Easiest way to use FMA instruction

[Johan]

On Thursday, 9 January 2020 at 22:50:37 UTC, Ben Jones wrote:
> On Thursday, 9 January 2020 at 20:57:10 UTC, Ben Jones wrote:
>> What's the easiest way to use the FMA instruction (fused 
>> multiply add that has nice rounding properties)?  The FMA 
>> function in Phobos just does a*b +c  which will round twice.
>>
>> Do any of the intrinsics libraries include this?  Should I 
>> write my own inline ASM?


Why do you want to use the FMA instruction?

If for performance:
Inline assembly is generally very bad for performance as it 
disables inlining and the compiler probably does not understand 
the instruction itself (hence cannot combine it with other 
optimizations). In this case you don't necessarily need the FMA 
instruction (instead you want whatever instruction is fastest), 
so you shouldn't force the compiler to use that instruction. Have 
a look at https://github.com/AuburnSounds/intel-intrinsics, FMA 
is not supported yet.


If only for the rounding behavior:
Then indeed you need to force the compiler to use the FMA 
instruction (also for non-optimized code, so cannot rely on 
optimizer). Inline assembly is a solution. GDC and LDC provide a 
better inline assembly method that preserves a.o. inlining 
potential and doesn't require hardcoded ABI details.
For LDC:
```
double fma(double a, double b, double c)
{
     import ldc.llvmasm;
     return __irEx!(
              `declare double @llvm.fma.f64(double %a, double %b, 
double %c)`,
              `%r = call double @llvm.fma.f64(double %0, double 
%1, double %2)
               ret double %r`,
              "",
              double, double, double, double)(a,b,c);
}
```
https://wiki.dlang.org/LDC_inline_IR , but it is a little 
outdated, see https://github.com/ldc-developers/ldc/issues/3271


cheers,
  Johan