[llvm] r207783 - Add an optimization that does CSE in a group of similar GEPs.
Eli Bendersky
eliben at google.com
Thu May 1 12:27:30 PDT 2014
I've seen the bot failures now - working on it.
On Thu, May 1, 2014 at 11:38 AM, Eli Bendersky <eliben at google.com> wrote:
> Author: eliben
> Date: Thu May 1 13:38:36 2014
> New Revision: 207783
>
> URL: http://llvm.org/viewvc/llvm-project?rev=207783&view=rev
> Log:
> Add an optimization that does CSE in a group of similar GEPs.
>
> This optimization merges the common part of a group of GEPs, so we can
> compute
> each pointer address by adding a simple offset to the common part.
>
> The optimization is currently only enabled for the NVPTX backend, where it
> has
> a large payoff on some benchmarks.
>
> Review: http://reviews.llvm.org/D3462
>
> Patch by Jingyue Wu.
>
>
>
> Added:
> llvm/trunk/lib/Transforms/Scalar/SeparateConstOffsetFromGEP.cpp
> llvm/trunk/test/Transforms/SeparateConstOffsetFromGEP/
> llvm/trunk/test/Transforms/SeparateConstOffsetFromGEP/NVPTX/
>
> llvm/trunk/test/Transforms/SeparateConstOffsetFromGEP/NVPTX/lit.local.cfg
>
> llvm/trunk/test/Transforms/SeparateConstOffsetFromGEP/NVPTX/split-gep-and-gvn.ll
>
> llvm/trunk/test/Transforms/SeparateConstOffsetFromGEP/NVPTX/split-gep.ll
> Modified:
> llvm/trunk/include/llvm/InitializePasses.h
> llvm/trunk/include/llvm/LinkAllPasses.h
> llvm/trunk/include/llvm/Transforms/Scalar.h
> llvm/trunk/lib/Target/NVPTX/NVPTXTargetMachine.cpp
> llvm/trunk/lib/Transforms/Scalar/Scalar.cpp
>
> Modified: llvm/trunk/include/llvm/InitializePasses.h
> URL:
> http://llvm.org/viewvc/llvm-project/llvm/trunk/include/llvm/InitializePasses.h?rev=207783&r1=207782&r2=207783&view=diff
>
> ==============================================================================
> --- llvm/trunk/include/llvm/InitializePasses.h (original)
> +++ llvm/trunk/include/llvm/InitializePasses.h Thu May 1 13:38:36 2014
> @@ -238,6 +238,7 @@ void initializeSimpleInlinerPass(PassReg
> void initializeRegisterCoalescerPass(PassRegistry&);
> void initializeSingleLoopExtractorPass(PassRegistry&);
> void initializeSinkingPass(PassRegistry&);
> +void initializeSeparateConstOffsetFromGEPPass(PassRegistry &);
> void initializeSlotIndexesPass(PassRegistry&);
> void initializeSpillPlacementPass(PassRegistry&);
> void initializeStackProtectorPass(PassRegistry&);
>
> Modified: llvm/trunk/include/llvm/LinkAllPasses.h
> URL:
> http://llvm.org/viewvc/llvm-project/llvm/trunk/include/llvm/LinkAllPasses.h?rev=207783&r1=207782&r2=207783&view=diff
>
> ==============================================================================
> --- llvm/trunk/include/llvm/LinkAllPasses.h (original)
> +++ llvm/trunk/include/llvm/LinkAllPasses.h Thu May 1 13:38:36 2014
> @@ -156,6 +156,7 @@ namespace {
> (void) llvm::createBBVectorizePass();
> (void) llvm::createPartiallyInlineLibCallsPass();
> (void) llvm::createScalarizerPass();
> + (void) llvm::createSeparateConstOffsetFromGEPPass();
>
> (void)new llvm::IntervalPartition();
> (void)new llvm::FindUsedTypes();
>
> Modified: llvm/trunk/include/llvm/Transforms/Scalar.h
> URL:
> http://llvm.org/viewvc/llvm-project/llvm/trunk/include/llvm/Transforms/Scalar.h?rev=207783&r1=207782&r2=207783&view=diff
>
> ==============================================================================
> --- llvm/trunk/include/llvm/Transforms/Scalar.h (original)
> +++ llvm/trunk/include/llvm/Transforms/Scalar.h Thu May 1 13:38:36 2014
> @@ -377,6 +377,12 @@ FunctionPass *createScalarizerPass();
> // AddDiscriminators - Add DWARF path discriminators to the IR.
