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

[dag at cray.com]

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

>     1) reciprocal.patch:
>         implement reciprocal rule:
>          1.1   X/C => X *(1/C) if 1/C is neither special value nor
> denormal.
>
>        This rule could incur noticeable rounding errors (sometimes up
> to 0.001% -- one testing case
>       in pollybench suite see a diff between 1.003 and 1.004, and the
> "huge" difference is caused
>       by a single transformation X/1000 => X * 0.001).

Is this rule controlled by strict fp or other options?  Users should be
able to disable it for precision.

>    2. fdiv.patch implement a slew of fdiv related rules:
>
>       2.1). (X/C1) / C2 => X * (1/(C2*C1))

Why not do (X/C1) / C2 => X / (C1 * C2) (under switch control) and let
1.1 clean it up if allowed by the previously-explained user switch?

>       2.2). X*C1 / C2 => X * (C1/C2)

This will need a strict or similar switch control for fp arithmetic.

>       2.3). (X/Y)/Z = > X/(Y*Z)  (at least one of Y and Z is symbolic
> value)
>       2.4). Z/(X/Y) = > (Z*Y)/X
>       2.5). C1/(X*C2) => (C1/C2) / X
>       2.6). C1/(X/C2) => (C1*C2) / X
>       2.7) C1/(C2/X) => (C1/C2) * X

And all of these too.

Every single one of these transformations can change fp arithmetic
answers and thus we must have a way to disable them on the command line.

                               -David