[LICM] Keep metadata on control equivalent hoists

Summary:
If the instruction we're hoisting out of a loop into its preheader is
guaranteed to have executed in the loop, then the metadata associated
with the instruction (e.g. !range or !dereferenceable) is valid in the
preheader.  This is because once we're in the preheader, we know we're
eventually going to reach the location the metadata was valid at.

This change makes LICM smarter around this, and helps it recognize cases
like these:

```
  do {
    int a = *ptr; !range !0
    ...
  } while (i++ < N);
```

to

```
  int a = *ptr; !range !0
  do {
    ...
  } while (i++ < N);
```

Earlier we'd drop the `!range` metadata after hoisting the load from
`ptr`.

Reviewers: igor-laevsky

Subscribers: mcrosier, llvm-commits

Differential Revision: http://reviews.llvm.org/D16669

llvm-svn: 259053

1 files changed, 18 insertions, 7 deletions

diff --git a/llvm/lib/Transforms/Scalar/LICM.cpp b/llvm/lib/Transforms/Scalar/LICM.cpp
index b866b25db4b..4fb06046c49 100644
--- a/llvm/lib/Transforms/Scalar/LICM.cpp
+++ b/llvm/lib/Transforms/Scalar/LICM.cpp
@@ -77,7 +77,8 @@ DisablePromotion("disable-licm-promotion", cl::Hidden,
 static bool inSubLoop(BasicBlock *BB, Loop *CurLoop, LoopInfo *LI);
 static bool isNotUsedInLoop(const Instruction &I, const Loop *CurLoop,
                             const LICMSafetyInfo *SafetyInfo);
-static bool hoist(Instruction &I, BasicBlock *Preheader);
+static bool hoist(Instruction &I, const DominatorTree *DT, const Loop *CurLoop,
+                  const LICMSafetyInfo *SafetyInfo);
 static bool sink(Instruction &I, const LoopInfo *LI, const DominatorTree *DT,
                  const Loop *CurLoop, AliasSetTracker *CurAST,
                  const LICMSafetyInfo *SafetyInfo);
@@ -397,7 +398,7 @@ bool llvm::hoistRegion(DomTreeNode *N, AliasAnalysis *AA, LoopInfo *LI,
           canSinkOrHoistInst(I, AA, DT, TLI, CurLoop, CurAST, SafetyInfo) &&
           isSafeToExecuteUnconditionally(I, DT, TLI, CurLoop, SafetyInfo,
                                  CurLoop->getLoopPreheader()->getTerminator()))
-        Changed |= hoist(I, CurLoop->getLoopPreheader());
+        Changed |= hoist(I, DT, CurLoop, SafetyInfo);
     }
 
   const std::vector<DomTreeNode*> &Children = N->getChildren();
@@ -716,16 +717,26 @@ static bool sink(Instruction &I, const LoopInfo *LI, const DominatorTree *DT,
 /// When an instruction is found to only use loop invariant operands that
 /// is safe to hoist, this instruction is called to do the dirty work.
 ///
-static bool hoist(Instruction &I, BasicBlock *Preheader) {
+static bool hoist(Instruction &I, const DominatorTree *DT, const Loop *CurLoop,
+                  const LICMSafetyInfo *SafetyInfo) {
+  auto *Preheader = CurLoop->getLoopPreheader();
   DEBUG(dbgs() << "LICM hoisting to " << Preheader->getName() << ": "
         << I << "\n");
+
+  // Metadata can be dependent on conditions we are hoisting above.
+  // Conservatively strip all metadata on the instruction unless we were
+  // guaranteed to execute I if we entered the loop, in which case the metadata
+  // is valid in the loop preheader.
+  if (I.hasMetadataOtherThanDebugLoc() &&
+      // The check on hasMetadataOtherThanDebugLoc is to prevent us from burning
+      // time in isGuaranteedToExecute if we don't actually have anything to
+      // drop.  It is a compile time optimization, not required for correctness.
+      !isGuaranteedToExecute(I, DT, CurLoop, SafetyInfo))
+    I.dropUnknownNonDebugMetadata();
+
   // Move the new node to the Preheader, before its terminator.
   I.moveBefore(Preheader->getTerminator());
 
-  // Metadata can be dependent on the condition we are hoisting above.
-  // Conservatively strip all metadata on the instruction.
-  I.dropUnknownNonDebugMetadata();
-
   if (isa<LoadInst>(I)) ++NumMovedLoads;
   else if (isa<CallInst>(I)) ++NumMovedCalls;
   ++NumHoisted;