[LICM] Report failing to hoist conditionally-executed loads

[anemet]

Summary

These are interesting again because the user may not be aware that this
is a common reason preventing LICM.

A const is removed from an instruction pointer declaration in order to
pass it to ORE.

[anemet]

Ping.

[anemet]

Rebase on top of the changes requested in D27938

[hfinkel]

This is a great idea! Users are very often unaware of this complication.

lib/Transforms/Scalar/LICM.cpp

797 ↗

(On Diff #83102)

I have the same question here as I did in D27939, is the CurLoop->isLoopInvariant check needed here? At least in the call on line 407, the call is guarded by CurLoop->hasLoopInvariantOperands.

[anemet]

lib/Transforms/Scalar/LICM.cpp

797 ↗

(On Diff #83102)

Similarly to D27939, I think, here too we need the check.

When isSafeToExecuteUnconditionally is called from canSinkOrHoistInst and we're sinking then the address is not necessarily loop-invariant.

[hfinkel]

LGTM

llvm/trunk/lib/Transforms/Scalar/LICM.cpp

static bool isNotUsedInLoop(const Instruction &I, const Loop *CurLoop,
                             const LoopSafetyInfo *SafetyInfo);
 static bool hoist(Instruction &I, const DominatorTree *DT, const Loop *CurLoop,
                   const LoopSafetyInfo *SafetyInfo,
                   OptimizationRemarkEmitter *ORE);
 static bool sink(Instruction &I, const LoopInfo *LI, const DominatorTree *DT,
                  const Loop *CurLoop, AliasSetTracker *CurAST,
                  const LoopSafetyInfo *SafetyInfo,
                  OptimizationRemarkEmitter *ORE);
-static bool isSafeToExecuteUnconditionally(const Instruction &Inst,
+static bool isSafeToExecuteUnconditionally(Instruction &Inst,
                                            const DominatorTree *DT,
                                            const Loop *CurLoop,
                                            const LoopSafetyInfo *SafetyInfo,
+                                           OptimizationRemarkEmitter *ORE,
                                            const Instruction *CtxI = nullptr);
 static bool pointerInvalidatedByLoop(Value *V, uint64_t Size,
                                      const AAMDNodes &AAInfo,
                                      AliasSetTracker *CurAST);
 static Instruction *
 CloneInstructionInExitBlock(Instruction &I, BasicBlock &ExitBlock, PHINode &PN,
                             const LoopInfo *LI,
                             const LoopSafetyInfo *SafetyInfo);
for (BasicBlock::iterator II = BB->begin(), E = BB->end(); II != E;) {
 
       // Try hoisting the instruction out to the preheader.  We can only do this
       // if all of the operands of the instruction are loop invariant and if it
       // is safe to hoist the instruction.
       //
       if (CurLoop->hasLoopInvariantOperands(&I) &&
           canSinkOrHoistInst(I, AA, DT, CurLoop, CurAST, SafetyInfo, ORE) &&
           isSafeToExecuteUnconditionally(
-              I, DT, CurLoop, SafetyInfo,
+              I, DT, CurLoop, SafetyInfo, ORE,
               CurLoop->getLoopPreheader()->getTerminator()))
         Changed |= hoist(I, DT, CurLoop, SafetyInfo, ORE);
     }
 
   const std::vector<DomTreeNode *> &Children = N->getChildren();
   for (DomTreeNode *Child : Children)
     Changed |=
         hoistRegion(Child, AA, LI, DT, TLI, CurLoop, CurAST, SafetyInfo, ORE);
else if (isa<CallInst>(I))
     ++NumMovedCalls;
   ++NumHoisted;
   return true;
 }
 
 /// Only sink or hoist an instruction if it is not a trapping instruction,
 /// or if the instruction is known not to trap when moved to the preheader.
 /// or if it is a trapping instruction and is guaranteed to execute.
-static bool isSafeToExecuteUnconditionally(const Instruction &Inst,
+static bool isSafeToExecuteUnconditionally(Instruction &Inst,
                                            const DominatorTree *DT,
                                            const Loop *CurLoop,
                                            const LoopSafetyInfo *SafetyInfo,
+                                           OptimizationRemarkEmitter *ORE,
                                            const Instruction *CtxI) {
   if (isSafeToSpeculativelyExecute(&Inst, CtxI, DT))
     return true;
 
