[cfe-dev] -fdenormal-fp-math

[Hal Finkel via cfe-dev]

On 03/09/2017 03:41 PM, Evandro Menezes wrote:
> On 03/09/2017 03:20 PM, Hal Finkel wrote:
>>
>> On 03/09/2017 03:06 PM, Evandro Menezes via cfe-dev wrote:
>>> Can you please clarify if -fdenormal-fp-math is implied by 
>>> -ffast-math?  If not, shouldn't it be?
>>
>> Why should it be?
> It'd make sense, since most targets hiccup when tripping at a 
> denormal, unless they're commanded to flush them to zero by flipping a 
> bit in a control register.

This is a good point. There is certainly a non-trivial amount of 
hardware that will dump into microcode if it needs to properly process 
denormals.

>>
>>>   Does the start-up code, if any, perhaps in compiler-rt, enable the 
>>> appropriate bits in the proper control register?  How can I help?
>>
>> No. Right now, I believe that flag is taken as a statement about what 
>> the compiler can assume about the FP environment. It is not something 
>> we attempt to ensure at startup. I imagine we could on some systems 
>> (as you say, by using some startup code).
> In GCC, though there's not equivalent option to -fdenormal-fp-math, 
> IIRC, on some targets using -ffast-math causes the CRT start up code 
> to enable flushing to zero.  Effectively, on these targets, 
> -ffast-math implies flushing denormals to zero.

Okay, interesting.

>
> Would this GCC behavior, -ffast-math implying -fdenormal-fp-math, be 
> interesting to carry over to Clang, if not universally, on specific 
> targets?

I think it would need to be target specific, but we could definitely let 
-ffast-math affect how linking is done so that you pick up some 
different startup files, or similar, on systems that provide such 
things. The startup files are generally associated with the libc 
implementation, however, so this might be a dual-pronged task.

If you could survey the GCC implementation and see what it does for this 
on various targets, that would certainly be a useful input to this 
discussion.

  -Hal

>
> Thank you,
>

-- 
Hal Finkel
Lead, Compiler Technology and Programming Languages
Leadership Computing Facility
Argonne National Laboratory