[llvm-commits] [PATCH][Fastmath, Instcombine] Enhancement of Fdiv

[dag at cray.com]

Shuxin Yang <shuxin.llvm at gmail.com> writes:

>> I would like to see this changed to produce X / (C1 * C2) precisely
>> because of the reciprocal.  X / (C1 * C2) may still be acceptable even
>> when the reciprocal is not.  In our compiler, for example, we have
>> multiple levels of fp math safety and users can pick what they want.  It
>> would be nice to map those to LLVM's optimizations when possible.
>>
>>                                      -David

> We don't have flags to control this. This xform is controlled by
> "fast-math", which implies
> "unsafe algebra" which is the highest relax level. Maybe we should
> introduce more flags to control
> the precision? Michael?

I'm not saying we need to add more flags.  It sounded like there is a
separate flag for reciprocal, but if not, it's not a big deal.  Don't
worry too much about it.

> BTW,  I realized my previous response to this issue is not quite
> to-the-point.
> I didn't divide X/C1/C2 => X * 1/(C1*C2) in two steps:
>    X/ (C1*C2) and optionally X * 1/(C1*C2)
> due to the structure of the my code.  It is more convenient to fold
> these two steps in one shot.

Yeah, I understand.

> I guess the reciprocal problem bug you as well. I will discuss with my
> coworker Michale soliciting his insight about how to do it right.

Again, it's not too big a deal.  I might go in later and add a couple
other flags that we could map to different optimization levels but that
wouldn't be for a while.  I'll need to gain some experience with the
code first.

What we did with 3.1 was add an FPOptLevel flag and mapped that to a few
checks in instcombine.  I haven't submitted that patch yet but I
probably will sometime this year.

Not your problem, so think no more about it.

                                    -David