[LoopSimplify] Re-instate r306081 with a bug fix w.r.t. indirectbr.

This was reverted in r306252, but I already had the bug fixed and was
just trying to form a test case.

The original commit factored the logic for forming dedicated exits
inside of LoopSimplify into a helper that could be used elsewhere and
with an approach that required fewer intermediate data structures. See
that commit for full details including the change to the statistic, etc.

The code looked fine to me and my reviewers, but in fact didn't handle
indirectbr correctly -- it left the 'InLoopPredecessors' vector dirty.

If you have code that looks *just* right, you can end up leaking these
predecessors into a subsequent rewrite, and crash deep down when trying
to update PHI nodes for predecessors that don't exist.

I've added an assert that makes the bug much more obvious, and then
changed the code to reliably clear the vector so we don't get this bug
again in some other form as the code changes.

I've also added a test case that *does* manage to catch this while also
giving some nice positive coverage in the face of indirectbr.

The real code that found this came out of what I think is CPython's
interpreter loop, but any code with really "creative" interpreter loops
mixing indirectbr and other exit paths could manage to tickle the bug.
I was hard to reduce the original test case because in addition to
having a particular pattern of IR, the whole thing depends on the order
of the predecessors which is in turn depends on use list order. The test
case added here was designed so that in multiple different predecessor
orderings it should always end up going down the same path and tripping
the same bug. I hope. At least, it tripped it for me without
manipulating the use list order which is better than anything bugpoint
could do...

llvm-svn: 306257

1 files changed, 65 insertions, 0 deletions

diff --git a/llvm/lib/Transforms/Utils/LoopUtils.cpp b/llvm/lib/Transforms/Utils/LoopUtils.cpp
index e545bc0..0ed3394 100644
--- a/llvm/lib/Transforms/Utils/LoopUtils.cpp
+++ b/llvm/lib/Transforms/Utils/LoopUtils.cpp
@@ -12,6 +12,7 @@
 //===----------------------------------------------------------------------===//
 
 #include "llvm/Transforms/Utils/LoopUtils.h"
+#include "llvm/ADT/ScopeExit.h"
 #include "llvm/Analysis/AliasAnalysis.h"
 #include "llvm/Analysis/BasicAliasAnalysis.h"
 #include "llvm/Analysis/GlobalsModRef.h"
@@ -29,6 +30,7 @@
 #include "llvm/IR/ValueHandle.h"
 #include "llvm/Pass.h"
 #include "llvm/Support/Debug.h"
+#include "llvm/Transforms/Utils/BasicBlockUtils.h"
 
 using namespace llvm;
 using namespace llvm::PatternMatch;
@@ -922,6 +924,69 @@ bool InductionDescriptor::isInductionPHI(PHINode *Phi, const Loop *TheLoop,
   return true;
 }
 
+bool llvm::formDedicatedExitBlocks(Loop *L, DominatorTree *DT, LoopInfo *LI,
+                                   bool PreserveLCSSA) {
+  bool Changed = false;
+
+  // We re-use a vector for the in-loop predecesosrs.
+  SmallVector<BasicBlock *, 4> InLoopPredecessors;
+
+  auto RewriteExit = [&](BasicBlock *BB) {
+    assert(InLoopPredecessors.empty() &&
+           "Must start with an empty predecessors list!");
+    auto Cleanup = make_scope_exit([&] { InLoopPredecessors.clear(); });
+
+    // See if there are any non-loop predecessors of this exit block and
+    // keep track of the in-loop predecessors.
+    bool IsDedicatedExit = true;
+    for (auto *PredBB : predecessors(BB))
+      if (L->contains(PredBB)) {
+        if (isa<IndirectBrInst>(PredBB->getTerminator()))
+          // We cannot rewrite exiting edges from an indirectbr.
+          return false;
+
+        InLoopPredecessors.push_back(PredBB);
+      } else {
+        IsDedicatedExit = false;
+      }
+
+    assert(!InLoopPredecessors.empty() && "Must have *some* loop predecessor!");
+
+    // Nothing to do if this is already a dedicated exit.
+    if (IsDedicatedExit)
+      return false;
+
+    auto *NewExitBB = SplitBlockPredecessors(
+        BB, InLoopPredecessors, ".loopexit", DT, LI, PreserveLCSSA);
+
+    if (!NewExitBB)
+      DEBUG(dbgs() << "WARNING: Can't create a dedicated exit block for loop: "
+                   << *L << "\n");
+    else
+      DEBUG(dbgs() << "LoopSimplify: Creating dedicated exit block "
+                   << NewExitBB->getName() << "\n");
+    return true;
+  };
+
+  // Walk the exit blocks directly rather than building up a data structure for
+  // them, but only visit each one once.
+  SmallPtrSet<BasicBlock *, 4> Visited;
+  for (auto *BB : L->blocks())
+    for (auto *SuccBB : successors(BB)) {
+      // We're looking for exit blocks so skip in-loop successors.
+      if (L->contains(SuccBB))
+        continue;
+
+      // Visit each exit block exactly once.
+      if (!Visited.insert(SuccBB).second)
+        continue;
+
+      Changed |= RewriteExit(SuccBB);
+    }
+
+  return Changed;
+}
+
 /// \brief Returns the instructions that use values defined in the loop.
 SmallVector<Instruction *, 8> llvm::findDefsUsedOutsideOfLoop(Loop *L) {
   SmallVector<Instruction *, 8> UsedOutside;