[LICM] Use ICFLoopSafetyInfo in LICM

This patch makes LICM use `ICFLoopSafetyInfo` that is a smarter version
of LoopSafetyInfo that leverages power of Implicit Control Flow Tracking
to keep track of throwing instructions and give less pessimistic answers
to queries related to throws.

The ICFLoopSafetyInfo itself has been introduced in rL344601. This patch
enables it in LICM only.

Differential Revision: https://reviews.llvm.org/D50377
Reviewed By: apilipenko

llvm-svn: 346201

7 files changed, 236 insertions, 29 deletions

diff --git a/llvm/include/llvm/Transforms/Utils/LoopUtils.h b/llvm/include/llvm/Transforms/Utils/LoopUtils.h
index c75a1de1137..f642852275c 100644
--- a/llvm/include/llvm/Transforms/Utils/LoopUtils.h
+++ b/llvm/include/llvm/Transforms/Utils/LoopUtils.h
@@ -109,7 +109,7 @@ bool formLCSSARecursively(Loop &L, DominatorTree &DT, LoopInfo *LI,
 /// arguments. Diagnostics is emitted via \p ORE. It returns changed status.
 bool sinkRegion(DomTreeNode *, AliasAnalysis *, LoopInfo *, DominatorTree *,
                 TargetLibraryInfo *, TargetTransformInfo *, Loop *,
-                AliasSetTracker *, LoopSafetyInfo *,
+                AliasSetTracker *, ICFLoopSafetyInfo *,
                 OptimizationRemarkEmitter *ORE);
 
 /// Walk the specified region of the CFG (defined by all blocks
@@ -122,7 +122,7 @@ bool sinkRegion(DomTreeNode *, AliasAnalysis *, LoopInfo *, DominatorTree *,
 /// ORE. It returns changed status.
 bool hoistRegion(DomTreeNode *, AliasAnalysis *, LoopInfo *, DominatorTree *,
                  TargetLibraryInfo *, Loop *, AliasSetTracker *,
-                 LoopSafetyInfo *, OptimizationRemarkEmitter *ORE);
+                 ICFLoopSafetyInfo *, OptimizationRemarkEmitter *ORE);
 
 /// This function deletes dead loops. The caller of this function needs to
 /// guarantee that the loop is infact dead.
@@ -151,7 +151,8 @@ bool promoteLoopAccessesToScalars(const SmallSetVector<Value *, 8> &,
                                   SmallVectorImpl<Instruction *> &,
                                   PredIteratorCache &, LoopInfo *,
                                   DominatorTree *, const TargetLibraryInfo *,
-                                  Loop *, AliasSetTracker *, LoopSafetyInfo *,
+                                  Loop *, AliasSetTracker *,
+                                  ICFLoopSafetyInfo *,
                                   OptimizationRemarkEmitter *);
 
 /// Does a BFS from a given node to all of its children inside a given loop.
diff --git a/llvm/lib/Transforms/Scalar/LICM.cpp b/llvm/lib/Transforms/Scalar/LICM.cpp
index d379808cd7f..d6dfdc7efed 100644
--- a/llvm/lib/Transforms/Scalar/LICM.cpp
+++ b/llvm/lib/Transforms/Scalar/LICM.cpp
@@ -103,10 +103,10 @@ static bool isNotUsedOrFreeInLoop(const Instruction &I, const Loop *CurLoop,
                                   const LoopSafetyInfo *SafetyInfo,
                                   TargetTransformInfo *TTI, bool &FreeInLoop);
 static void hoist(Instruction &I, const DominatorTree *DT, const Loop *CurLoop,
-                  LoopSafetyInfo *SafetyInfo,
+                  ICFLoopSafetyInfo *SafetyInfo,
                   OptimizationRemarkEmitter *ORE);
 static bool sink(Instruction &I, LoopInfo *LI, DominatorTree *DT,
-                 const Loop *CurLoop, LoopSafetyInfo *SafetyInfo,
+                 const Loop *CurLoop, ICFLoopSafetyInfo *SafetyInfo,
                  OptimizationRemarkEmitter *ORE, bool FreeInLoop);
 static bool isSafeToExecuteUnconditionally(Instruction &Inst,
                                            const DominatorTree *DT,
@@ -123,7 +123,8 @@ CloneInstructionInExitBlock(Instruction &I, BasicBlock &ExitBlock, PHINode &PN,
                             const LoopInfo *LI,
                             const LoopSafetyInfo *SafetyInfo);
 
