LICM: hoist calls to global_init functions

Global initializers are executed only once.
Therefore it's possible to hoist such an initializer call to a loop pre-header - in case there are no conflicting side-effects in the loop before the call.
Also, the call must post-dominate the loop pre-header. Otherwise it would be executed speculatively.

Author: eeckstein

lib/SILOptimizer/LoopTransforms/LICM.cpp

@@ -146,6 +146,57 @@ static bool mayWriteTo(AliasAnalysis *AA, SideEffectAnalysis *SEA,
   return false;
 }
 
+/// Returns true if \p sideEffectInst cannot be reordered with a call to a
+/// global initialier.
+static bool mayConflictWithGlobalInit(AliasAnalysis *AA,
+                    SILInstruction *sideEffectInst, ApplyInst *globalInitCall) {
+  if (auto *SI = dyn_cast<StoreInst>(sideEffectInst)) {
+    return AA->mayReadOrWriteMemory(globalInitCall, SI->getDest());
+  }
+  if (auto *LI = dyn_cast<LoadInst>(sideEffectInst)) {
+    return AA->mayWriteToMemory(globalInitCall, LI->getOperand());
+  }
+  return true;
+}
+
+/// Returns true if any of the instructions in \p sideEffectInsts which are
+/// post-dominated by a call to a global initialier cannot be reordered with
+/// the call.
+static bool mayConflictWithGlobalInit(AliasAnalysis *AA,
+                       InstSet &sideEffectInsts,
+                       ApplyInst *globalInitCall,
+                       SILBasicBlock *preHeader, PostDominanceInfo *PD) {
+  if (!PD->dominates(globalInitCall->getParent(), preHeader))
+    return true;
+
+  SILBasicBlock *globalInitBlock = globalInitCall->getParent();
+  for (auto *seInst : sideEffectInsts) {
+    // Only check instructions in blocks which are "before" (i.e. post-dominated
+    // by) the block which contains the init-call.
+    // Instructions which are before the call in the same block have already
+    // been checked.
+    if (PD->properlyDominates(globalInitBlock, seInst->getParent())) {
+      if (mayConflictWithGlobalInit(AA, seInst, globalInitCall))
+        return true;
+    }
+  }
+  return false;
+}
+
+/// Returns true if any of the instructions in \p sideEffectInsts cannot be
+/// reordered with a call to a global initialier (which is in the same basic
+/// block).
+static bool mayConflictWithGlobalInit(AliasAnalysis *AA,
+                       ArrayRef<SILInstruction *> sideEffectInsts,
+                       ApplyInst *globalInitCall) {
+  for (auto *seInst : sideEffectInsts) {
+    assert(seInst->getParent() == globalInitCall->getParent());
+    if (mayConflictWithGlobalInit(AA, seInst, globalInitCall))
+      return true;
+  }
+  return false;
+}
+
 // When Hoisting / Sinking,
 // Don't descend into control-dependent code.
 // Only traverse into basic blocks that dominate all exits.
@@ -409,6 +460,8 @@ class LoopTreeOptimization {
   AliasAnalysis *AA;
   SideEffectAnalysis *SEA;
   DominanceInfo *DomTree;
+  PostDominanceAnalysis *PDA;
+  PostDominanceInfo *postDomTree = nullptr;
   AccessedStorageAnalysis *ASA;
   bool Changed;
 
