[PATCH] [AArch64] Improve and enable the SeparateConstOffsetFromGEP for AArch64 backend.

[Hao Liu]

>>! In D5864#5, @hfinkel wrote:
>> And most important is that it will also transform complex GEPs into a "ptrtoint + arithmetic + inttoptr" form, so that it is able to find CSE opportunities across basic blocks. 
> 
> You'll need to disable this transformation when AA is enabled in CodeGen. You can do this just like CGP does (essentially by calling TM->getSubtarget<TargetSubtargetInfo>().useAA()). CGP splits the GEPs into simpler GEPs when AA is in use, and uses inttoptr otherwise. This all relates to the way that we use BasicAA to override TBAA and catch cases of basic type punning -- but BasicAA does not analyze inttoptr, and so can no longer fulfill this need after new inttoptrs are introduced. You could try using simpler GEPs like CGP does when AA is in use, or just disable this part of the transformation for now. Either is fine with me.

Hi Hal,

Thanks for the explanation on useAA(). We do transformation because using simpler GEPs to replace a complex GEP like CGP does has small impact. Such transformation is not in CGP, because we want to make use of the constant extraction in SeparateConstOffsetFromGEP pass, and we also want to call other passes like EarlyCSE to remove common sub-expressions after such transformation. For the performance concern, I prefer to enable such transformation in AArch64 backend.
But unfortunately, AArch64 backend enables AA for both cortex-a53 and cortex-a57:
      bool useAA() const override { return isCortexA53() || isCortexA57(); }
This was introduced recently in http://reviews.llvm.org/D5103. I find most of the backend including X86,ARM doesn't enable AA. AArch64 backend also disables AA when CPU is cyclone. As Sanjin also says enabling AA for CortexA57 doesn't have performance changes in D5103, I think we can just remove CortexA57 from useAA(), so that we can get benefit from such transformation and has no regression.

For CortexA53, I don't test how much benefit from enabling AA in code gen. Maybe we can test following situations and find the best solution:
  (1) disable AA, enable Transformation.
  (2) enable AA, disable Transformation.
  (3) enable AA and Transformation.
Anyway, such test need a lot of time. So I think we'd better only consider about A57, so I've attached a new patch (diff ID 15234) following your suggestion and remove A57 form useAA(). 

Review please.

Thanks,
-Hao

http://reviews.llvm.org/D5864