[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

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;

llvm/test/Transforms/LICM/hoist-deref-load.ll

@@ -432,5 +432,42 @@ for.end:                                          ; preds = %for.inc, %entry
   ret void
 }
 
+define void @test11(i32* noalias %a, i32* %b, i32** dereferenceable(8) %cptr, i32 %n) #0 {
+; CHECK-LABEL: @test11(
+entry:
+  %cmp11 = icmp sgt i32 %n, 0
+  br i1 %cmp11, label %for.body, label %for.end
+
+; CHECK: for.body.preheader:
+; CHECK:  %c = load i32*, i32** %cptr, !dereferenceable !0
+; CHECK:  %d = load i32, i32* %c, align 4
+
+
+for.body:                                         ; preds = %entry, %for.inc
+  %indvars.iv = phi i64 [ %indvars.iv.next, %for.inc ], [ 0, %entry ]
+  %arrayidx = getelementptr inbounds i32, i32* %a, i64 %indvars.iv
+  %0 = load i32, i32* %arrayidx, align 4
+  %cmp1 = icmp sgt i32 %0, 0
+  %c = load i32*, i32** %cptr, !dereferenceable !0
+  br i1 %cmp1, label %if.then, label %for.inc
+
+if.then:                                          ; preds = %for.body
+  %d = load i32, i32* %c, align 4
+  %arrayidx3 = getelementptr inbounds i32, i32* %b, i64 %indvars.iv
+  %e = load i32, i32* %arrayidx3, align 4
+  %mul = mul nsw i32 %e, %d
+  store i32 %mul, i32* %arrayidx, align 4
+  br label %for.inc
+
+for.inc:                                          ; preds = %for.body, %if.then
+  %indvars.iv.next = add nuw nsw i64 %indvars.iv, 1
+  %lftr.wideiv = trunc i64 %indvars.iv.next to i32
+  %exitcond = icmp eq i32 %lftr.wideiv, %n
+  br i1 %exitcond, label %for.end, label %for.body
+
+for.end:                                          ; preds = %for.inc, %entry
+  ret void
+}
+
 attributes #0 = { nounwind uwtable }
 !0 = !{i64 4}