[PATCH] D12882: [SimplifyCFG] do not speculate fdiv by default; it's expensive (PR24818)

[Sanjay Patel via llvm-commits]

spatel added a comment.

In http://reviews.llvm.org/D12882#251209, @hfinkel wrote:

> > Is it art or science?
>
>
> Both.


Aha, now it's all starting to make more sense. That's much more fun. :)
Many thanks to you and Philip for explaining this.

My plan to push this forward is:

1. Fix up the TTI cost model (this patch and possibly others).
2. Add code to CGP that uses the cost model to undo this sort of speculation for targets that don't want it.
3. Change this part of SimplifyCFG to not use the TTI cost model for purposes of speculative transforms.

I've just started looking at CGP for the first time...it has this ominous comment:

  // This pass munges the code in the input function to better prepare it for
  // SelectionDAG-based code generation. This works around limitations in it's
  // basic-block-at-a-time approach. It should eventually be removed.

Is there a proposed replacement for CGP?


http://reviews.llvm.org/D12882