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-rw-r--r--lib/Transforms/InstCombine/InstCombineLoadStoreAlloca.cpp285
1 files changed, 285 insertions, 0 deletions
diff --git a/lib/Transforms/InstCombine/InstCombineLoadStoreAlloca.cpp b/lib/Transforms/InstCombine/InstCombineLoadStoreAlloca.cpp
index 88e16e9..0579c27 100644
--- a/lib/Transforms/InstCombine/InstCombineLoadStoreAlloca.cpp
+++ b/lib/Transforms/InstCombine/InstCombineLoadStoreAlloca.cpp
@@ -16,10 +16,20 @@
#include "llvm/Analysis/Loads.h"
#include "llvm/IR/DataLayout.h"
#include "llvm/IR/IntrinsicInst.h"
+#include "llvm/Support/CommandLine.h"
#include "llvm/Transforms/Utils/BasicBlockUtils.h"
#include "llvm/Transforms/Utils/Local.h"
using namespace llvm;
+/// Hidden option to stress test load slicing, i.e., when this option
+/// is enabled, load slicing bypasses most of its profitability guards.
+/// It will also generate, uncanonalized form of slicing.
+static cl::opt<bool>
+StressLoadSlicing("instcombine-stress-load-slicing", cl::Hidden,
+ cl::desc("Bypass the profitability model of load "
+ "slicing"),
+ cl::init(false));
+
STATISTIC(NumDeadStore, "Number of dead stores eliminated");
STATISTIC(NumGlobalCopies, "Number of allocas copied from constant global");
@@ -337,6 +347,274 @@ static Instruction *InstCombineLoadCast(InstCombiner &IC, LoadInst &LI,
return 0;
}
+namespace {
+ /// \brief Helper structure used to slice a load in smaller loads.
+ struct LoadedSlice {
+ // The last instruction that represent the slice. This should be a
+ // truncate instruction.
+ Instruction *Inst;
+ // The original load instruction.
+ LoadInst *Origin;
+ // The right shift amount in bits from the original load.
+ unsigned Shift;
+
+ LoadedSlice(Instruction *Inst = NULL, LoadInst *Origin = NULL,
+ unsigned Shift = 0)
+ : Inst(Inst), Origin(Origin), Shift(Shift) {}
+
+ LoadedSlice(const LoadedSlice& LS) : Inst(LS.Inst), Origin(LS.Origin),
+ Shift(LS.Shift) {}
+
+ /// \brief Get the bits used in a chunk of bits \p BitWidth large.
+ /// \return Result is \p BitWidth and has used bits set to 1 and
+ /// not used bits set to 0.
+ APInt getUsedBits() const {
+ // Reproduce the trunc(lshr) sequence:
+ // - Start from the truncated value.
+ // - Zero extend to the desired bit width.
+ // - Shift left.
+ assert(Origin && "No original load to compare against.");
+ unsigned BitWidth = Origin->getType()->getPrimitiveSizeInBits();
+ assert(Inst && "This slice is not bound to an instruction");
+ assert(Inst->getType()->getPrimitiveSizeInBits() <= BitWidth &&
+ "Extracted slice is smaller than the whole type!");
+ APInt UsedBits(Inst->getType()->getPrimitiveSizeInBits(), 0);
+ UsedBits.setAllBits();
+ UsedBits = UsedBits.zext(BitWidth);
+ UsedBits <<= Shift;
+ return UsedBits;
+ }
+
+ /// \brief Get the size of the slice to be loaded in bytes.
+ unsigned getLoadedSize() const {
+ unsigned SliceSize = getUsedBits().countPopulation();
+ assert(!(SliceSize & 0x7) && "Size is not a multiple of a byte.");
+ return SliceSize / 8;
+ }
+
+ /// \brief Get the offset in bytes of this slice in the original chunk of
+ /// bits, whose layout is defined by \p IsBigEndian.
+ uint64_t getOffsetFromBase(bool IsBigEndian) const {
+ assert(!(Shift & 0x7) && "Shifts not aligned on Bytes are not support.");
+ uint64_t Offset = Shift / 8;
+ unsigned TySizeInBytes = Origin->getType()->getPrimitiveSizeInBits() / 8;
+ assert(!(Origin->getType()->getPrimitiveSizeInBits() & 0x7) &&
+ "The size of the original loaded type is not a multiple of a"
+ " byte.");
+ // If Offset is bigger than TySizeInBytes, it means we are loading all
+ // zeros. This should have been optimized before in the process.
+ assert(TySizeInBytes > Offset &&
+ "Invalid shift amount for given loaded size");
+ if (IsBigEndian)
+ Offset = TySizeInBytes - Offset - getLoadedSize();
+ return Offset;
+ }
+
+ /// \brief Generate the sequence of instructions to load the slice
+ /// represented by this object and redirect the uses of this slice to
+ /// this new sequence of instructions.
