[cfe-dev] -fdenormal-fp-math

[Hal Finkel via cfe-dev]

On 03/17/2017 09:37 AM, Evandro Menezes wrote:
> On 03/16/2017 02:09 PM, Hal Finkel wrote:
>>
>>
>> On 03/16/2017 12:43 PM, Evandro Menezes wrote:
>>> Hal,
>>>
>>> Would it be feasible for clang to generate crtfastmath.o on the fly 
>>> at link time, by using LLVM to emit the object file?
>>
>> We could. We could also just inject the necessary IR into each module 
>> as a linkonce_odr initialization function. I'm not sure it is 
>> worthwhile; we depend on compiler-rt for a lot of optional features.
>
> OTOH, by making sure that a module contains this code when any 
> function is relaxing the FP semantics, even if the link command line 
> doesn't include -ffast-math, the expected behavior can be guaranteed.

I'm not sure that's desirable. The problem is that these are global 
settings, and so we need to decide whether the flag on any translation 
unit will affect all others, or if we need the flag at link time for 
something that affects the entire application. I lean toward the latter.

  -Hal

>
>>
>>>
>>> On 03/16/2017 12:40 PM, Hal Finkel wrote:
>>>>
>>>> On 03/16/2017 11:35 AM, Evandro Menezes wrote:
>>>>> Hi, Hal.
>>>>>
>>>>> For the benefit of all, on foil 14 in this presentation by Intel, 
>>>>> https://is.gd/7Ptgwx, the issues of underflow and denormals in 
>>>>> IEEE754 are explained.
>>>>>
>>>>> Here's what I found out in GCC:
>>>>>
>>>>> On {aarch64,alpha,arm,i386,ia64,mips,sparc}, when 
>>>>> {-Ofast,-ffast-math,-funsafe-math-optimizations} is specified, the 
>>>>> file crtfastmath.o is added to the link line.
>>>>>
>>>>> crtfastmath.c contains a static function with the attribute 
>>>>> "constructor" that sets the appropriate flag in a control register 
>>>>> to round denormals to zero.  Some targets set other bits too (like 
>>>>> Alpha and i386, also to round underflows to zero, and MIPS, also 
>>>>> to disable exceptions).
>>>>>
>>>>> Sun's SPARC compiler also flushes underflows and denormals to zero 
>>>>> when the option {-fast,-fnonstd,-fns} is specified.  The reason 
>>>>> given (v. https://is.gd/6jxW97) is also performance degradation.
>>>>>
>>>>> Since clang has a specific flag to guide the behavior of 
>>>>> denormals, -fdenormal-fp-math, it could be used to make sure that 
>>>>> the FPU is set to the same specified behavior: 
>>>>> {ieee,preserve-sign,positive-zero}.
>>>>
>>>> I don't know if we can use ctrfastmath.o for this, independent of 
>>>> -ffast-math, essentially because we have no control over what else 
>>>> it might do, but I can certainly see shipping (e.g. as part of 
>>>> compiler-rt) some similar files that specifically deal with denormals.
>>>>
>>>>  -Hal
>>>>
>>>>>   Again, specific targets might decide to imply a value other than 
>>>>> "ieee" for -fdenormal-fp-math when -funsafe-math-optimizations is 
>>>>> specified, either explicitly or implicitly.
>>>>>
>>>>> Thank you,
>>>>>
>
> Thank you,
>
>

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