-static void eraseInstruction(Instruction &I, AliasSetTracker *AST);
+static void eraseInstruction(Instruction &I, ICFLoopSafetyInfo &SafetyInfo,
+                             AliasSetTracker *AST);
 
 namespace {
 struct LoopInvariantCodeMotion {
@@ -269,7 +270,7 @@ bool LoopInvariantCodeMotion::runOnLoop(
   BasicBlock *Preheader = L->getLoopPreheader();
 
   // Compute loop safety information.
-  SimpleLoopSafetyInfo SafetyInfo;
+  ICFLoopSafetyInfo SafetyInfo(DT);
   SafetyInfo.computeLoopSafetyInfo(L);
 
   // We want to visit all of the instructions in this loop... that are not parts
@@ -376,7 +377,7 @@ bool LoopInvariantCodeMotion::runOnLoop(
 bool llvm::sinkRegion(DomTreeNode *N, AliasAnalysis *AA, LoopInfo *LI,
                       DominatorTree *DT, TargetLibraryInfo *TLI,
                       TargetTransformInfo *TTI, Loop *CurLoop,
-                      AliasSetTracker *CurAST, LoopSafetyInfo *SafetyInfo,
+                      AliasSetTracker *CurAST, ICFLoopSafetyInfo *SafetyInfo,
                       OptimizationRemarkEmitter *ORE) {
 
   // Verify inputs.
@@ -406,7 +407,7 @@ bool llvm::sinkRegion(DomTreeNode *N, AliasAnalysis *AA, LoopInfo *LI,
         LLVM_DEBUG(dbgs() << "LICM deleting dead inst: " << I << '\n');
         salvageDebugInfo(I);
         ++II;
-        eraseInstruction(I, CurAST);
+        eraseInstruction(I, *SafetyInfo, CurAST);
         Changed = true;
         continue;
       }
@@ -423,7 +424,7 @@ bool llvm::sinkRegion(DomTreeNode *N, AliasAnalysis *AA, LoopInfo *LI,
         if (sink(I, LI, DT, CurLoop, SafetyInfo, ORE, FreeInLoop)) {
           if (!FreeInLoop) {
             ++II;
-            eraseInstruction(I, CurAST);
+            eraseInstruction(I, *SafetyInfo, CurAST);
           }
           Changed = true;
         }
@@ -440,7 +441,7 @@ bool llvm::sinkRegion(DomTreeNode *N, AliasAnalysis *AA, LoopInfo *LI,
 ///
 bool llvm::hoistRegion(DomTreeNode *N, AliasAnalysis *AA, LoopInfo *LI,
                        DominatorTree *DT, TargetLibraryInfo *TLI, Loop *CurLoop,
-                       AliasSetTracker *CurAST, LoopSafetyInfo *SafetyInfo,
+                       AliasSetTracker *CurAST, ICFLoopSafetyInfo *SafetyInfo,
                        OptimizationRemarkEmitter *ORE) {
   // Verify inputs.
   assert(N != nullptr && AA != nullptr && LI != nullptr && DT != nullptr &&
@@ -481,7 +482,7 @@ bool llvm::hoistRegion(DomTreeNode *N, AliasAnalysis *AA, LoopInfo *LI,
         CurAST->copyValue(&I, C);
         I.replaceAllUsesWith(C);
         if (isInstructionTriviallyDead(&I, TLI))
-          eraseInstruction(I, CurAST);
+          eraseInstruction(I, *SafetyInfo, CurAST);
         Changed = true;
         continue;
       }
@@ -510,14 +511,16 @@ bool llvm::hoistRegion(DomTreeNode *N, AliasAnalysis *AA, LoopInfo *LI,
         auto One = llvm::ConstantFP::get(Divisor->getType(), 1.0);
         auto ReciprocalDivisor = BinaryOperator::CreateFDiv(One, Divisor);
         ReciprocalDivisor->setFastMathFlags(I.getFastMathFlags());
+        SafetyInfo->insertInstructionTo(I.getParent());
         ReciprocalDivisor->insertBefore(&I);
 