> FunctionPass *createAddDiscriminatorsPass();
>
>
> +//===----------------------------------------------------------------------===//
> +//
> +// SeparateConstOffsetFromGEP - Split GEPs for better CSE
> +//
> +FunctionPass *createSeparateConstOffsetFromGEPPass();
> +
> } // End llvm namespace
>
> #endif
>
> Modified: llvm/trunk/lib/Target/NVPTX/NVPTXTargetMachine.cpp
> URL:
> http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Target/NVPTX/NVPTXTargetMachine.cpp?rev=207783&r1=207782&r2=207783&view=diff
>
> ==============================================================================
> --- llvm/trunk/lib/Target/NVPTX/NVPTXTargetMachine.cpp (original)
> +++ llvm/trunk/lib/Target/NVPTX/NVPTXTargetMachine.cpp Thu May 1 13:38:36
> 2014
> @@ -147,10 +147,23 @@ void NVPTXPassConfig::addIRPasses() {
> addPass(createNVPTXAssignValidGlobalNamesPass());
> addPass(createGenericToNVVMPass());
> addPass(createNVPTXFavorNonGenericAddrSpacesPass());
> - // The FavorNonGenericAddrSpaces pass may remove instructions and leave
> some
> - // values unused. Therefore, we run a DCE pass right afterwards. We
> could
> - // remove unused values in an ad-hoc manner, but it requires manual
> work and
> - // might be error-prone.
> + addPass(createSeparateConstOffsetFromGEPPass());
> + // The SeparateConstOffsetFromGEP pass creates variadic bases that can
> be used
> + // by multiple GEPs. Run GVN or EarlyCSE to really reuse them. GVN
> generates
> + // significantly better code than EarlyCSE for some of our benchmarks.
> + if (getOptLevel() == CodeGenOpt::Aggressive)
> + addPass(createGVNPass());
> + else
> + addPass(createEarlyCSEPass());
> + // Both FavorNonGenericAddrSpaces and SeparateConstOffsetFromGEP may
> leave
> + // some dead code. We could remove dead code in an ad-hoc manner, but
> that
> + // requires manual work and might be error-prone.
> + //
> + // The FavorNonGenericAddrSpaces pass shortcuts unnecessary
> addrspacecasts,
> + // and leave them unused.
> + //
> + // SeparateConstOffsetFromGEP rebuilds a new index from the old index,
> and the
> + // old index and some of its intermediate results may become unused.
> addPass(createDeadCodeEliminationPass());
> }
>
>
> Modified: llvm/trunk/lib/Transforms/Scalar/Scalar.cpp
> URL:
> http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Transforms/Scalar/Scalar.cpp?rev=207783&r1=207782&r2=207783&view=diff
>
> ==============================================================================
> --- llvm/trunk/lib/Transforms/Scalar/Scalar.cpp (original)
> +++ llvm/trunk/lib/Transforms/Scalar/Scalar.cpp Thu May 1 13:38:36 2014
> @@ -64,6 +64,7 @@ void llvm::initializeScalarOpts(PassRegi
> initializeStructurizeCFGPass(Registry);
> initializeSinkingPass(Registry);
> initializeTailCallElimPass(Registry);
> + initializeSeparateConstOffsetFromGEPPass(Registry);
> }
>
> void LLVMInitializeScalarOpts(LLVMPassRegistryRef R) {
>
> Added: llvm/trunk/lib/Transforms/Scalar/SeparateConstOffsetFromGEP.cpp
> URL:
> http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Transforms/Scalar/SeparateConstOffsetFromGEP.cpp?rev=207783&view=auto
>
> ==============================================================================
> --- llvm/trunk/lib/Transforms/Scalar/SeparateConstOffsetFromGEP.cpp (added)
> +++ llvm/trunk/lib/Transforms/Scalar/SeparateConstOffsetFromGEP.cpp Thu
> May 1 13:38:36 2014
> @@ -0,0 +1,583 @@
> +//===-- SeparateConstOffsetFromGEP.cpp - ------------------------*- C++
> -*-===//
> +//
> +// The LLVM Compiler Infrastructure
> +//
> +// This file is distributed under the University of Illinois Open Source
> +// License. See LICENSE.TXT for details.
