[llvm] r259869 - [LoopUnrolling] Try harder to avoid rebuilding LCSSA when possible.

[Michael Zolotukhin via llvm-commits]

Author: mzolotukhin
Date: Thu Feb  4 20:17:36 2016
New Revision: 259869

URL: http://llvm.org/viewvc/llvm-project?rev=259869&view=rev
Log:
[LoopUnrolling] Try harder to avoid rebuilding LCSSA when possible.

In r255133 (reapplied r253126) we started to avoid redundant
recomputation of LCSSA after loop-unrolling. This patch moves one step
further in this direction - now we can avoid it for much wider range of
loops, as we start to look at IR and try to figure out if the
transformation actually breaks LCSSA phis or makes it necessary to
insert new ones.

Differential Revision: http://reviews.llvm.org/D16838

Modified:
    llvm/trunk/lib/Transforms/Utils/LoopUnroll.cpp

Modified: llvm/trunk/lib/Transforms/Utils/LoopUnroll.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Transforms/Utils/LoopUnroll.cpp?rev=259869&r1=259868&r2=259869&view=diff
==============================================================================
--- llvm/trunk/lib/Transforms/Utils/LoopUnroll.cpp (original)
+++ llvm/trunk/lib/Transforms/Utils/LoopUnroll.cpp Thu Feb  4 20:17:36 2016
@@ -126,6 +126,31 @@ FoldBlockIntoPredecessor(BasicBlock *BB,
   return OnlyPred;
 }
 
+/// Check if unrolling created a situation where we need to insert phi nodes to
+/// preserve LCSSA form.
+/// \param Blocks is a vector of basic blocks representing unrolled loop.
+/// \param L is the outer loop.
+/// It's possible that some of the blocks are in L, and some are not. In this
+/// case, if there is a use is outside L, and definition is inside L, we need to
+/// insert a phi-node, otherwise LCSSA will be broken.
+/// The function is just a helper function for llvm::UnrollLoop that returns
+/// true if this situation occurs, indicating that LCSSA needs to be fixed.
+static bool needToInsertPhisForLCSSA(Loop *L, std::vector<BasicBlock *> Blocks,
+                                     LoopInfo *LI) {
+  for (BasicBlock *BB : Blocks) {
+    if (LI->getLoopFor(BB) == L)
+      continue;
+    for (Instruction &I : *BB) {
+      for (Use &U : I.operands()) {
+        if (auto Def = dyn_cast<Instruction>(U))
+          if (LI->getLoopFor(Def->getParent()) == L)
+            return true;
+      }
+    }
+  }
+  return false;
+}
+
 /// Unroll the given loop by Count. The loop must be in LCSSA form. Returns true
 /// if unrolling was successful, or false if the loop was unmodified. Unrolling
 /// can only fail when the loop's latch block is not terminated by a conditional
@@ -218,10 +243,16 @@ bool llvm::UnrollLoop(Loop *L, unsigned
   bool CompletelyUnroll = Count == TripCount;
   SmallVector<BasicBlock *, 4> ExitBlocks;
   L->getExitBlocks(ExitBlocks);
-  Loop *ParentL = L->getParentLoop();
-  bool AllExitsAreInsideParentLoop = !ParentL ||
-      std::all_of(ExitBlocks.begin(), ExitBlocks.end(),
-                  [&](BasicBlock *BB) { return ParentL->contains(BB); });
+
+  // Go through all exits of L and see if there are any phi-nodes there. We just
+  // conservatively assume that they're inserted to preserve LCSSA form, which
+  // means that complete unrolling might break this form. We need to either fix
+  // it in-place after the transformation, or entirely rebuild LCSSA. TODO: For
+  // now we just recompute LCSSA for the outer loop, but it should be possible
+  // to fix it in-place.
+  bool NeedToFixLCSSA = PreserveLCSSA && CompletelyUnroll &&
+      std::any_of(ExitBlocks.begin(), ExitBlocks.end(),
+                  [&](BasicBlock *BB) { return isa<PHINode>(BB->begin()); });
 