         auto Product =
             BinaryOperator::CreateFMul(I.getOperand(0), ReciprocalDivisor);
         Product->setFastMathFlags(I.getFastMathFlags());
+        SafetyInfo->insertInstructionTo(I.getParent());
         Product->insertAfter(&I);
         I.replaceAllUsesWith(Product);
-        eraseInstruction(I, CurAST);
+        eraseInstruction(I, *SafetyInfo, CurAST);
 
         hoist(*ReciprocalDivisor, DT, CurLoop, SafetyInfo, ORE);
         Changed = true;
@@ -886,9 +889,11 @@ CloneInstructionInExitBlock(Instruction &I, BasicBlock &ExitBlock, PHINode &PN,
   return New;
 }
 
-static void eraseInstruction(Instruction &I, AliasSetTracker *AST) {
+static void eraseInstruction(Instruction &I, ICFLoopSafetyInfo &SafetyInfo,
+                             AliasSetTracker *AST) {
   if (AST)
     AST->deleteValue(&I);
+  SafetyInfo.removeInstruction(&I);
   I.eraseFromParent();
 }
 
@@ -999,7 +1004,7 @@ static void splitPredecessorsOfLoopExit(PHINode *PN, DominatorTree *DT,
 /// position, and may either delete it or move it to outside of the loop.
 ///
 static bool sink(Instruction &I, LoopInfo *LI, DominatorTree *DT,
-                 const Loop *CurLoop, LoopSafetyInfo *SafetyInfo,
+                 const Loop *CurLoop, ICFLoopSafetyInfo *SafetyInfo,
                  OptimizationRemarkEmitter *ORE, bool FreeInLoop) {
   LLVM_DEBUG(dbgs() << "LICM sinking instruction: " << I << "\n");
   ORE->emit([&]() {
@@ -1090,7 +1095,7 @@ static bool sink(Instruction &I, LoopInfo *LI, DominatorTree *DT,
     Instruction *New = sinkThroughTriviallyReplaceablePHI(PN, &I, LI, SunkCopies,
                                                           SafetyInfo, CurLoop);
     PN->replaceAllUsesWith(New);
-    eraseInstruction(*PN, nullptr);
+    eraseInstruction(*PN, *SafetyInfo, nullptr);
     Changed = true;
   }
   return Changed;
@@ -1100,7 +1105,7 @@ static bool sink(Instruction &I, LoopInfo *LI, DominatorTree *DT,
 /// is safe to hoist, this instruction is called to do the dirty work.
 ///
 static void hoist(Instruction &I, const DominatorTree *DT, const Loop *CurLoop,
-                  LoopSafetyInfo *SafetyInfo, OptimizationRemarkEmitter *ORE) {
+                  ICFLoopSafetyInfo *SafetyInfo, OptimizationRemarkEmitter *ORE) {
   auto *Preheader = CurLoop->getLoopPreheader();
   LLVM_DEBUG(dbgs() << "LICM hoisting to " << Preheader->getName() << ": " << I
                     << "\n");
@@ -1120,6 +1125,8 @@ static void hoist(Instruction &I, const DominatorTree *DT, const Loop *CurLoop,
       !SafetyInfo->isGuaranteedToExecute(I, DT, CurLoop))
     I.dropUnknownNonDebugMetadata();
 
