[PATCH] D92933: [ARM][MachineOutliner] Fix costs model.

Mon Dec 14 01:08:50 PST 2020

yroux added inline comments.

================
Comment at: llvm/lib/Target/ARM/ARMBaseInstrInfo.cpp:5823
+      // outline this by saving all of its bytes.
+      else
+        NumBytesNoStackCalls += SequenceSize;
----------------
samparker wrote:
> yroux wrote:
> > samparker wrote:
> > > We were returning here previously, and now it's possible that there's a Candidate that hasn't been inserted into CandidatesWithoutStackFixups. When we update RepeatedSequences on line 5832, are we guaranteed to have not missed some candidates?
> > OK, in fact what I tried to do here is just to fix the costs given to the candidates which was wrongly set to Calldefault for all candidates when SP wasn't used and at least one of them was called with save/restore of LR into the stack.
> > 
> > We were returning here without outling these candidates, if the need of stack fixup was detected, but that wasn't necessary the instructions which migh need such a fixup were marked as illegal for outlining in the code lines 6055-6070
> > 
> > So, here we are just preparing the next step which will handle the fixups, it is the same logic which is used in the AArch64 implementation.
> > 
> > I'll renane and change the description of this revision
> Okay, thanks for reminding me how this plugs together. So I'm sorry if I'm being dense... but couldn't this still mean we could outline an instruction that is dependent upon one that isn't outlined? If we're iterating through a sequence of instructions, effectively copying them back into RepeatedSequenceLocs, what is stopping us from skipping over an instruction which a following instruction maybe dependent upon?
Problematic instructions (like predicated ones, PIC related ones, etc...) are marked as illegal inside getOutliningType function, then the MachineOutliner algorithm search the repeated patterns of legal instructions, thus RepatedSequenceLocs only contain Candidates composed by safe instructions and here we are just adding info about their kind and costs.  Hope it is clearer

Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D92933/new/

https://reviews.llvm.org/D92933