Re: [PATCH] powerpc: Fix feraiseexcept and feclearexcept macros

[Florian Weimer <fweimer at redhat dot com>]

* Adhemerval Zanella:

> On 03/03/2020 15:20, Matheus Castanho wrote:
>> A recent change to fenvinline.h modified the check if __e is a
>> a power of 2 inside feraiseexcept and feclearexcept macros.  It
>> introduced the use of the powerof2 macro but also removed the
>> if statement checking whether __e != 0 before issuing an mtfsb*
>> instruction.  This is problematic because powerof2 (0) evaluates
>> to 1 and without the removed if __e is allowed to be 0 when
>> __builtin_clz is called.  In that case the value 32 is passed
>> to __MTFSB*, which is invalid.
>> 
>> This commit uses __builtin_popcount instead of powerof2 to fix this
>> issue and avoid the extra check for __e != 0.  This was the approach
>> used by the initial versions of that previous patch.

Sorry about that.  I convinced myself that the change was valid, even
after remembering that there was something weird with the powerof2
macro.

> This code is becoming convoluted and I think these micro-optimization
> are hardly wildly used and even more being a possible hotspot in 
> realword cases (non-default rounding are used only on specific cases 
> and excepting handling are done most likely only on exceptions cases).

It's not an optimization because it's a compile-time-evaluated
expression.  The question is how to express this in a succinct way, now
that the powerof2 macro is out.  __builtin_popcount (e) == 1 does not
seem to be unreasonable to express this condition.

Or do you object to the existence of the feclearexcept macro as a whole?

Thanks,
Florian