+  SafetyInfo->removeInstruction(&I);
+  SafetyInfo->insertInstructionTo(Preheader);
   // Move the new node to the Preheader, before its terminator.
   I.moveBefore(Preheader->getTerminator());
 
@@ -1180,6 +1187,7 @@ class LoopPromoter : public LoadAndStorePromoter {
   int Alignment;
   bool UnorderedAtomic;
   AAMDNodes AATags;
+  ICFLoopSafetyInfo &SafetyInfo;
 
   Value *maybeInsertLCSSAPHI(Value *V, BasicBlock *BB) const {
     if (Instruction *I = dyn_cast<Instruction>(V))
@@ -1202,11 +1210,13 @@ public:
                SmallVectorImpl<BasicBlock *> &LEB,
                SmallVectorImpl<Instruction *> &LIP, PredIteratorCache &PIC,
                AliasSetTracker &ast, LoopInfo &li, DebugLoc dl, int alignment,
-               bool UnorderedAtomic, const AAMDNodes &AATags)
+               bool UnorderedAtomic, const AAMDNodes &AATags,
+               ICFLoopSafetyInfo &SafetyInfo)
       : LoadAndStorePromoter(Insts, S), SomePtr(SP), PointerMustAliases(PMA),
         LoopExitBlocks(LEB), LoopInsertPts(LIP), PredCache(PIC), AST(ast),
         LI(li), DL(std::move(dl)), Alignment(alignment),
-        UnorderedAtomic(UnorderedAtomic), AATags(AATags) {}
+        UnorderedAtomic(UnorderedAtomic), AATags(AATags), SafetyInfo(SafetyInfo)
+      {}
 
   bool isInstInList(Instruction *I,
                     const SmallVectorImpl<Instruction *> &) const override {
@@ -1243,7 +1253,10 @@ public:
     // Update alias analysis.
     AST.copyValue(LI, V);
   }
-  void instructionDeleted(Instruction *I) const override { AST.deleteValue(I); }
+  void instructionDeleted(Instruction *I) const override {
+    SafetyInfo.removeInstruction(I);
+    AST.deleteValue(I);
+  }
 };
 
 
@@ -1281,7 +1294,7 @@ bool llvm::promoteLoopAccessesToScalars(
     SmallVectorImpl<BasicBlock *> &ExitBlocks,
     SmallVectorImpl<Instruction *> &InsertPts, PredIteratorCache &PIC,
     LoopInfo *LI, DominatorTree *DT, const TargetLibraryInfo *TLI,
-    Loop *CurLoop, AliasSetTracker *CurAST, LoopSafetyInfo *SafetyInfo,
+    Loop *CurLoop, AliasSetTracker *CurAST, ICFLoopSafetyInfo *SafetyInfo,
     OptimizationRemarkEmitter *ORE) {
   // Verify inputs.
   assert(LI != nullptr && DT != nullptr && CurLoop != nullptr &&
@@ -1500,7 +1513,7 @@ bool llvm::promoteLoopAccessesToScalars(
   SSAUpdater SSA(&NewPHIs);
   LoopPromoter Promoter(SomePtr, LoopUses, SSA, PointerMustAliases, ExitBlocks,
                         InsertPts, PIC, *CurAST, *LI, DL, Alignment,
-                        SawUnorderedAtomic, AATags);
+                        SawUnorderedAtomic, AATags, *SafetyInfo);
 
   // Set up the preheader to have a definition of the value.  It is the live-out
   // value from the preheader that uses in the loop will use.
@@ -1520,7 +1533,7 @@ bool llvm::promoteLoopAccessesToScalars(
 
