[PATCH] D11393: [X86] Allow X86::COND_NE_OR_P and X86::COND_NP_OR_E to be reversed.

Cong Hou via llvm-commits llvm-commits at lists.llvm.org
Fri Jan 29 16:36:55 PST 2016

congh added a comment.

In http://reviews.llvm.org/D11393#338709, @DavidKreitzer wrote:

> Thanks for the fixes and for cleaning up the unnecessary conditions. I still think the FBB==nullptr case is broken in the COND_E_AND_NP detection code, but otherwise, this looks good.
> Did you get reproducers for the failures that people were seeing with this patch? Do these fixes solve them?

Yes. The issue is that I get the incorrect FBB in InsertBranch() when the passed-in FBB is null. What I did before is using the layout fallthrough BB as the FBB, but this may be incorrect because at that moment the user may have already modifed the layout, making the fallthrough BB not the FBB. Instead, I searched the successor list of MBB to find the FBB (the successor that is not the TBB or EHPad).

> Thanks,

> Dave

Comment at: llvm/trunk/lib/Target/X86/X86InstrInfo.cpp:4071
@@ +4070,3 @@
+      // See comments above for X86::COND_P_AND_NE.
+      BranchCode = X86::COND_E_AND_NP;
+    } else
DavidKreitzer wrote:
> congh wrote:
> > DavidKreitzer wrote:
> > > I think you need to verify here that NewTBB == FBB before making this transformation. If FBB is nullptr, you'll need to compute the fallthrough block and check that it matches NewTBB.
> > > 
> > When FBB is nullptr, we could not safely let the fallthrough block be FBB. This is because there is a use case of AnalyzeBranch in block-placement where MBBs are reordered before this function is called, in which case the fallthrough MBB may have nothing to do with the branch.
> > 
> > But we can do this check if FBB is not null.
> To be clear, I wasn't suggesting that you actually compute & set FBB here in the case when it was initially nullptr. Rather, I am saying that you cannot legally use COND_E_AND_NP here without proving that the target of the first branch is the same as the fall-through target.
> What is to prevent this code from analyzing the sequence:
> JNE B1
> JNP B2
> ... fallthrough to some mystery block B3 (FBB == nullptr) ...
> and returning this?
> BranchCond = COND_E_AND_NP
> TBB = B2
> FBB = nullptr
> The branch to B1 is lost.
You are right! However, even we have 

(fallthough to B1)

we are not 100% certain that B1 will be FBB. The user of AnalyzeBranch() can do anything before calling it, making the fallthrough block not the actual FBB. This happens in the block-placement pass. A possible solution is iterating the successor list of the MBB and finding the correct FBB, which is done in the updated patch. In the function InsertBranch() I did the same thing: when the passed-in FBB is false, I try to find it by checking the successor list of the MBB. But there is an another potential issue: the user passed-in TBB/FBB may not be the successors of the MBB. This happens in the tail-duplication pass. So I think it is very easy to misuse InsertBranch().


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