler for SIGFPE?


---
Fred J. Tydeman        Tydeman Consulting
tydeman at tybor.com      Testing, numerics, programming
+1 (775) 287-5904      Vice-chair of PL22.11 (ANSI "C")
Sample C99+FPCE tests: http://www.tybor.com
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