   // If the SSAUpdater didn't use the load in the preheader, just zap it now.
   if (PreheaderLoad->use_empty())
-    eraseInstruction(*PreheaderLoad, CurAST);
+    eraseInstruction(*PreheaderLoad, *SafetyInfo, CurAST);
 
   return true;
 }
diff --git a/llvm/test/CodeGen/AMDGPU/build-vector-insert-elt-infloop.ll b/llvm/test/CodeGen/AMDGPU/build-vector-insert-elt-infloop.ll
index 865dccb2791..fd81c0438d6 100644
--- a/llvm/test/CodeGen/AMDGPU/build-vector-insert-elt-infloop.ll
+++ b/llvm/test/CodeGen/AMDGPU/build-vector-insert-elt-infloop.ll
@@ -18,7 +18,7 @@ bb1:
   %tmp4 = bitcast half %tmp3 to i16
   %tmp5 = insertelement <2 x i16> <i16 0, i16 undef>, i16 %tmp4, i32 1
   %tmp6 = bitcast i16* %arg to half*
-  store half %tmp2, half* %tmp6, align 2
+  store volatile half %tmp2, half* %tmp6, align 2
   %tmp7 = bitcast <2 x i16> %tmp to <2 x half>
   %tmp8 = extractelement <2 x half> %tmp7, i32 0
   br label %bb1
diff --git a/llvm/test/Transforms/LICM/guards.ll b/llvm/test/Transforms/LICM/guards.ll
index b2f672104f8..b37c4189284 100644
--- a/llvm/test/Transforms/LICM/guards.ll
+++ b/llvm/test/Transforms/LICM/guards.ll
@@ -85,15 +85,15 @@ loop:
 }
 
 
-; TODO: We can also hoist this load and guard from mustexec non-header block.
+; TODO: We can also hoist this guard from mustexec non-header block.
 define void @test4(i1 %c, i32* %p) {
 
 ; CHECK-LABEL: @test4(
 ; CHECK-LABEL: entry:
-; CHECK-LABEL: loop:
-; CHECK-LABEL: backedge:
 ; CHECK:       %a = load i32, i32* %p
 ; CHECK:       %invariant_cond = icmp ne i32 %a, 100
+; CHECK-LABEL: loop:
+; CHECK-LABEL: backedge:
 ; CHECK:       call void (i1, ...) @llvm.experimental.guard(i1 %invariant_cond)
 
