[PATCH] D142924: [llvm][IfConversion] update successor list when merging INLINEASM_BR

[Nick Desaulniers via Phabricator via llvm-commits]

nickdesaulniers added inline comments.


================
Comment at: llvm/lib/CodeGen/IfConversion.cpp:2254
+          if (MO.isMBB() && !ToBBI.BB->isSuccessor(MO.getMBB()))
+            ToBBI.BB->addSuccessor(MO.getMBB(), BranchProbability::getZero());
+
----------------
nickdesaulniers wrote:
> nickdesaulniers wrote:
> > void wrote:
> > > void wrote:
> > > > nickdesaulniers wrote:
> > > > > nickdesaulniers wrote:
> > > > > > Perhaps it might be best to see if the prior branch had a branch probability, and if so, reuse that?
> > > > > > 
> > > > > > 
> > > > > > See calls to `getEdgeProbability` below.
> > > > > eh maybe not. Pretty sure we only create these edges with 0% probability. The fallthough BB is not an explicit operand.
> > > > +1. I think we mark all indirect edges as "cold" by default.
> > > Do we already move the indirect blocks to the end of the function? Or would that be too pessimistic?
> > I imagine that `Branch Probability Basic Block Placement (block-placement)` would, or `ARM block placement (arm-block-placement)`.  But there's no such potential optimization at least for this test case.
> > 
> > ---
> > 
> > That said, tailduplication seems broken on this test case...we have 2 unconditional branches to return instructions that don't get replaced with just return instructions...
> Probably the fault of the `if (PredBB->succ_size() > 1)` guard in `TailDuplicator::canTailDuplicate`
```
diff --git a/llvm/lib/CodeGen/TailDuplicator.cpp b/llvm/lib/CodeGen/TailDuplicator.cpp
index 865add28f781..5cc70979a9d3 100644
--- a/llvm/lib/CodeGen/TailDuplicator.cpp
+++ b/llvm/lib/CodeGen/TailDuplicator.cpp
@@ -791,7 +791,7 @@ bool TailDuplicator::duplicateSimpleBB(
 bool TailDuplicator::canTailDuplicate(MachineBasicBlock *TailBB,
                                       MachineBasicBlock *PredBB) {
   // EH edges are ignored by analyzeBranch.
-  if (PredBB->succ_size() > 1)
+  if (PredBB->succ_size() > 1 && !PredBB->mayHaveInlineAsmBr())
     return false;
 
   MachineBasicBlock *PredTBB = nullptr, *PredFBB = nullptr;
@@ -896,7 +896,7 @@ bool TailDuplicator::tailDuplicate(bool IsSimple, MachineBasicBlock *TailBB,
 
     // Update the CFG.
     PredBB->removeSuccessor(PredBB->succ_begin());
-    assert(PredBB->succ_empty() &&
+    assert((PredBB->succ_empty() || PredBB->mayHaveInlineAsmBr()) &&
            "TailDuplicate called on block with multiple successors!");
     for (MachineBasicBlock *Succ : TailBB->successors())
       PredBB->addSuccessor(Succ, MBPI->getEdgeProbability(TailBB, Succ));
```
produces what I want (from the test case in this patch, though this whole thread is orthogonal to this patch).


Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D142924/new/

https://reviews.llvm.org/D142924