[X86,SimplifyCFG] Support hoisting load/store with conditional faulting (Part I) (#96878)

This is simplifycfg part of #95515 In this PR, we support hoisting load/store with conditional faulting in `SimplifyCFGOpt::speculativelyExecuteBB` to eliminate conditional branches. This is for cases like ``` void test (int a, int *b) { if (a) *b = a; } ``` In the following patches, we will support the hoist in `SimplifyCFGOpt::hoistCommonCodeFromSuccessors`. That is for cases like ``` void test (int a, int *c, int *d) { if (a) *c = a; else *d = a; } ```

Author: KanRobert

llvm/include/llvm/Transforms/Utils/SimplifyCFGOptions.h

@@ -27,6 +27,7 @@ struct SimplifyCFGOptions {
  bool ConvertSwitchToLookupTable = false;
  bool NeedCanonicalLoop = true;
  bool HoistCommonInsts = false;
+ bool HoistLoadsStoresWithCondFaulting = false;
  bool SinkCommonInsts = false;
  bool SimplifyCondBranch = true;
  bool SpeculateBlocks = true;
@@ -59,6 +60,10 @@ struct SimplifyCFGOptions {
  HoistCommonInsts = B;
  return *this;
  }
+ SimplifyCFGOptions &hoistLoadsStoresWithCondFaulting(bool B) {
+ HoistLoadsStoresWithCondFaulting = B;
+ return *this;
+ }
  SimplifyCFGOptions &sinkCommonInsts(bool B) {
  SinkCommonInsts = B;
  return *this;

llvm/lib/Passes/PassBuilder.cpp

@@ -848,6 +848,8 @@ Expected<SimplifyCFGOptions> parseSimplifyCFGOptions(StringRef Params) {
  Result.needCanonicalLoops(Enable);
  } else if (ParamName == "hoist-common-insts") {
  Result.hoistCommonInsts(Enable);
+ } else if (ParamName == "hoist-loads-stores-with-cond-faulting") {
+ Result.hoistLoadsStoresWithCondFaulting(Enable);
  } else if (ParamName == "sink-common-insts") {
  Result.sinkCommonInsts(Enable);
  } else if (ParamName == "speculate-unpredictables") {

llvm/lib/Passes/PassBuilderPipelines.cpp

@@ -1534,9 +1534,11 @@ PassBuilder::buildModuleOptimizationPipeline(OptimizationLevel Level,
 
  // LoopSink (and other loop passes since the last simplifyCFG) might have
  // resulted in single-entry-single-exit or empty blocks. Clean up the CFG.
- OptimizePM.addPass(SimplifyCFGPass(SimplifyCFGOptions()
- .convertSwitchRangeToICmp(true)
- .speculateUnpredictables(true)));
+ OptimizePM.addPass(
+ SimplifyCFGPass(SimplifyCFGOptions()
+ .convertSwitchRangeToICmp(true)
+ .speculateUnpredictables(true)
+ .hoistLoadsStoresWithCondFaulting(true)));
 
  // Add the core optimizing pipeline.
  MPM.addPass(createModuleToFunctionPassAdaptor(std::move(OptimizePM),

llvm/lib/Transforms/Scalar/SimplifyCFGPass.cpp

@@ -73,6 +73,11 @@ static cl::opt<bool> UserHoistCommonInsts(
  "hoist-common-insts", cl::Hidden, cl::init(false),
  cl::desc("hoist common instructions (default = false)"));
 