 entry:
diff --git a/llvm/test/Transforms/LICM/hoist-mustexec.ll b/llvm/test/Transforms/LICM/hoist-mustexec.ll
index 5bce1fbce1c..53f78e88f72 100644
--- a/llvm/test/Transforms/LICM/hoist-mustexec.ll
+++ b/llvm/test/Transforms/LICM/hoist-mustexec.ll
@@ -456,3 +456,150 @@ backedge:
 exit:
   ret void
 }
+
+; Check that we can hoist a mustexecute load from backedge even if something
+; throws after it.
+define void @test_hoist_from_backedge_01(i32* %p, i32 %n) {
+
+; CHECK-LABEL: @test_hoist_from_backedge_01(
+; CHECK:       entry:
+; CHECK-NEXT:  %load = load i32, i32* %p
+; CHECK-NOT:   load i32
+
+entry:
+  br label %loop
+
+loop:
+  %iv = phi i32 [ 0, %entry ], [ %iv.next, %backedge ]
+  %dummy = phi i32 [ 0, %entry ], [ %merge, %backedge ]
+  %cond = icmp slt i32 %iv, %n
+  br i1 %cond, label %if.true, label %if.false
+
+if.true:
+  %a = add i32 %iv, %iv
+  br label %backedge
+
+if.false:
+  %b = mul i32 %iv, %iv
+  br label %backedge
+
+backedge:
+  %merge = phi i32 [ %a, %if.true ], [ %b, %if.false ]
+  %iv.next = add i32 %iv, %merge
+  %load = load i32, i32* %p
+  call void @may_throw()
+  %loop.cond = icmp ult i32 %iv.next, %load
+  br i1 %loop.cond, label %loop, label %exit
+
+exit:
+  ret void
+}
+
+; Check that we don't hoist the load if something before it can throw.
+define void @test_hoist_from_backedge_02(i32* %p, i32 %n) {
+
+; CHECK-LABEL: @test_hoist_from_backedge_02(
+; CHECK:       entry:
+; CHECK:       loop:
+; CHECK:       %load = load i32, i32* %p
+
+entry:
+  br label %loop
+
+loop:
+  %iv = phi i32 [ 0, %entry ], [ %iv.next, %backedge ]
+  %dummy = phi i32 [ 0, %entry ], [ %merge, %backedge ]
+  %cond = icmp slt i32 %iv, %n
+  br i1 %cond, label %if.true, label %if.false
+
+if.true:
+  %a = add i32 %iv, %iv
+  br label %backedge
+
+if.false:
+  %b = mul i32 %iv, %iv
+  br label %backedge
+
+backedge:
+  %merge = phi i32 [ %a, %if.true ], [ %b, %if.false ]
+  %iv.next = add i32 %iv, %merge
+  call void @may_throw()
+  %load = load i32, i32* %p
+  %loop.cond = icmp ult i32 %iv.next, %load
+  br i1 %loop.cond, label %loop, label %exit
+
+exit:
+  ret void
+}
+
+define void @test_hoist_from_backedge_03(i32* %p, i32 %n) {
+
+; CHECK-LABEL: @test_hoist_from_backedge_03(
+; CHECK:       entry:
+; CHECK:       loop:
+; CHECK:       %load = load i32, i32* %p
+
+entry:
+  br label %loop
+
+loop:
+  %iv = phi i32 [ 0, %entry ], [ %iv.next, %backedge ]
+  %dummy = phi i32 [ 0, %entry ], [ %merge, %backedge ]
+  %cond = icmp slt i32 %iv, %n
+  br i1 %cond, label %if.true, label %if.false
+
+if.true:
+  %a = add i32 %iv, %iv
+  br label %backedge
+
+if.false:
+  %b = mul i32 %iv, %iv
+  call void @may_throw()
+  br label %backedge
+
+backedge:
+  %merge = phi i32 [ %a, %if.true ], [ %b, %if.false ]
+  %iv.next = add i32 %iv, %merge
+  %load = load i32, i32* %p
+  %loop.cond = icmp ult i32 %iv.next, %load
+  br i1 %loop.cond, label %loop, label %exit
+
+exit:
+  ret void
+}
+
+define void @test_hoist_from_backedge_04(i32* %p, i32 %n) {
+
+; CHECK-LABEL: @test_hoist_from_backedge_04(
+; CHECK:       entry:
+; CHECK:       loop:
+; CHECK:       %load = load i32, i32* %p
+
+entry:
+  br label %loop
+
+loop:
+  %iv = phi i32 [ 0, %entry ], [ %iv.next, %backedge ]
+  %dummy = phi i32 [ 0, %entry ], [ %merge, %backedge ]
+  call void @may_throw()
+  %cond = icmp slt i32 %iv, %n
+  br i1 %cond, label %if.true, label %if.false
+
+if.true:
+  %a = add i32 %iv, %iv
+  br label %backedge
+
+if.false:
+  %b = mul i32 %iv, %iv
+  br label %backedge
+
+backedge:
+  %merge = phi i32 [ %a, %if.true ], [ %b, %if.false ]
+  %iv.next = add i32 %iv, %merge
+  %load = load i32, i32* %p
+  %loop.cond = icmp ult i32 %iv.next, %load
+  br i1 %loop.cond, label %loop, label %exit
+
+exit:
+  ret void
+}
diff --git a/llvm/test/Transforms/LICM/hoist-nounwind.ll b/llvm/test/Transforms/LICM/hoist-nounwind.ll
index 9fc4903b830..d53e4043af1 100644
--- a/llvm/test/Transforms/LICM/hoist-nounwind.ll
+++ b/llvm/test/Transforms/LICM/hoist-nounwind.ll
@@ -49,14 +49,16 @@ for.cond.cleanup:
   ret i32 0
 }
 
