[SimlifyCFG] Prevent passes from destroying canonical loop structure, especially for nested loops

When eliminating or merging almost empty basic blocks, the existence of non-trivial PHI nodes
is currently used to recognize potential loops of which the block is the header and keep the block.
However, the current algorithm fails if the loops' exit condition is evaluated only with volatile
values hence no PHI nodes in the header. Especially when such a loop is an outer loop of a nested
loop, the loop is collapsed into a single loop which prevent later optimizations from being 
applied (e.g., transforming nested loops into simplified forms and loop vectorization).

The patch augments the existing PHI node-based check by adding a pre-test if the BB actually 
belongs to a set of loop headers and not eliminating it if yes. 

llvm-svn: 264596

1 files changed, 4 insertions, 1 deletions

diff --git a/llvm/lib/Transforms/Scalar/JumpThreading.cpp b/llvm/lib/Transforms/Scalar/JumpThreading.cpp
index c79e756f572..2def0b54c6d 100644
--- a/llvm/lib/Transforms/Scalar/JumpThreading.cpp
+++ b/llvm/lib/Transforms/Scalar/JumpThreading.cpp
@@ -245,10 +245,13 @@ bool JumpThreading::runOnFunction(Function &F) {
       // Can't thread an unconditional jump, but if the block is "almost
       // empty", we can replace uses of it with uses of the successor and make
       // this dead.
+      // We should not eliminate the loop header either, because eliminating
+      // a loop header might later prevent LoopSimplify from transforming nested
+      // loops into simplified form.
       if (BI && BI->isUnconditional() &&
           BB != &BB->getParent()->getEntryBlock() &&
           // If the terminator is the only non-phi instruction, try to nuke it.
-          BB->getFirstNonPHIOrDbg()->isTerminator()) {
+          BB->getFirstNonPHIOrDbg()->isTerminator() && !LoopHeaders.count(BB)) {
         // Since TryToSimplifyUncondBranchFromEmptyBlock may delete the
         // block, we have to make sure it isn't in the LoopHeaders set.  We
         // reinsert afterward if needed.