+static cl::opt<bool> UserHoistLoadsStoresWithCondFaulting(
+ "hoist-loads-stores-with-cond-faulting", cl::Hidden, cl::init(false),
+ cl::desc("Hoist loads/stores if the target supports conditional faulting "
+ "(default = false)"));
+
 static cl::opt<bool> UserSinkCommonInsts(
  "sink-common-insts", cl::Hidden, cl::init(false),
  cl::desc("Sink common instructions (default = false)"));
@@ -326,6 +331,9 @@ static void applyCommandLineOverridesToOptions(SimplifyCFGOptions &Options) {
  Options.NeedCanonicalLoop = UserKeepLoops;
  if (UserHoistCommonInsts.getNumOccurrences())
  Options.HoistCommonInsts = UserHoistCommonInsts;
+ if (UserHoistLoadsStoresWithCondFaulting.getNumOccurrences())
+ Options.HoistLoadsStoresWithCondFaulting =
+ UserHoistLoadsStoresWithCondFaulting;
  if (UserSinkCommonInsts.getNumOccurrences())
  Options.SinkCommonInsts = UserSinkCommonInsts;
  if (UserSpeculateUnpredictables.getNumOccurrences())
@@ -354,6 +362,8 @@ void SimplifyCFGPass::printPipeline(
  << "switch-to-lookup;";
  OS << (Options.NeedCanonicalLoop ? "" : "no-") << "keep-loops;";
  OS << (Options.HoistCommonInsts ? "" : "no-") << "hoist-common-insts;";
+ OS << (Options.HoistLoadsStoresWithCondFaulting ? "" : "no-")
+ << "hoist-loads-stores-with-cond-faulting;";
  OS << (Options.SinkCommonInsts ? "" : "no-") << "sink-common-insts;";
  OS << (Options.SpeculateBlocks ? "" : "no-") << "speculate-blocks;";
  OS << (Options.SimplifyCondBranch ? "" : "no-") << "simplify-cond-branch;";

llvm/lib/Transforms/Utils/SimplifyCFG.cpp

@@ -117,6 +117,18 @@ static cl::opt<bool>
  HoistCommon("simplifycfg-hoist-common", cl::Hidden, cl::init(true),
  cl::desc("Hoist common instructions up to the parent block"));
 
+static cl::opt<bool> HoistLoadsStoresWithCondFaulting(
+ "simplifycfg-hoist-loads-stores-with-cond-faulting", cl::Hidden,
+ cl::init(true),
+ cl::desc("Hoist loads/stores if the target supports "
+ "conditional faulting"));
+
+static cl::opt<unsigned> HoistLoadsStoresWithCondFaultingThreshold(
+ "hoist-loads-stores-with-cond-faulting-threshold", cl::Hidden, cl::init(6),
+ cl::desc("Control the maximal conditonal load/store that we are willing "
+ "to speculatively execute to eliminate conditional branch "
+ "(default = 6)"));
+
 static cl::opt<unsigned>
  HoistCommonSkipLimit("simplifycfg-hoist-common-skip-limit", cl::Hidden,
  cl::init(20),
@@ -2986,6 +2998,25 @@ static bool isProfitableToSpeculate(const BranchInst *BI, bool Invert,
  return BIEndProb < Likely;
 }
 
+static bool isSafeCheapLoadStore(const Instruction *I,
+ const TargetTransformInfo &TTI) {
+ // Not handle volatile or atomic.
+ if (auto *L = dyn_cast<LoadInst>(I)) {
+ if (!L->isSimple())
+ return false;
+ } else if (auto *S = dyn_cast<StoreInst>(I)) {
+ if (!S->isSimple())
+ return false;
+ } else
+ return false;
+
+ // llvm.masked.load/store use i32 for alignment while load/store use i64.
+ // That's why we have the alignment limitation.
+ // FIXME: Update the prototype of the intrinsics?
+ return TTI.hasConditionalLoadStoreForType(getLoadStoreType(I)) &&
+ getLoadStoreAlignment(I) < Value::MaximumAlignment;
+}
+
 /// Speculate a conditional basic block flattening the CFG.
 ///
 /// Note that this is a very risky transform currently. Speculating
@@ -3060,6 +3091,9 @@ bool SimplifyCFGOpt::speculativelyExecuteBB(BranchInst *BI,
  SmallVector<Instruction *, 4> SpeculatedDbgIntrinsics;
 
