[LPM] Fix PR18616 where the shifts to the loop pass manager to extract

LCSSA from it caused a crasher with the LoopUnroll pass.

This crasher is really nasty. We destroy LCSSA form in a suprising way.
When unrolling a loop into an outer loop, we not only need to restore
LCSSA form for the outer loop, but for all children of the outer loop.
This is somewhat obvious in retrospect, but hey!

While this seems pretty heavy-handed, it's not that bad. Fundamentally,
we only do this when we unroll a loop, which is already a heavyweight
operation. We're unrolling all of these hypothetical inner loops as
well, so their size and complexity is already on the critical path. This
is just adding another pass over them to re-canonicalize.

I have a test case from PR18616 that is great for reproducing this, but
pretty useless to check in as it relies on many 10s of nested empty
loops that get unrolled and deleted in just the right order. =/ What's
worse is that investigating this has exposed another source of failure
that is likely to be even harder to test. I'll try to come up with test
cases for these fixes, but I want to get the fixes into the tree first
as they're causing crashes in the wild.

llvm-svn: 200273

include/llvm/Transforms/Utils/LoopUtils.h

@@ -47,6 +47,17 @@ bool simplifyLoop(Loop *L, DominatorTree *DT, LoopInfo *LI, Pass *PP,
 /// Returns true if any modifications are made to the loop.
 bool formLCSSA(Loop &L, DominatorTree &DT, ScalarEvolution *SE = 0);
 
+/// \brief Put a loop nest into LCSSA form.
+///
+/// This recursively forms LCSSA for a loop nest.
+///
+/// LoopInfo and DominatorTree are required and preserved.
+///
+/// If ScalarEvolution is passed in, it will be preserved.
+///
+/// Returns true if any modifications are made to the loop.
+bool formLCSSARecursively(Loop &L, DominatorTree &DT, ScalarEvolution *SE = 0);
+
 }
 
 #endif

lib/Transforms/Utils/LCSSA.cpp

@@ -226,6 +226,19 @@ bool llvm::formLCSSA(Loop &L, DominatorTree &DT, ScalarEvolution *SE) {
   return Changed;
 }
 
+/// Process a loop nest depth first.
+bool llvm::formLCSSARecursively(Loop &L, DominatorTree &DT,
+                                ScalarEvolution *SE) {
+  bool Changed = false;
+
+  // Recurse depth-first through inner loops.
+  for (Loop::iterator LI = L.begin(), LE = L.end(); LI != LE; ++LI)
+    Changed |= formLCSSARecursively(**LI, DT, SE);
+
+  Changed |= formLCSSA(L, DT, SE);
+  return Changed;
+}
+
 namespace {
 struct LCSSA : public FunctionPass {
   static char ID; // Pass identification, replacement for typeid
@@ -278,23 +291,11 @@ bool LCSSA::runOnFunction(Function &F) {
 
   // Simplify each loop nest in the function.
   for (LoopInfo::iterator I = LI->begin(), E = LI->end(); I != E; ++I)
-    Changed |= processLoop(**I);
+    Changed |= formLCSSARecursively(**I, *DT, SE);
 
   return Changed;
 }
 
-/// Process a loop nest depth first.
-bool LCSSA::processLoop(Loop &L) {
-  bool Changed = false;
-
-  // Recurse depth-first through inner loops.
-  for (Loop::iterator LI = L.begin(), LE = L.end(); LI != LE; ++LI)
-    Changed |= processLoop(**LI);
-
-  Changed |= formLCSSA(L, *DT, SE);
-  return Changed;
-}
-
 static void verifyLoop(Loop &L, DominatorTree &DT) {
   // Recurse depth-first through inner loops.
   for (Loop::iterator LI = L.begin(), LE = L.end(); LI != LE; ++LI)

lib/Transforms/Utils/LoopUnroll.cpp

@@ -468,9 +468,11 @@ bool llvm::UnrollLoop(Loop *L, unsigned Count, unsigned TripCount,
   if (PP && DT) {
     if (!OuterL && !CompletelyUnroll)
       OuterL = L;
-    if (OuterL)
-      simplifyLoop(OuterL, DT, LI, PP, /*AliasAnalysis*/ 0,
-                   PP->getAnalysisIfAvailable<ScalarEvolution>());
+    if (OuterL) {
+      ScalarEvolution *SE = PP->getAnalysisIfAvailable<ScalarEvolution>();
+      simplifyLoop(OuterL, DT, LI, PP, /*AliasAnalysis*/ 0, SE);
+      formLCSSARecursively(*OuterL, *DT, SE);
+    }
   }
 
   return true;