[cfe-dev] -fdenormal-fp-math

[Hal Finkel via cfe-dev]

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,
>

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