  unsigned SpeculatedInstructions = 0;
+ bool HoistLoadsStores = HoistLoadsStoresWithCondFaulting &&
+ Options.HoistLoadsStoresWithCondFaulting;
+ SmallVector<Instruction *, 2> SpeculatedConditionalLoadsStores;
  Value *SpeculatedStoreValue = nullptr;
  StoreInst *SpeculatedStore = nullptr;
  EphemeralValueTracker EphTracker;
@@ -3088,22 +3122,33 @@ bool SimplifyCFGOpt::speculativelyExecuteBB(BranchInst *BI,
 
  // Only speculatively execute a single instruction (not counting the
  // terminator) for now.
- ++SpeculatedInstructions;
+ bool IsSafeCheapLoadStore = HoistLoadsStores &&
+ isSafeCheapLoadStore(&I, TTI) &&
+ SpeculatedConditionalLoadsStores.size() <
+ HoistLoadsStoresWithCondFaultingThreshold;
+ // Not count load/store into cost if target supports conditional faulting
+ // b/c it's cheap to speculate it.
+ if (IsSafeCheapLoadStore)
+ SpeculatedConditionalLoadsStores.push_back(&I);
+ else
+ ++SpeculatedInstructions;
+
  if (SpeculatedInstructions > 1)
  return false;
 
  // Don't hoist the instruction if it's unsafe or expensive.
- if (!isSafeToSpeculativelyExecute(&I) &&
- !(HoistCondStores && (SpeculatedStoreValue = isSafeToSpeculateStore(
- &I, BB, ThenBB, EndBB))))
+ if (!IsSafeCheapLoadStore && !isSafeToSpeculativelyExecute(&I) &&
+ !(HoistCondStores && !SpeculatedStoreValue &&
+ (SpeculatedStoreValue =
+ isSafeToSpeculateStore(&I, BB, ThenBB, EndBB))))
  return false;
- if (!SpeculatedStoreValue &&
+ if (!IsSafeCheapLoadStore && !SpeculatedStoreValue &&
  computeSpeculationCost(&I, TTI) >
  PHINodeFoldingThreshold * TargetTransformInfo::TCC_Basic)
  return false;
 
  // Store the store speculation candidate.
- if (SpeculatedStoreValue)
+ if (!SpeculatedStore && SpeculatedStoreValue)
  SpeculatedStore = cast<StoreInst>(&I);
 
  // Do not hoist the instruction if any of its operands are defined but not
@@ -3130,11 +3175,11 @@ bool SimplifyCFGOpt::speculativelyExecuteBB(BranchInst *BI,
 
  // Check that we can insert the selects and that it's not too expensive to do
  // so.
- bool Convert = SpeculatedStore != nullptr;
+ bool Convert =
+ SpeculatedStore != nullptr || !SpeculatedConditionalLoadsStores.empty();
  InstructionCost Cost = 0;
  Convert |= validateAndCostRequiredSelects(BB, ThenBB, EndBB,
- SpeculatedInstructions,
- Cost, TTI);
+ SpeculatedInstructions, Cost, TTI);
  if (!Convert || Cost > Budget)
  return false;
 
@@ -3222,6 +3267,107 @@ bool SimplifyCFGOpt::speculativelyExecuteBB(BranchInst *BI,
  BB->splice(BI->getIterator(), ThenBB, ThenBB->begin(),
  std::prev(ThenBB->end()));
 
