[PATCH] D76066: [ARM][MachineOutliner] Add Machine Outliner support for ARM

Wed Apr 8 23:57:57 PDT 2020

samparker added inline comments.

================
Comment at: llvm/lib/Target/ARM/ARMTargetMachine.cpp:553
   addPass(createARMConstantIslandPass());
-  addPass(createARMLowOverheadLoopsPass());
+  if (!MachineOutlinerEnabled)
+    addPass(createARMLowOverheadLoopsPass());
----------------
yroux wrote:
> samparker wrote:
> > We'll need the LowOverheadLoops pass to run for correctness, so we should instead only add the MachineOutliner if the subtarget doesn't support LOB.
> What do you mean by "for correctness" ?
> 
> I think that it makes more sense that until MachineOutliner and LowOverheadLoops can work together, we have loloops enabled on targets which have LOB support unless it is explicitly disabled by -disable-arm-loloops flag or if the user wants machine outlining with the flag -moutline.   If we do that in the opposite way it means that passing the flag -moutline will have no impact on such targets unless the -disable-arm-loloops flag is used
> 
Ok. Well, the HardwareLoops pass inserts intrinsics, which are lowered to pseudos and then finalised by ARMLowOverheadLoops so the compiler could crash with this logic. TTI controls whether we generate a low-overhead loop and it also controls whether the vectorizer tries to optimise for those loops too... So, I think TTI will need to understand when we're trying to use the outliner too.

Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D76066/new/

https://reviews.llvm.org/D76066