indicator for fast FMA

[simonbyrne]

Now we have a fma function, it would be useful to have some method for determining whether or not this is more efficient than the naive x*y+z, particularly for use in algorithms that use double-double style arithmetic (C provides the FP_FAST_FMA macro for this purpose).

From #8112 (comment), it seems that the best option is to expose TargetLowering::isFMAFasterThanFMulAndFAdd (presumably in base/sysinfo.jl?)

[ivarne]

Isn't this the problem muladd from #9840 is intended to solve?

[simonbyrne]

Not quite: they are related but distinct.

• The purpose muladd is to compute x*y+z in the fastest way possible (e.g. in a Horner evaluation scheme for a polynomial).
• The purpose of this is to determine whether fma is fast (i.e. in hardware), or slow (i.e. in software) in which case you might want to use a different approach altogether. An example is here: if you have a hardware fma, it can reduce to 5 operations, otherwise you need 12 (using a software fma would most likely be even slower).

[simonbyrne]

As a rough point of reference, a software fma is about 10x slower than a (non-fused) multiply-add on my computer.

[nalimilan]

Couldn't a function be designed so that it would automatically use the fastest solution? Maybe I'm being naive though.

[simonbyrne]

@nalimilan I'm not sure what you mean: do you mean something like specifying two code paths and letting the compiler pick the one it likes the most?

[nalimilan]

@simonbyrne No, I wonder whether u+muladd(fma(-u,f,2(f-u)),g,q) couldn't be automatically translated to the detailed steps you show here if fma is known to be slow. But I guess it's more complex than that.

[simonbyrne]

Ah I see: you could do that, but it would probably be around twice as slow as the alternative: a decent software fma (e.g. openlibm) does a few extra operations to avoid things like overflow and double rounding, which are not needed in this particular case.

[eschnett]

Another point: fma is guaranteed to avoid rounding the intermediate result, which improves accuracy, and this is what allows alternative algorithms to be used. muladd, on the other hand, may be fast, but may still round the intermediate result. For example, ARM has both vmla and fma instructions -- the former rounds the intermediate result, the latter doesn't.

[JeffreySarnoff]

When using double-double algorithms for extended precision math or to get Float64 results from Float32 friendly GPUs, fma is essential (it is possible to do without -- but everything takes much, much longer and gets more complicated). It is that the fma only-rounds-once that matters for this.

After trying it both ways, I am using fma everywhere possible -- whether or not the fma is slow (software emulated); the alternative is backwards facing and confounding for careful numerics.

[musm]

how can TargetLowering::isFMAFasterThanFMulAndFAdd be exposed, its a cpp fun?