+ // If the target supports conditional faulting,
+ // we look for the following pattern:
+ // \code
+ // BB:
+ // ...
+ // %cond = icmp ult %x, %y
+ // br i1 %cond, label %TrueBB, label %FalseBB
+ // FalseBB:
+ // store i32 1, ptr %q, align 4
+ // ...
+ // TrueBB:
+ // %maskedloadstore = load i32, ptr %b, align 4
+ // store i32 %maskedloadstore, ptr %p, align 4
+ // ...
+ // \endcode
+ //
+ // and transform it into:
+ //
+ // \code
+ // BB:
+ // ...
+ // %cond = icmp ult %x, %y
+ // %maskedloadstore = cload i32, ptr %b, %cond
+ // cstore i32 %maskedloadstore, ptr %p, %cond
+ // cstore i32 1, ptr %q, ~%cond
+ // br i1 %cond, label %TrueBB, label %FalseBB
+ // FalseBB:
+ // ...
+ // TrueBB:
+ // ...
+ // \endcode
+ //
+ // where cload/cstore are represented by llvm.masked.load/store intrinsics,
+ // e.g.
+ //
+ // \code
+ // %vcond = bitcast i1 %cond to <1 x i1>
+ // %v0 = call <1 x i32> @llvm.masked.load.v1i32.p0
+ // (ptr %b, i32 4, <1 x i1> %vcond, <1 x i32> poison)
+ // %maskedloadstore = bitcast <1 x i32> %v0 to i32
+ // call void @llvm.masked.store.v1i32.p0
+ // (<1 x i32> %v0, ptr %p, i32 4, <1 x i1> %vcond)
+ // %cond.not = xor i1 %cond, true
+ // %vcond.not = bitcast i1 %cond.not to <1 x i>
+ // call void @llvm.masked.store.v1i32.p0
+ // (<1 x i32> <i32 1>, ptr %q, i32 4, <1x i1> %vcond.not)
+ // \endcode
+ //
+ // So we need to turn hoisted load/store into cload/cstore.
+ auto &Context = BI->getParent()->getContext();
+ auto *VCondTy = FixedVectorType::get(Type::getInt1Ty(Context), 1);
+ auto *Cond = BI->getOperand(0);
+ Value *Mask = nullptr;
+ // Construct the condition if needed.
+ if (!SpeculatedConditionalLoadsStores.empty()) {
+ IRBuilder<> Builder(SpeculatedConditionalLoadsStores.back());
+ Mask = Builder.CreateBitCast(
+ Invert ? Builder.CreateXor(Cond, ConstantInt::getTrue(Context)) : Cond,
+ VCondTy);
+ }
+ for (auto *I : SpeculatedConditionalLoadsStores) {
+ IRBuilder<> Builder(I);
+ // We currently assume conditional faulting load/store is supported for
+ // scalar types only when creating new instructions. This can be easily
+ // extended for vector types in the future.
+ assert(!getLoadStoreType(I)->isVectorTy() && "not implemented");
+ auto *Op0 = I->getOperand(0);
+ Instruction *MaskedLoadStore = nullptr;
+ if (auto *LI = dyn_cast<LoadInst>(I)) {
+ // Handle Load.
+ auto *Ty = I->getType();
+ MaskedLoadStore = Builder.CreateMaskedLoad(FixedVectorType::get(Ty, 1),
+ Op0, LI->getAlign(), Mask);
+ I->replaceAllUsesWith(Builder.CreateBitCast(MaskedLoadStore, Ty));
+ } else {
+ // Handle Store.
+ auto *StoredVal =
+ Builder.CreateBitCast(Op0, FixedVectorType::get(Op0->getType(), 1));
+ MaskedLoadStore = Builder.CreateMaskedStore(
+ StoredVal, I->getOperand(1), cast<StoreInst>(I)->getAlign(), Mask);
+ }
+ // For non-debug metadata, only !annotation, !range, !nonnull and !align are
+ // kept when hoisting (see Instruction::dropUBImplyingAttrsAndMetadata).
+ //
+ // !nonnull, !align : Not support pointer type, no need to keep.
+ // !range: Load type is changed from scalar to vector, but the metadata on
+ // vector specifies a per-element range, so the semantics stay the
+ // same. Keep it.
+ // !annotation: Not impact semantics. Keep it.
+ I->dropUBImplyingAttrsAndUnknownMetadata(
+ {LLVMContext::MD_range, LLVMContext::MD_annotation});
+ // FIXME: DIAssignID is not supported for masked store yet.
+ // (Verifier::visitDIAssignIDMetadata)
+ at::deleteAssignmentMarkers(I);
+ I->eraseMetadataIf([](unsigned MDKind, MDNode *Node) {
+ return Node->getMetadataID() == Metadata::DIAssignIDKind;
+ });
+ MaskedLoadStore->copyMetadata(*I);
+ I->eraseFromParent();
+ }
+
  // Insert selects and rewrite the PHI operands.
  IRBuilder<NoFolder> Builder(BI);
  for (PHINode &PN : EndBB->phis()) {