@@ -435,10 +488,11 @@ class LoopTreeOptimization {
 public:
   LoopTreeOptimization(SILLoop *TopLevelLoop, SILLoopInfo *LI,
                        AliasAnalysis *AA, SideEffectAnalysis *SEA,
-                       DominanceInfo *DT, AccessedStorageAnalysis *ASA,
+                       DominanceInfo *DT, PostDominanceAnalysis *PDA,
+                       AccessedStorageAnalysis *ASA,
                        bool RunsOnHighLevelSil)
-      : LoopInfo(LI), AA(AA), SEA(SEA), DomTree(DT), ASA(ASA), Changed(false),
-        RunsOnHighLevelSIL(RunsOnHighLevelSil) {
+      : LoopInfo(LI), AA(AA), SEA(SEA), DomTree(DT), PDA(PDA), ASA(ASA),
+        Changed(false), RunsOnHighLevelSIL(RunsOnHighLevelSil) {
     // Collect loops for a recursive bottom-up traversal in the loop tree.
     BotUpWorkList.push_back(TopLevelLoop);
     for (unsigned i = 0; i < BotUpWorkList.size(); ++i) {
@@ -556,9 +610,11 @@ static bool isSafeReadOnlyApply(SideEffectAnalysis *SEA, ApplyInst *AI) {
 }
 
 static void checkSideEffects(swift::SILInstruction &Inst,
-                             InstSet &SideEffectInsts) {
+                      InstSet &SideEffectInsts,
+                      SmallVectorImpl<SILInstruction *> &sideEffectsInBlock) {
   if (Inst.mayHaveSideEffects()) {
     SideEffectInsts.insert(&Inst);
+    sideEffectsInBlock.push_back(&Inst);
   }
 }
 
@@ -708,13 +764,15 @@ void LoopTreeOptimization::analyzeCurrentLoop(
 
   // Interesting instructions in the loop:
   SmallVector<ApplyInst *, 8> ReadOnlyApplies;
+  SmallVector<ApplyInst *, 8> globalInitCalls;
   SmallVector<LoadInst *, 8> Loads;
   SmallVector<StoreInst *, 8> Stores;
   SmallVector<FixLifetimeInst *, 8> FixLifetimes;
   SmallVector<BeginAccessInst *, 8> BeginAccesses;
   SmallVector<FullApplySite, 8> fullApplies;
 
   for (auto *BB : Loop->getBlocks()) {
+    SmallVector<SILInstruction *, 8> sideEffectsInBlock;
     for (auto &Inst : *BB) {
       switch (Inst.getKind()) {
       case SILInstructionKind::FixLifetimeInst: {
@@ -731,12 +789,12 @@ void LoopTreeOptimization::analyzeCurrentLoop(
       case SILInstructionKind::StoreInst: {
         Stores.push_back(cast<StoreInst>(&Inst));
         LoadsAndStores.push_back(&Inst);
-        checkSideEffects(Inst, sideEffects);
+        checkSideEffects(Inst, sideEffects, sideEffectsInBlock);
         break;
       }
       case SILInstructionKind::BeginAccessInst:
         BeginAccesses.push_back(cast<BeginAccessInst>(&Inst));
-        checkSideEffects(Inst, sideEffects);
+        checkSideEffects(Inst, sideEffects, sideEffectsInBlock);
         break;
       case SILInstructionKind::RefElementAddrInst:
         SpecialHoist.push_back(cast<RefElementAddrInst>(&Inst));
@@ -747,12 +805,21 @@ void LoopTreeOptimization::analyzeCurrentLoop(
         // cond_fail that would have protected (executed before) a memory access
         // must - after hoisting - also be executed before said access.
         HoistUp.insert(&Inst);
-        checkSideEffects(Inst, sideEffects);
+        checkSideEffects(Inst, sideEffects, sideEffectsInBlock);
         break;
       case SILInstructionKind::ApplyInst: {
         auto *AI = cast<ApplyInst>(&Inst);
         if (isSafeReadOnlyApply(SEA, AI)) {
           ReadOnlyApplies.push_back(AI);
+        } else if (SILFunction *callee = AI->getReferencedFunctionOrNull()) {
+          // Calls to global inits are different because we don't care about
+          // side effects which are "after" the call in the loop.
+          if (callee->isGlobalInit() &&
+              // Check against side-effects within the same block.
+              // Side-effects in other blocks are checked later (after we
+              // scanned all blocks of the loop).
+              !mayConflictWithGlobalInit(AA, sideEffectsInBlock, AI))
+            globalInitCalls.push_back(AI);
         }
         // check for array semantics and side effects - same as default
         LLVM_FALLTHROUGH;
@@ -761,7 +828,7 @@ void LoopTreeOptimization::analyzeCurrentLoop(
         if (auto fullApply = FullApplySite::isa(&Inst)) {
           fullApplies.push_back(fullApply);
         }
-        checkSideEffects(Inst, sideEffects);
+        checkSideEffects(Inst, sideEffects, sideEffectsInBlock);
         if (canHoistUpDefault(&Inst, Loop, DomTree, RunsOnHighLevelSIL)) {
           HoistUp.insert(&Inst);
         }
@@ -780,6 +847,23 @@ void LoopTreeOptimization::analyzeCurrentLoop(
       HoistUp.insert(LI);
     }
   }
+
+  if (!globalInitCalls.empty()) {
+    if (!postDomTree) {
+      postDomTree = PDA->get(Preheader->getParent());
+    }
+    if (postDomTree->getRootNode()) {
+      for (ApplyInst *ginitCall : globalInitCalls) {
+        // Check against side effects which are "before" (i.e. post-dominated
+        // by) the global initializer call.
+        if (!mayConflictWithGlobalInit(AA, sideEffects, ginitCall, Preheader,
+             postDomTree)) {
+          HoistUp.insert(ginitCall);
+        }
+      }
+    }
+  }
+
   // Collect memory locations for which we can move all loads and stores out
   // of the loop.
   for (StoreInst *SI : Stores) {
@@ -1041,6 +1125,7 @@ class LICM : public SILFunctionTransform {
     }
 