   // We assume a run-time trip count if the compiler cannot
   // figure out the loop trip count and the unroll-runtime
@@ -308,6 +339,7 @@ bool llvm::UnrollLoop(Loop *L, unsigned
   LoopBlocksDFS::RPOIterator BlockBegin = DFS.beginRPO();
   LoopBlocksDFS::RPOIterator BlockEnd = DFS.endRPO();
 
+  std::vector<BasicBlock*> UnrolledLoopBlocks = L->getBlocks();
   for (unsigned It = 1; It != Count; ++It) {
     std::vector<BasicBlock*> NewBlocks;
     SmallDenseMap<const Loop *, Loop *, 4> NewLoops;
@@ -387,6 +419,7 @@ bool llvm::UnrollLoop(Loop *L, unsigned
         Latches.push_back(New);
 
       NewBlocks.push_back(New);
+      UnrolledLoopBlocks.push_back(New);
     }
 
     // Remap all instructions in the most recent iteration
@@ -476,8 +509,13 @@ bool llvm::UnrollLoop(Loop *L, unsigned
     if (Term->isUnconditional()) {
       BasicBlock *Dest = Term->getSuccessor(0);
       if (BasicBlock *Fold = FoldBlockIntoPredecessor(Dest, LI, SE,
-                                                      ForgottenLoops))
+                                                      ForgottenLoops)) {
+        // Dest has been folded into Fold. Update our worklists accordingly.
         std::replace(Latches.begin(), Latches.end(), Dest, Fold);
+        UnrolledLoopBlocks.erase(std::remove(UnrolledLoopBlocks.begin(),
+                                             UnrolledLoopBlocks.end(), Dest),
+                                 UnrolledLoopBlocks.end());
+      }
     }
   }
 
@@ -530,6 +568,17 @@ bool llvm::UnrollLoop(Loop *L, unsigned
   if (CompletelyUnroll)
     LI->markAsRemoved(L);
 
+  // After complete unrolling most of the blocks should be contained in OuterL.
+  // However, some of them might happen to be out of OuterL (e.g. if they
+  // precede a loop exit). In this case we might need to insert PHI nodes in
+  // order to preserve LCSSA form.
+  // We don't need to check this if we already know that we need to fix LCSSA
+  // form.
+  // TODO: For now we just recompute LCSSA for the outer loop in this case, but
+  // it should be possible to fix it in-place.
+  if (PreserveLCSSA && OuterL && CompletelyUnroll && !NeedToFixLCSSA)
+    NeedToFixLCSSA |= ::needToInsertPhisForLCSSA(OuterL, UnrolledLoopBlocks, LI);
+
   // If we have a pass and a DominatorTree we should re-simplify impacted loops
   // to ensure subsequent analyses can rely on this form. We want to simplify
   // at least one layer outside of the loop that was unrolled so that any
@@ -538,7 +587,7 @@ bool llvm::UnrollLoop(Loop *L, unsigned
     if (!OuterL && !CompletelyUnroll)
       OuterL = L;
     if (OuterL) {
-      bool Simplified = simplifyLoop(OuterL, DT, LI, SE, AC, PreserveLCSSA);
+      simplifyLoop(OuterL, DT, LI, SE, AC, PreserveLCSSA);
 
       // LCSSA must be performed on the outermost affected loop. The unrolled
       // loop's last loop latch is guaranteed to be in the outermost loop after
@@ -548,7 +597,7 @@ bool llvm::UnrollLoop(Loop *L, unsigned
         while (OuterL->getParentLoop() != LatchLoop)
           OuterL = OuterL->getParentLoop();
 
-      if (CompletelyUnroll && (!AllExitsAreInsideParentLoop || Simplified))
+      if (NeedToFixLCSSA)
         formLCSSARecursively(*OuterL, *DT, LI, SE);
       else
         assert(OuterL->isLCSSAForm(*DT) &&