> +//
>
> +//===----------------------------------------------------------------------===//
> +//
> +// Loop unrolling may create many similar GEPs for array accesses.
> +// e.g., a 2-level loop
> +//
> +// float a[32][32]; // global variable
> +//
> +// for (int i = 0; i < 2; ++i) {
> +// for (int j = 0; j < 2; ++j) {
> +// ...
> +// ... = a[x + i][y + j];
> +// ...
> +// }
> +// }
> +//
> +// will probably be unrolled to:
> +//
> +// gep %a, 0, %x, %y; load
> +// gep %a, 0, %x, %y + 1; load
> +// gep %a, 0, %x + 1, %y; load
> +// gep %a, 0, %x + 1, %y + 1; load
> +//
> +// LLVM's GVN does not use partial redundancy elimination yet, and is thus
> +// unable to reuse (gep %a, 0, %x, %y). As a result, this misoptimization
> incurs
> +// significant slowdown in targets with limited addressing modes. For
> instance,
> +// because the PTX target does not support the reg+reg addressing mode,
> the
> +// NVPTX backend emits PTX code that literally computes the pointer
> address of
> +// each GEP, wasting tons of registers. It emits the following PTX for the
> +// first load and similar PTX for other loads.
> +//
> +// mov.u32 %r1, %x;
> +// mov.u32 %r2, %y;
> +// mul.wide.u32 %rl2, %r1, 128;
> +// mov.u64 %rl3, a;
> +// add.s64 %rl4, %rl3, %rl2;
> +// mul.wide.u32 %rl5, %r2, 4;
> +// add.s64 %rl6, %rl4, %rl5;
> +// ld.global.f32 %f1, [%rl6];
> +//
> +// To reduce the register pressure, the optimization implemented in this
> file
> +// merges the common part of a group of GEPs, so we can compute each
> pointer
> +// address by adding a simple offset to the common part, saving many
> registers.
> +//
> +// It works by splitting each GEP into a variadic base and a constant
> offset.
> +// The variadic base can be computed once and reused by multiple GEPs,
> and the
> +// constant offsets can be nicely folded into the reg+immediate
> addressing mode
> +// (supported by most targets) without using any extra register.
> +//
> +// For instance, we transform the four GEPs and four loads in the above
> example
> +// into:
> +//
> +// base = gep a, 0, x, y
> +// load base
> +// laod base + 1 * sizeof(float)
> +// load base + 32 * sizeof(float)
> +// load base + 33 * sizeof(float)
> +//
> +// Given the transformed IR, a backend that supports the reg+immediate
> +// addressing mode can easily fold the pointer arithmetics into the
> loads. For
> +// example, the NVPTX backend can easily fold the pointer arithmetics
> into the
> +// ld.global.f32 instructions, and the resultant PTX uses much fewer
> registers.
> +//
> +// mov.u32 %r1, %tid.x;
> +// mov.u32 %r2, %tid.y;
> +// mul.wide.u32 %rl2, %r1, 128;
> +// mov.u64 %rl3, a;
> +// add.s64 %rl4, %rl3, %rl2;
> +// mul.wide.u32 %rl5, %r2, 4;
> +// add.s64 %rl6, %rl4, %rl5;
> +// ld.global.f32 %f1, [%rl6]; // so far the same as unoptimized PTX
> +// ld.global.f32 %f2, [%rl6+4]; // much better
> +// ld.global.f32 %f3, [%rl6+128]; // much better
> +// ld.global.f32 %f4, [%rl6+132]; // much better
> +//
>
> +//===----------------------------------------------------------------------===//
> +
> +#include "llvm/Analysis/TargetTransformInfo.h"
> +#include "llvm/Analysis/ValueTracking.h"
> +#include "llvm/IR/Constants.h"
> +#include "llvm/IR/DataLayout.h"
> +#include "llvm/IR/Instructions.h"
> +#include "llvm/IR/LLVMContext.h"
> +#include "llvm/IR/Module.h"
> +#include "llvm/IR/Operator.h"
> +#include "llvm/Support/CommandLine.h"
> +#include "llvm/Support/raw_ostream.h"
> +#include "llvm/Transforms/Scalar.h"
> +
> +using namespace llvm;
> +
> +static cl::opt<bool> DisableSeparateConstOffsetFromGEP(
> + "disable-separate-const-offset-from-gep", cl::init(false),
> + cl::desc("Do not separate the constant offset from a GEP
> instruction"),
> + cl::Hidden);
> +
> +namespace {
> +
> +/// \brief A helper class for separating a constant offset from a GEP
> index.
