[SimplifyCFG] Defer folding unconditional branches to LateSimplifyCFG if it can destroy canonical loop structure.

Summary:
When simplifying unconditional branches from empty blocks, we pre-test if the
BB belongs to a set of loop headers and keep the block to prevent passes from
destroying canonical loop structure. However, the current algorithm fails if
the destination of the branch is a loop header. Especially when such a loop's
latch block is folded into loop header it results in additional backedges and
LoopSimplify turns it into a nested loop which prevent later optimizations
from being applied (e.g., loop  unrolling and loop interleaving).

This patch augments the existing algorithm by further checking if the
destination of the branch belongs to a set of loop headers and defer
eliminating it if yes to LateSimplifyCFG.

Fixes PR33605: https://bugs.llvm.org/show_bug.cgi?id=33605

Reviewers: efriedma, mcrosier, pacxx, hsung, davidxl

Reviewed By: efriedma

Subscribers: ashutosh.nema, gberry, javed.absar, llvm-commits

Differential Revision: https://reviews.llvm.org/D35411

llvm-svn: 308422

1 files changed, 9 insertions, 7 deletions

diff --git a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
index dee658f9839..8784b970214 100644
--- a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
+++ b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
@@ -5656,20 +5656,22 @@ static bool TryToMergeLandingPad(LandingPadInst *LPad, BranchInst *BI,
 bool SimplifyCFGOpt::SimplifyUncondBranch(BranchInst *BI,
                                           IRBuilder<> &Builder) {
   BasicBlock *BB = BI->getParent();
+  BasicBlock *Succ = BI->getSuccessor(0);
 
   if (SinkCommon && SinkThenElseCodeToEnd(BI))
     return true;
 
   // If the Terminator is the only non-phi instruction, simplify the block.
-  // if LoopHeader is provided, check if the block is a loop header
-  // (This is for early invocations before loop simplify and vectorization
-  // to keep canonical loop forms for nested loops.
-  // These blocks can be eliminated when the pass is invoked later
-  // in the back-end.)
+  // if LoopHeader is provided, check if the block or its successor is a loop
+  // header (This is for early invocations before loop simplify and
+  // vectorization to keep canonical loop forms for nested loops. These blocks
+  // can be eliminated when the pass is invoked later in the back-end.)
+  bool NeedCanonicalLoop =
+      !LateSimplifyCFG &&
+      (LoopHeaders && (LoopHeaders->count(BB) || LoopHeaders->count(Succ)));
   BasicBlock::iterator I = BB->getFirstNonPHIOrDbg()->getIterator();
   if (I->isTerminator() && BB != &BB->getParent()->getEntryBlock() &&
-      (!LoopHeaders || !LoopHeaders->count(BB)) &&
-      TryToSimplifyUncondBranchFromEmptyBlock(BB))
+      !NeedCanonicalLoop && TryToSimplifyUncondBranchFromEmptyBlock(BB))
     return true;
 
   // If the only instruction in the block is a seteq/setne comparison