-  return isGuaranteedToExecute(Inst, DT, CurLoop, SafetyInfo);
+  bool GuaranteedToExecute =
+      isGuaranteedToExecute(Inst, DT, CurLoop, SafetyInfo);
+
+  if (!GuaranteedToExecute) {
+    auto *LI = dyn_cast<LoadInst>(&Inst);
+    if (LI && CurLoop->isLoopInvariant(LI->getPointerOperand()))
+      ORE->emit(OptimizationRemarkMissed(
+                    DEBUG_TYPE, "LoadWithLoopInvariantAddressCondExecuted", LI)
+                << "failed to hoist load with loop-invariant address "
+                   "because load is conditionally executed");
+  }
+
+  return GuaranteedToExecute;
 }
 
 namespace {
 class LoopPromoter : public LoadAndStorePromoter {
   Value *SomePtr; // Designated pointer to store to.
   SmallPtrSetImpl<Value *> &PointerMustAliases;
   SmallVectorImpl<BasicBlock *> &LoopExitBlocks;
   SmallVectorImpl<Instruction *> &LoopInsertPts;
for (const auto &ASI : AS) {
     for (User *U : ASIV->users()) {
       // Ignore instructions that are outside the loop.
       Instruction *UI = dyn_cast<Instruction>(U);
       if (!UI || !CurLoop->contains(UI))
         continue;
 
       // If there is an non-load/store instruction in the loop, we can't promote
       // it.
-      if (const LoadInst *Load = dyn_cast<LoadInst>(UI)) {
+      if (LoadInst *Load = dyn_cast<LoadInst>(UI)) {
         assert(!Load->isVolatile() && "AST broken");
         if (!Load->isSimple())
           return false;
 
         if (!DereferenceableInPH)
           DereferenceableInPH = isSafeToExecuteUnconditionally(
-              *Load, DT, CurLoop, SafetyInfo, Preheader->getTerminator());
+              *Load, DT, CurLoop, SafetyInfo, ORE, Preheader->getTerminator());
       } else if (const StoreInst *Store = dyn_cast<StoreInst>(UI)) {
         // Stores *of* the pointer are not interesting, only stores *to* the
         // pointer.
         if (UI->getOperand(1) != ASIV)
           continue;
         assert(!Store->isVolatile() && "AST broken");
         if (!Store->isSimple())
           return false;

llvm/trunk/test/Transforms/LICM/opt-remarks-conditional-load.ll

+; RUN: opt < %s -licm -pass-remarks-missed=licm -o /dev/null 2>&1 | FileCheck %s
+; RUN: opt -aa-pipeline=basic-aa -passes='require<aa>,require<targetir>,require<scalar-evolution>,require<opt-remark-emit>,loop(licm)' %s -o /dev/null -pass-remarks-missed=licm 2>&1 | FileCheck %s
+target datalayout = "E-p:64:64:64-a0:0:8-f32:32:32-f64:64:64-i1:8:8-i8:8:8-i16:16:16-i32:32:32-i64:32:64-v64:64:64-v128:128:128"
+
+; With the load from %p conditional, we can't optmize this and the remark
+; should tell us about it.
+
+define void @test(i32* %array, i32* noalias %p) {
+Entry:
+  br label %Loop
+
+Loop:
+  %j = phi i32 [ 0, %Entry ], [ %Next, %else]
+  %addr = getelementptr i32, i32* %array, i32 %j
+  %a = load i32, i32* %addr
+  %c = icmp eq i32 %a, 0
+  br i1 %c, label %then, label %else
+
+then:
+; CHECK: remark: /tmp/kk.c:2:20: failed to hoist load with loop-invariant address because load is conditionally executed
+  %b = load i32, i32* %p, !dbg !8
+  %a2 = add i32 %a, %b
+  store i32 %a2, i32* %addr
+  br label %else
+
+else:
+  %Next = add i32 %j, 1
+  %cond = icmp eq i32 %Next, 0
+  br i1 %cond, label %Out, label %Loop
+
+Out:
+  ret void
+}
+
+!llvm.dbg.cu = !{!0}
+!llvm.module.flags = !{!3, !4}
+!llvm.ident = !{!5}
+
+!0 = distinct !DICompileUnit(language: DW_LANG_C99, file: !1, producer: "clang version 3.9.0 ", isOptimized: true, runtimeVersion: 0, emissionKind: NoDebug, enums: !2)
+!1 = !DIFile(filename: "/tmp/kk.c", directory: "/tmp")
+!2 = !{}
+!3 = !{i32 2, !"Debug Info Version", i32 3}
+!4 = !{i32 1, !"PIC Level", i32 2}
+!5 = !{!"clang version 3.9.0 "}
+!6 = distinct !DISubprogram(name: "success", scope: !1, file: !1, line: 1, type: !7, isLocal: false, isDefinition: true, scopeLine: 1, flags: DIFlagPrototyped, isOptimized: true, unit: !0, variables: !2)
+!7 = !DISubroutineType(types: !2)
+!8 = !DILocation(line: 2, column: 20, scope: !6)