[cfe-dev] -fdenormal-fp-math

[Evandro Menezes via cfe-dev]

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.
>
>>   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.

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

Thank you,

-- 
Evandro Menezes