[IfConversion] Keep the CFG updated incrementally in IfConvertTriangle

Summary:
Instead of using RemoveExtraEdges (which uses analyzeBranch, which cannot
always be trusted) at the end to fixup the CFG we keep the CFG updated as
we go along and remove or add branches and merge blocks.

This way we won't have any problems if the involved MBBs contain
unanalyzable instructions.

This fixes PR32721.

In that case we had a triangle

   EBB
   | \
   |  |
   | TBB
   |  /
   FBB

where FBB didn't have any successors at all since it ended with an
unconditional return. Then TBB and FBB were be merged into EBB, but EBB
would still keep its successors, and the use of analyzeBranch and
CorrectExtraCFGEdges wouldn't help to remove them since the return
instruction is not analyzable (at least not on ARM).

Reviewers: kparzysz, iteratee, MatzeB

Reviewed By: iteratee

Subscribers: aemerson, rengolin, javed.absar, llvm-commits

Differential Revision: https://reviews.llvm.org/D33037

llvm-svn: 302876

1 files changed, 21 insertions, 6 deletions

diff --git a/llvm/lib/CodeGen/IfConversion.cpp b/llvm/lib/CodeGen/IfConversion.cpp
index 46c3911..628d599 100644
--- a/llvm/lib/CodeGen/IfConversion.cpp
+++ b/llvm/lib/CodeGen/IfConversion.cpp
@@ -1588,22 +1588,32 @@ bool IfConverter::IfConvertTriangle(BBInfo &BBI, IfcvtKind Kind) {
     BBCvt = MBPI->getEdgeProbability(BBI.BB, &CvtMBB);
   }
 
+  // To be able to insert code freely at the end of BBI we sometimes remove
+  // the branch from BBI to NextMBB temporarily. Remember if this happened.
+  bool RemovedBranchToNextMBB = false;
   if (CvtMBB.pred_size() > 1) {
     BBI.NonPredSize -= TII->removeBranch(*BBI.BB);
     // Copy instructions in the true block, predicate them, and add them to
     // the entry block.
     CopyAndPredicateBlock(BBI, *CvtBBI, Cond, true);
 
-    // RemoveExtraEdges won't work if the block has an unanalyzable branch, so
-    // explicitly remove CvtBBI as a successor.
+    // Keep the CFG updated.
     BBI.BB->removeSuccessor(&CvtMBB, true);
   } else {
     // Predicate the 'true' block after removing its branch.
     CvtBBI->NonPredSize -= TII->removeBranch(CvtMBB);
     PredicateBlock(*CvtBBI, CvtMBB.end(), Cond);
 
-    // Now merge the entry of the triangle with the true block.
+    // Remove the branch from the entry of the triangle to NextBB to be able to
+    // do the merge below. Keep the CFG updated, but remember we removed the
+    // branch since we do want to execute NextMBB, either by introducing a
+    // branch to it again, or merging it into the entry block.
+    // How it's handled is decided further down.
     BBI.NonPredSize -= TII->removeBranch(*BBI.BB);
+    BBI.BB->removeSuccessor(&NextMBB, true);
+    RemovedBranchToNextMBB = true;
+
+    // Now merge the entry of the triangle with the true block.
     MergeBlocks(BBI, *CvtBBI, false);
   }
 
@@ -1641,12 +1651,19 @@ bool IfConverter::IfConvertTriangle(BBInfo &BBI, IfcvtKind Kind) {
     // block. By not merging them, we make it possible to iteratively
     // ifcvt the blocks.
     if (!HasEarlyExit &&
-        NextMBB.pred_size() == 1 && !NextBBI->HasFallThrough &&
+        // We might have removed BBI from NextMBB's predecessor list above but
+        // we want it to be there, so consider that too.
+        (NextMBB.pred_size() == (RemovedBranchToNextMBB ? 0 : 1)) &&
+        !NextBBI->HasFallThrough &&
         !NextMBB.hasAddressTaken()) {
+      // We will merge NextBBI into BBI, and thus remove the current
+      // fallthrough from BBI into CvtBBI.
+      BBI.BB->removeSuccessor(&CvtMBB, true);
       MergeBlocks(BBI, *NextBBI);
       FalseBBDead = true;
     } else {
       InsertUncondBranch(*BBI.BB, NextMBB, TII);
+      BBI.BB->addSuccessor(&NextMBB);
       BBI.HasFallThrough = false;
     }
     // Mixed predicated and unpredicated code. This cannot be iteratively
@@ -1654,8 +1671,6 @@ bool IfConverter::IfConvertTriangle(BBInfo &BBI, IfcvtKind Kind) {
     IterIfcvt = false;
   }
 
-  RemoveExtraEdges(BBI);
-
   // Update block info. BB can be iteratively if-converted.
   if (!IterIfcvt)
     BBI.IsDone = true;