> +///
> +/// In real programs, a GEP index may be more complicated than a simple
> addition
> +/// of something and a constant integer which can be trivially splitted.
> For
> +/// example, to split ((a << 3) | 5) + b, we need to search deeper for the
> +/// constant offset, so that we can seperate the index to (a << 3) + b
> and 5.
> +///
> +/// Therefore, this class looks into the expression that computes a given
> GEP
> +/// index, and tries to find a constant integer that can be hoisted to the
> +/// outermost level of the expression as an addition. Not every constant
> in an
> +/// expression can jump out. e.g., we cannot transform (b * (a + 5)) to
> (b * a +
> +/// 5); nor can we transform (3 * (a + 5)) to (3 * a + 5), however in
> this case,
> +/// -instcombine probably already optimized (3 * (a + 5)) to (3 * a + 15).
> +class ConstantOffsetExtractor {
> + public:
> + /// Extracts a constant offset from the given GEP index. It outputs the
> + /// numeric value of the extracted constant offset (0 if failed), and a
> + /// new index representing the remainder (equal to the original index
> minus
> + /// the constant offset).
> + /// \p Idx The given GEP index
> + /// \p NewIdx The new index to replace
> + /// \p DL The datalayout of the module
> + /// \p IP Calculating the new index requires new instructions. IP
> indicates
> + /// where to insert them (typically right before the GEP).
> + static int64_t Extract(Value *Idx, Value *&NewIdx, const DataLayout *DL,
> + Instruction *IP);
> + /// Looks for a constant offset without extracting it. The meaning of
> the
> + /// arguments and the return value are the same as Extract.
> + static int64_t Find(Value *Idx, const DataLayout *DL);
> +
> + private:
> + ConstantOffsetExtractor(const DataLayout *Layout, Instruction
> *InsertionPt)
> + : DL(Layout), IP(InsertionPt) {}
> + /// Searches the expression that computes V for a constant offset. If
> the
> + /// searching is successful, update UserChain as a path from V to the
> constant
> + /// offset.
> + int64_t find(Value *V);
> + /// A helper function to look into both operands of a binary operator U.
> + /// \p IsSub Whether U is a sub operator. If so, we need to negate the
> + /// constant offset at some point.
> + int64_t findInEitherOperand(User *U, bool IsSub);
> + /// After finding the constant offset and how it is reached from the GEP
> + /// index, we build a new index which is a clone of the old one except
> the
> + /// constant offset is removed. For example, given (a + (b + 5)) and
> knowning
> + /// the constant offset is 5, this function returns (a + b).
> + ///
> + /// We cannot simply change the constant to zero because the expression
> that
> + /// computes the index or its intermediate result may be used by others.
> + Value *rebuildWithoutConstantOffset();
> + // A helper function for rebuildWithoutConstantOffset that rebuilds the
> direct
> + // user (U) of the constant offset (C).
> + Value *rebuildLeafWithoutConstantOffset(User *U, Value *C);
> + /// Returns a clone of U except the first occurrence of From with To.
> + Value *cloneAndReplace(User *U, Value *From, Value *To);
> +
> + /// Returns true if LHS and RHS have no bits in common, i.e., LHS | RHS
> == 0.
> + bool NoCommonBits(Value *LHS, Value *RHS) const;
> + /// Computes which bits are known to be one or zero.
> + /// \p KnownOne Mask of all bits that are known to be one.