llvm/test/Other/new-pm-print-pipeline.ll

@@ -49,8 +49,8 @@
 ; RUN: opt -disable-output -disable-verify -print-pipeline-passes -passes='function(print<stack-lifetime><may>,print<stack-lifetime><must>)' < %s | FileCheck %s --match-full-lines --check-prefixes=CHECK-17
 ; CHECK-17: function(print<stack-lifetime><may>,print<stack-lifetime><must>)
 
-; RUN: opt -disable-output -disable-verify -print-pipeline-passes -passes='function(simplifycfg<bonus-inst-threshold=5;forward-switch-cond;switch-to-lookup;keep-loops;hoist-common-insts;sink-common-insts;speculate-blocks;simplify-cond-branch;speculate-unpredictables>,simplifycfg<bonus-inst-threshold=7;no-forward-switch-cond;no-switch-to-lookup;no-keep-loops;no-hoist-common-insts;no-sink-common-insts;no-speculate-blocks;no-simplify-cond-branch;no-speculate-unpredictables>)' < %s | FileCheck %s --match-full-lines --check-prefixes=CHECK-18
-; CHECK-18: function(simplifycfg<bonus-inst-threshold=5;forward-switch-cond;no-switch-range-to-icmp;switch-to-lookup;keep-loops;hoist-common-insts;sink-common-insts;speculate-blocks;simplify-cond-branch;speculate-unpredictables>,simplifycfg<bonus-inst-threshold=7;no-forward-switch-cond;no-switch-range-to-icmp;no-switch-to-lookup;no-keep-loops;no-hoist-common-insts;no-sink-common-insts;no-speculate-blocks;no-simplify-cond-branch;no-speculate-unpredictables>)
+; RUN: opt -disable-output -disable-verify -print-pipeline-passes -passes='function(simplifycfg<bonus-inst-threshold=5;forward-switch-cond;switch-to-lookup;keep-loops;hoist-common-insts;hoist-loads-stores-with-cond-faulting;sink-common-insts;speculate-blocks;simplify-cond-branch;speculate-unpredictables>,simplifycfg<bonus-inst-threshold=7;no-forward-switch-cond;no-switch-to-lookup;no-keep-loops;no-hoist-common-insts;no-hoist-loads-stores-with-cond-faulting;no-sink-common-insts;no-speculate-blocks;no-simplify-cond-branch;no-speculate-unpredictables>)' < %s | FileCheck %s --match-full-lines --check-prefixes=CHECK-18
+; CHECK-18: function(simplifycfg<bonus-inst-threshold=5;forward-switch-cond;no-switch-range-to-icmp;switch-to-lookup;keep-loops;hoist-common-insts;hoist-loads-stores-with-cond-faulting;sink-common-insts;speculate-blocks;simplify-cond-branch;speculate-unpredictables>,simplifycfg<bonus-inst-threshold=7;no-forward-switch-cond;no-switch-range-to-icmp;no-switch-to-lookup;no-keep-loops;no-hoist-common-insts;no-hoist-loads-stores-with-cond-faulting;no-sink-common-insts;no-speculate-blocks;no-simplify-cond-branch;no-speculate-unpredictables>)
 