     DominanceAnalysis *DA = PM->getAnalysis<DominanceAnalysis>();
+    PostDominanceAnalysis *PDA = PM->getAnalysis<PostDominanceAnalysis>();
     AliasAnalysis *AA = PM->getAnalysis<AliasAnalysis>();
     SideEffectAnalysis *SEA = PM->getAnalysis<SideEffectAnalysis>();
     AccessedStorageAnalysis *ASA = getAnalysis<AccessedStorageAnalysis>();
@@ -1051,8 +1136,8 @@ class LICM : public SILFunctionTransform {
 
     for (auto *TopLevelLoop : *LoopInfo) {
       if (!DomTree) DomTree = DA->get(F);
-      LoopTreeOptimization Opt(TopLevelLoop, LoopInfo, AA, SEA, DomTree, ASA,
-                               RunsOnHighLevelSil);
+      LoopTreeOptimization Opt(TopLevelLoop, LoopInfo, AA, SEA, DomTree, PDA,
+                               ASA, RunsOnHighLevelSil);
       Changed |= Opt.optimize();
     }
 

test/SILOptimizer/global_hoisting_crash.swift

@@ -0,0 +1,24 @@
+// RUN: %empty-directory(%t) 
+// RUN: %target-build-swift -O %s -o %t/a.out
+// RUN: %target-run %t/a.out | %FileCheck %s
+
+// REQUIRES: executable_test
+
+struct Teststruct {
+  static let s = Teststruct()
+
+  @inline(never)
+  init() {
+    let set = Set<String>()
+    for _ in set {
+      // Check that the global initializer is not hoisted out of this loop,
+      // resulting in a dispatch_once re-retrance crash.
+      _ = Teststruct.s
+    }
+  }
+}
+
+// CHECK: Teststruct
+print(Teststruct.s)
+
+

test/SILOptimizer/global_init_opt.swift

@@ -16,3 +16,18 @@ var gg: Int = {
 public func cse() -> Int {
   return gg + gg
 }
+
+// CHECK-LABEL: sil @$s4test4licmSiyF
+// CHECK: bb0:
+// CHECK:   builtin "once"
+// CHECK: bb1:
+// CHECK-NOT:   builtin "once"
+// CHECK: } // end sil function '$s4test4licmSiyF'
+public func licm() -> Int {
+  var s = 0
+  for _ in 0..<100 {
+    s += gg
+  }
+  return s
+}
+