-; Don't hoist load past volatile load.
+; Hoist a non-volatile load past volatile load.
 define i32 @test3(i32* noalias nocapture readonly %a, i32* %v) nounwind uwtable {
 ; CHECK-LABEL: @test3(
 entry:
   br label %for.body
 
+; CHECK: load i32
+; CHECK: for.body:
 ; CHECK: load volatile i32
-; CHECK-NEXT: load i32
+; CHECK-NOT: load
 for.body:
   %i.06 = phi i32 [ 0, %entry ], [ %inc, %for.body ]
   %x.05 = phi i32 [ 0, %entry ], [ %add, %for.body ]
@@ -70,3 +72,26 @@ for.body:
 for.cond.cleanup:
   ret i32 %add
 }
+
+; Don't a volatile load past volatile load.
+define i32 @test4(i32* noalias nocapture readonly %a, i32* %v) nounwind uwtable {
+; CHECK-LABEL: @test4(
+entry:
+  br label %for.body
+
+; CHECK: for.body:
+; CHECK: load volatile i32
+; CHECK-NEXT: load volatile i32
+for.body:
+  %i.06 = phi i32 [ 0, %entry ], [ %inc, %for.body ]
+  %x.05 = phi i32 [ 0, %entry ], [ %add, %for.body ]
+  %xxx = load volatile i32, i32* %v, align 4
+  %i1 = load volatile i32, i32* %a, align 4
+  %add = add nsw i32 %i1, %x.05
+  %inc = add nuw nsw i32 %i.06, 1
+  %exitcond = icmp eq i32 %inc, 1000
+  br i1 %exitcond, label %for.cond.cleanup, label %for.body
+
+for.cond.cleanup:
+  ret i32 %add
+}
\ No newline at end of file
diff --git a/llvm/test/Transforms/LICM/preheader-safe.ll b/llvm/test/Transforms/LICM/preheader-safe.ll
index 0bfe123862c..03a7258df11 100644
--- a/llvm/test/Transforms/LICM/preheader-safe.ll
+++ b/llvm/test/Transforms/LICM/preheader-safe.ll
@@ -112,11 +112,31 @@ loop-if:
 exit:
   ret void
 }
+
+; Positive test - can hoist something that happens before thrower.
+define void @nothrow_header_pos(i64 %x, i64 %y, i1 %cond) {
+; CHECK-LABEL: nothrow_header_pos
+; CHECK-LABEL: entry
+; CHECK: %div = udiv i64 %x, %y
+; CHECK-LABEL: loop
+; CHECK: call void @use(i64 %div)
+entry:
+  br label %loop
+loop:                                         ; preds = %entry, %for.inc
+  br label %loop-if
+loop-if:
+  %div = udiv i64 %x, %y
+  call void @use(i64 %div)
+  br label %loop
+}
+
+
 ; Negative test - can't move out of throwing block
 define void @nothrow_header_neg(i64 %x, i64 %y, i1 %cond) {
 ; CHECK-LABEL: nothrow_header_neg
 ; CHECK-LABEL: entry
 ; CHECK-LABEL: loop
+; CHECK: call void @maythrow()
 ; CHECK: %div = udiv i64 %x, %y
 ; CHECK: call void @use(i64 %div)
 entry:
@@ -124,6 +144,7 @@ entry:
 loop:                                         ; preds = %entry, %for.inc
   br label %loop-if
 loop-if:
+  call void @maythrow()
   %div = udiv i64 %x, %y
   call void @use(i64 %div)
   br label %loop