> + /// \p KnownZero Mask of all bits that are known to be zero.
> + void ComputeKnownBits(Value *V, APInt &KnownOne, APInt &KnownZero)
> const;
> + /// Finds the first use of Used in U. Returns -1 if not found.
> + static unsigned FindFirstUse(User *U, Value *Used);
> +
> + /// The path from the constant offset to the old GEP index. e.g., if
> the GEP
> + /// index is "a * b + (c + 5)". After running function find,
> UserChain[0] will
> + /// be the constant 5, UserChain[1] will be the subexpression "c + 5",
> and
> + /// UserChain[2] will be the entire expression "a * b + (c + 5)".
> + ///
> + /// This path helps rebuildWithoutConstantOffset rebuild the new GEP
> index.
> + SmallVector<User *, 8> UserChain;
> + /// The data layout of the module. Used in ComputeKnownBits.
> + const DataLayout *DL;
> + Instruction *IP; /// Insertion position of cloned instructions.
> +};
> +
> +/// \brief A pass that tries to split every GEP in the function into a
> variadic
> +/// base and a constant offset. It is a FuntionPass because searching for
> the
> +/// constant offset may inspect other basic blocks.
> +class SeparateConstOffsetFromGEP : public FunctionPass {
> + public:
> + static char ID;
> + SeparateConstOffsetFromGEP() : FunctionPass(ID) {
> +
> initializeSeparateConstOffsetFromGEPPass(*PassRegistry::getPassRegistry());
> + }
> +
> + void getAnalysisUsage(AnalysisUsage &AU) const override {
> + AU.addRequired<DataLayoutPass>();
> + AU.addRequired<TargetTransformInfo>();
> + }
> + bool runOnFunction(Function &F) override;
> +
> + private:
> + /// Tries to split the given GEP into a variadic base and a constant
> offset,
> + /// and returns true if the splitting succeeds.
> + bool splitGEP(GetElementPtrInst *GEP);
> + /// Finds the constant offset within each index, and accumulates them.
> This
> + /// function only inspects the GEP without changing it. The output
> + /// NeedsExtraction indicates whether we can extract a non-zero constant
> + /// offset from any index.
> + int64_t accumulateByteOffset(GetElementPtrInst *GEP, const DataLayout
> *DL,
> + bool &NeedsExtraction);
> +};
> +} // anonymous namespace
> +
> +char SeparateConstOffsetFromGEP::ID = 0;
> +INITIALIZE_PASS_BEGIN(
> + SeparateConstOffsetFromGEP, "separate-const-offset-from-gep",
> + "Split GEPs to a variadic base and a constant offset for better CSE",
> false,
> + false)
> +INITIALIZE_AG_DEPENDENCY(TargetTransformInfo)
> +INITIALIZE_PASS_DEPENDENCY(DataLayoutPass)
> +INITIALIZE_PASS_END(
> + SeparateConstOffsetFromGEP, "separate-const-offset-from-gep",
> + "Split GEPs to a variadic base and a constant offset for better CSE",
> false,
> + false)
> +
> +FunctionPass *llvm::createSeparateConstOffsetFromGEPPass() {
> + return new SeparateConstOffsetFromGEP();
> +}
> +
> +int64_t ConstantOffsetExtractor::findInEitherOperand(User *U, bool IsSub)
> {
> + assert(U->getNumOperands() == 2);
> + int64_t ConstantOffset = find(U->getOperand(0));
> + // If we found a constant offset in the left operand, stop and return
> that.
> + // This shortcut might cause us to miss opportunities of combining the
> + // constant offsets in both operands, e.g., (a + 4) + (b + 5) => (a +
> b) + 9.
> + // However, such cases are probably already handled by -instcombine,
> + // given this pass runs after the standard optimizations.
> + if (ConstantOffset != 0) return ConstantOffset;
> + ConstantOffset = find(U->getOperand(1));
> + // If U is a sub operator, negate the constant offset found in the right
> + // operand.
> + return IsSub ? -ConstantOffset : ConstantOffset;
> +}
> +
> +int64_t ConstantOffsetExtractor::find(Value *V) {
> + // TODO(jingyue): We can even trace into integer/pointer casts, such as
> + // inttoptr, ptrtoint, bitcast, and addrspacecast. We choose to handle
> only
> + // integers because it gives good enough results for our benchmarks.
