[PATCH] D12345: [Reassociate]: Add intermediate subtract instructions created while negating to be redone later for more reassociate opportunities

[Owen Anderson via llvm-commits]

> On Nov 13, 2015, at 7:58 AM, Daniel Berlin <dberlin at dberlin.org> wrote:
> 
> 
> 
> On Thu, Nov 12, 2015 at 11:55 PM, Owen Anderson <resistor at mac.com <mailto:resistor at mac.com>> wrote:
> resistor added a comment.
> 
> In http://reviews.llvm.org/D12345#288471 <http://reviews.llvm.org/D12345#288471>, @dberlin wrote:
> 
> Would it be reasonable to stage that investigation?  AFAICT the changes already proposed (with Chad's comments incorporated) are a strict improvement, and I at least am seeing pretty severe performance regressions from their absence.  Would you be alright with going ahead and landing those?
> 
> 
> I'm not opposed, I'd like to see compile time numbers first since it transforms reassociate into an iterative algorithm.

I’ll see what I can get you on that.

> For my use case, at least, N-ary reassociation is not really appropriate as I also heavily depend on reassociation of floating point arithmetic. 
> 
> I'm not sure why these two statements go together ;-)
> What am i missing?

N-ary reassociation depends heavily on SCEV, both in practice and according to Jingyue’s design doc.  SCEV does not work on floating point today, and it seems has an immense undertaking to get it to work on it.

—Owen
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