 ; RUN: opt -disable-output -disable-verify -print-pipeline-passes -passes='function(loop-vectorize<no-interleave-forced-only;no-vectorize-forced-only>,loop-vectorize<interleave-forced-only;vectorize-forced-only>)' < %s | FileCheck %s --match-full-lines --check-prefixes=CHECK-19
 ; CHECK-19: function(loop-vectorize<no-interleave-forced-only;no-vectorize-forced-only;>,loop-vectorize<interleave-forced-only;vectorize-forced-only;>)

llvm/test/Transforms/PhaseOrdering/X86/masked-memory-ops-with-cf.ll

@@ -0,0 +1,40 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
+; RUN: opt < %s -mtriple=x86_64 -mattr=+cf -O1 -S | FileCheck %s
+
+;; Test masked.load/store.v1* is generated in simplifycfg and not falls back to branch+load/store in following passes.
+define void @basic(i1 %cond, ptr %b, ptr %p, ptr %q) {
+; CHECK-LABEL: @basic(
+; CHECK-NEXT: entry:
+; CHECK-NEXT: [[TMP0:%.*]] = bitcast i1 [[COND:%.*]] to <1 x i1>
+; CHECK-NEXT: [[TMP1:%.*]] = call <1 x i16> @llvm.masked.load.v1i16.p0(ptr [[P:%.*]], i32 2, <1 x i1> [[TMP0]], <1 x i16> poison)
+; CHECK-NEXT: [[TMP2:%.*]] = bitcast <1 x i16> [[TMP1]] to i16
+; CHECK-NEXT: [[TMP3:%.*]] = call <1 x i32> @llvm.masked.load.v1i32.p0(ptr [[Q:%.*]], i32 4, <1 x i1> [[TMP0]], <1 x i32> poison)
+; CHECK-NEXT: [[TMP4:%.*]] = bitcast <1 x i32> [[TMP3]] to i32
+; CHECK-NEXT: [[TMP5:%.*]] = call <1 x i64> @llvm.masked.load.v1i64.p0(ptr [[B:%.*]], i32 8, <1 x i1> [[TMP0]], <1 x i64> poison)
+; CHECK-NEXT: [[TMP6:%.*]] = bitcast <1 x i64> [[TMP5]] to i64
+; CHECK-NEXT: [[TMP7:%.*]] = bitcast i16 [[TMP2]] to <1 x i16>
+; CHECK-NEXT: call void @llvm.masked.store.v1i16.p0(<1 x i16> [[TMP7]], ptr [[B]], i32 2, <1 x i1> [[TMP0]])
+; CHECK-NEXT: [[TMP8:%.*]] = bitcast i32 [[TMP4]] to <1 x i32>
+; CHECK-NEXT: call void @llvm.masked.store.v1i32.p0(<1 x i32> [[TMP8]], ptr [[P]], i32 4, <1 x i1> [[TMP0]])
+; CHECK-NEXT: [[TMP9:%.*]] = bitcast i64 [[TMP6]] to <1 x i64>
+; CHECK-NEXT: call void @llvm.masked.store.v1i64.p0(<1 x i64> [[TMP9]], ptr [[Q]], i32 8, <1 x i1> [[TMP0]])
+; CHECK-NEXT: ret void
+;
+entry:
+ br i1 %cond, label %if.true, label %if.false
+
+if.false:
+ br label %if.end
+
+if.true:
+ %pv = load i16, ptr %p, align 2
+ %qv = load i32, ptr %q, align 4
+ %bv = load i64, ptr %b, align 8
+ store i16 %pv, ptr %b, align 2
+ store i32 %qv, ptr %p, align 4
+ store i64 %bv, ptr %q, align 8
+ br label %if.false
+
+if.end:
+ ret void
+}