+ /// \pre this->Inst && this->Origin are valid Instructions.
+ /// \return The last instruction of the sequence used to load the slice.
+ Instruction *loadSlice(InstCombiner::BuilderTy &Builder,
+ bool IsBigEndian) const {
+ assert(Inst && Origin && "Unable to replace a non-existing slice.");
+ Value *BaseAddr = Origin->getOperand(0);
+ unsigned Alignment = Origin->getAlignment();
+ Builder.SetInsertPoint(Origin);
+ // Assume we are looking at a chunk of bytes.
+ // BaseAddr = (i8*)BaseAddr.
+ BaseAddr = Builder.CreateBitCast(BaseAddr, Builder.getInt8PtrTy(),
+ "raw_cast");
+ // Get the offset in that chunk of bytes w.r.t. the endianess.
+ uint64_t Offset = getOffsetFromBase(IsBigEndian);
+ if (Offset) {
+ APInt APOffset(64, Offset);
+ // BaseAddr = BaseAddr + Offset.
+ BaseAddr = Builder.CreateInBoundsGEP(BaseAddr, Builder.getInt(APOffset),
+ "raw_idx");
+ }
+
+ // Create the type of the loaded slice according to its size.
+ Type *SliceType =
+ Type::getIntNTy(Origin->getContext(), getLoadedSize() * 8);
+
+ // Bit cast the raw pointer to the pointer type of the slice.
+ BaseAddr = Builder.CreateBitCast(BaseAddr, SliceType->getPointerTo(),
+ "cast");
+
+ // Compute the new alignment.
+ if (Offset != 0)
+ Alignment = MinAlign(Alignment, Alignment + Offset);
+
+ // Create the load for the slice.
+ Instruction *LastInst = Builder.CreateAlignedLoad(BaseAddr, Alignment,
+ Inst->getName()+".val");
+ // If the final type is not the same as the loaded type, this means that
+ // we have to pad with zero. Create a zero extend for that.
+ Type * FinalType = Inst->getType();
+ if (SliceType != FinalType)
+ LastInst = cast<Instruction>(Builder.CreateZExt(LastInst, FinalType));
+
+ // Update the IR to reflect the new access to the slice.
+ Inst->replaceAllUsesWith(LastInst);
+
+ return LastInst;
+ }
+
+ /// \brief Check if it would be profitable to expand this slice as an
+ /// independant load.
+ bool isProfitable() const {
+ // Slicing is assumed to be profitable iff the chains leads to arithmetic
+ // operations.
+ SmallVector<const Instruction *, 8> Uses;
+ Uses.push_back(Inst);
+ do {
+ const Instruction *Use = Uses.pop_back_val();
+ for (Value::const_use_iterator UseIt = Use->use_begin(),
+ UseItEnd = Use->use_end(); UseIt != UseItEnd; ++UseIt) {
+ const Instruction *UseOfUse = cast<Instruction>(*UseIt);
+ // Consider these instructions as arithmetic operations.
+ if (isa<BinaryOperator>(UseOfUse) ||
+ isa<CastInst>(UseOfUse) ||
+ isa<PHINode>(UseOfUse) ||
+ isa<GetElementPtrInst>(UseOfUse))
+ return true;
+ // No need to check if the Use has already been checked as we do not
+ // insert any PHINode.
+ Uses.push_back(UseOfUse);
+ }
+ } while (!Uses.empty());
+ DEBUG(dbgs() << "IC: Not a profitable slice " << *Inst << '\n');
+ return false;
+ }
+ };
+}
+
+/// \brief Check the profitability of all involved LoadedSlice.
+/// Unless StressLoadSlicing is specified, this also returns false
+/// when slicing is not in the canonical form.
+/// The canonical form of sliced load is (1) two loads,
+/// which are (2) next to each other in memory.
+///
+/// FIXME: We may want to allow more slices to be created but
+/// this means other passes should know how to deal with all those
+/// slices.
+/// FIXME: We may want to split loads to different types, e.g.,
+/// int vs. float.
+static bool
+isSlicingProfitable(const SmallVectorImpl<LoadedSlice> &LoadedSlices,
+ const APInt &UsedBits) {
+ unsigned NbOfSlices = LoadedSlices.size();
+ // Check (1).
+ if (!StressLoadSlicing && NbOfSlices != 2)
+ return false;
+
+ // Check (2).
+ if (!StressLoadSlicing && !UsedBits.isAllOnesValue()) {
+ // Get rid of the unused bits on the right.
+ APInt MemoryLayout = UsedBits.lshr(UsedBits.countTrailingZeros());
+ // Get rid of the unused bits on the left.
+ if (MemoryLayout.countLeadingZeros())
+ MemoryLayout = MemoryLayout.trunc(MemoryLayout.getActiveBits());
+ // Check that the chunk of memory is completely used.
