[PATCH RFC] <math.h> issignaling

[Joseph S. Myers]

On Wed, 6 Mar 2013, Thomas Schwinge wrote:

> Currently I'm providing a __issignalingl symbol (aliased to
> __issignaling) if NO_LONG_DOUBLE is defined.  The <math.h> issignaling
> macro definition on the other hand adds long double support based on
> __NO_LONG_DOUBLE_MATH being defined.  I don't completely understand yet
> what exactly each of NO_LONG_DOUBLE, __NO_LONG_DOUBLE_MATH, and then
> __LONG_DOUBLE_MATH_OPTIONAL are to achieve, and which I have to care for;
> also whether I need to do anything with sysdeps/ieee754/ldbl-opt and
> sysdeps/ieee754/ldbl-64-128 (don't think so)?

I don't think an __issignalingl alias should be provided.  If double and 
long double have the same representation, the header macro should never 
call __issignalingl.  By not providing it, you avoid needing special 
symbol versioning if any architectures move in future to having distinct 
long double.

> On 32-bit x86, the __issignalingf and __issignaling functions already
> "fail" at the call site:
> 
>      804a130:       d9 05 18 e1 04 08       flds   0x804e118
>      804a136:       d9 1c 24                fstps  (%esp)
>      804a139:       e8 d2 eb ff ff          call   8048d10 <__issignalingf@plt>

Does this depend on compiler options (you don't say what specific code 
shows this problem)?  A test I did of passing a float variable to a 
function used integer load/store to move it to the stack, not 
floating-point.

> For float and double (but not long double), per the x86 ISA, this will
> load the sNaN variable (0x804e118), but then turn it into a qNaN and
> raise the INVALID exception.  (Thus, the __issignalingf function will
> never be able to detect a sNaN.)  I hope to get around this by adding
> always_inline definitions for the __issignaling functions to the
> respective mathinline.h files.  (But what about someone using <math.h>

Or the macro could use a union to pass the value to a function defined to 
take an integer value, if that helps.

-- 
Joseph S. Myers
joseph@codesourcery.com