[Cfp-interest 2126] Re: CFP question with FLT_EVAL_METHOD

[Rajan Bhakta]

Hi Aaron,

I am forwarding this question on to the CFP group (cc'd on the list), but I
can also give you my opinion of the intent right now as well.

The FLT_EVAL_METHOD is intended to be a once per TU setting and it was
expected that compile time flags or switches would change the value. In
practice, we haven't seen an implementation really do that. Furthermore, in
actual practice what we've heard is that a lot of implementations set it
wrong (and would possibly be better served by the macro expanding to the
value of -1).

That being said, I don't think there is any real wording preventing a
modification to the value via another pragma, just as there is not for many
other macros like FLT_RADIX, and even non floating point related ones we
traditionally consider TU level constants such as CHAR_BIT, INT_MAX, etc.
i.e. Nothing special about FLT_EVAL_METHOD in this way.

Regards,

Rajan Bhakta
z/OS XL C/C++ Compiler Technical Architect
ISO C Standards Representative (Canada, USA), PL22.11 Chair
C/C++ Compiler Development
rbhakta at us.ibm.com

IBM



From:	"Aaron Ballman" <aaron at aaronballman.com>
To:	"Rajan Bhakta" <rbhakta at us.ibm.com>
Date:	08/30/2021 01:57 PM
Subject:	[EXTERNAL] CFP question with FLT_EVAL_METHOD



I ran into an implementation question with FLT_EVAL_METHOD and I was
wondering if I could pick your considerable brains on what you think
the right answer is. Imagine code like:

#include <stdio.h>
#include <float.h>

#ifdef ON
#pragma float_control(source, on, push)
#endif

int main() {
  printf("FLT_EVAL_METHOD = %d\n", FLT_EVAL_METHOD);
}

Would you expect the FLT_EVAL_METHOD macro to expand to a different
value depending on whether ON is defined or not? We're trying to
figure out whether the standard requires FLT_EVAL_METHOD to report
back transitional values that may be under control of a #pragma, or
whether we have the latitude to decide to only report one value for
FLT_EVAL_METHOD for the entire TU.

Thanks!

~Aaron


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