+ if (!MemoryLayout.isAllOnesValue())
+ return false;
+ }
+
+ unsigned NbOfProfitableSlices = 0;
+ for (unsigned CurrSlice = 0; CurrSlice < NbOfSlices; ++CurrSlice) {
+ if (LoadedSlices[CurrSlice].isProfitable())
+ ++NbOfProfitableSlices;
+ else if (!StressLoadSlicing)
+ return false;
+ }
+ // In Stress mode, we may have 0 profitable slice.
+ // Check that here.
+ // In non-Stress mode, all the slices are profitable at this point.
+ return NbOfProfitableSlices > 0;
+}
+
+/// \brief If the given load, \p LI, is used only by trunc or trunc(lshr)
+/// operations, split it in the various pieces being extracted.
+///
+/// This sort of thing is introduced by SROA.
+/// This slicing takes care not to insert overlapping loads.
+/// \pre LI is a simple load (i.e., not an atomic or volatile load).
+static Instruction *sliceUpLoadInst(LoadInst &LI,
+ InstCombiner::BuilderTy &Builder,
+ DataLayout &TD) {
+ assert(LI.isSimple() && "We are trying to transform a non-simple load!");
+
+ // FIXME: If we want to support floating point and vector types, we should
+ // support bitcast and extract/insert element instructions.
+ Type *LITy = LI.getType();
+ if (!LITy->isIntegerTy()) return 0;
+
+ // Keep track of already used bits to detect overlapping values.
+ // In that case, we will just abort the transformation.
+ APInt UsedBits(LITy->getPrimitiveSizeInBits(), 0);
+
+ SmallVector<LoadedSlice, 4> LoadedSlices;
+
+ // Check if this load is used as several smaller chunks of bits.
+ // Basically, look for uses in trunc or trunc(lshr) and record a new chain
+ // of computation for each trunc.
+ for (Value::use_iterator UI = LI.use_begin(), UIEnd = LI.use_end();
+ UI != UIEnd; ++UI) {
+ Instruction *User = cast<Instruction>(*UI);
+ unsigned Shift = 0;
+
+ // Check if this is a trunc(lshr).
+ if (User->getOpcode() == Instruction::LShr && User->hasOneUse() &&
+ isa<ConstantInt>(User->getOperand(1))) {
+ Shift = cast<ConstantInt>(User->getOperand(1))->getZExtValue();
+ User = User->use_back();
+ }
+
+ // At this point, User is a TruncInst, iff we encountered, trunc or
+ // trunc(lshr).
+ if (!isa<TruncInst>(User))
+ return 0;
+
+ // The width of the type must be a power of 2 and greater than 8-bits.
+ // Otherwise the load cannot be represented in LLVM IR.
+ // Moreover, if we shifted with a non 8-bits multiple, the slice
+ // will be accross several bytes. We do not support that.
+ unsigned Width = User->getType()->getPrimitiveSizeInBits();
+ if (Width < 8 || !isPowerOf2_32(Width) || (Shift & 0x7))
+ return 0;
+
+ // Build the slice for this chain of computations.
+ LoadedSlice LS(User, &LI, Shift);
+ APInt CurrentUsedBits = LS.getUsedBits();
+
+ // Check if this slice overlaps with another.
+ if ((CurrentUsedBits & UsedBits) != 0)
+ return 0;
+ // Update the bits used globally.
+ UsedBits |= CurrentUsedBits;
+
+ // Record the slice.
+ LoadedSlices.push_back(LS);
+ }
+
+ // Abort slicing if it does not seem to be profitable.
+ if (!isSlicingProfitable(LoadedSlices, UsedBits))
+ return 0;
+
+ // Rewrite each chain to use an independent load.
+ // By construction, each chain can be represented by a unique load.
+ bool IsBigEndian = TD.isBigEndian();
+ for (SmallVectorImpl<LoadedSlice>::const_iterator LSIt = LoadedSlices.begin(),
+ LSItEnd = LoadedSlices.end(); LSIt != LSItEnd; ++LSIt) {
+ Instruction *SliceInst = LSIt->loadSlice(Builder, IsBigEndian);
+ (void)SliceInst;
+ DEBUG(dbgs() << "IC: Replacing " << *LSIt->Inst << "\n"
+ " with " << *SliceInst << '\n');
+ }
+ return 0; // Don't do anything with LI.
+}
+
Instruction *InstCombiner::visitLoadInst(LoadInst &LI) {
Value *Op = LI.getOperand(0);
@@ -443,6 +721,13 @@ Instruction *InstCombiner::visitLoadInst(LoadInst &LI) {
}
}
}
+
+ // Try to split a load in smaller non-overlapping loads to expose independant
+ // chain of computations and get rid of trunc/lshr sequence of code.
+ // The data layout is required for that operation, as code generation will
+ // change with respect to endianess.
+ if (TD)
+ return sliceUpLoadInst(LI, *Builder, *TD);
return 0;
}
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