> + assert(V->getType()->isIntegerTy());
> +
> + User *U = dyn_cast<User>(V);
> + // We cannot do much with Values that are not a User, such as
> BasicBlock and
> + // MDNode.
> + if (U == nullptr) return 0;
> +
> + int64_t ConstantOffset = 0;
> + if (ConstantInt *CI = dyn_cast<ConstantInt>(U)) {
> + // Hooray, we found it!
> + ConstantOffset = CI->getSExtValue();
> + } else if (Operator *O = dyn_cast<Operator>(U)) {
> + // The GEP index may be more complicated than a simple addition of a
> + // varaible and a constant. Therefore, we trace into subexpressions
> for more
> + // hoisting opportunities.
> + switch (O->getOpcode()) {
> + case Instruction::Add: {
> + ConstantOffset = findInEitherOperand(U, false);
> + break;
> + }
> + case Instruction::Sub: {
> + ConstantOffset = findInEitherOperand(U, true);
> + break;
> + }
> + case Instruction::Or: {
> + // If LHS and RHS don't have common bits, (LHS | RHS) is
> equivalent to
> + // (LHS + RHS).
> + if (NoCommonBits(U->getOperand(0), U->getOperand(1)))
> + ConstantOffset = findInEitherOperand(U, false);
> + break;
> + }
> + case Instruction::SExt: {
> + // For safety, we trace into sext only when its operand is marked
> + // "nsw" because xxx.nsw guarantees no signed wrap. e.g., we can
> safely
> + // transform "sext (add nsw a, 5)" into "add nsw (sext a), 5".
> + if (BinaryOperator *BO =
> dyn_cast<BinaryOperator>(U->getOperand(0))) {
> + if (BO->hasNoSignedWrap())
> + ConstantOffset = find(U->getOperand(0));
> + }
> + break;
> + }
> + case Instruction::ZExt: {
> + // Similarly, we trace into zext only when its operand is marked
> with
> + // "nuw" because zext (add nuw a, b) == add nuw (zext a), (zext
> b).
> + if (BinaryOperator *BO =
> dyn_cast<BinaryOperator>(U->getOperand(0))) {
> + if (BO->hasNoUnsignedWrap())
> + ConstantOffset = find(U->getOperand(0));
> + }
> + break;
> + }
> + }
> + }
> + // If we found a non-zero constant offset, adds it to the path for
> future
> + // transformation (rebuildWithoutConstantOffset). Zero is a valid
> constant
> + // offset, but doesn't help this optimization.
> + if (ConstantOffset != 0)
> + UserChain.push_back(U);
> + return ConstantOffset;
> +}
> +
> +unsigned ConstantOffsetExtractor::FindFirstUse(User *U, Value *Used) {
> + for (unsigned I = 0, E = U->getNumOperands(); I < E; ++I) {
> + if (U->getOperand(I) == Used)
> + return I;
> + }
> + return -1;
> +}
> +
> +Value *ConstantOffsetExtractor::cloneAndReplace(User *U, Value *From,
> + Value *To) {
> + // Finds in U the first use of From. It is safe to ignore future
> occurrences
> + // of From, because findInEitherOperand similarly stops searching the
> right
> + // operand when the first operand has a non-zero constant offset.
> + unsigned OpNo = FindFirstUse(U, From);
> + assert(OpNo != (unsigned)-1 && "UserChain wasn't built correctly");
> +
> + // ConstantOffsetExtractor::find only follows Operators (i.e.,
> Instructions
> + // and ConstantExprs). Therefore, U is either an Instruction or a
> + // ConstantExpr.
> + if (Instruction *I = dyn_cast<Instruction>(U)) {
> + Instruction *Clone = I->clone();
> + Clone->setOperand(OpNo, To);
> + Clone->insertBefore(IP);
> + return Clone;
> + }
> + // cast<Constant>(To) is safe because a ConstantExpr only uses
> Constants.
> + return cast<ConstantExpr>(U)
> + ->getWithOperandReplaced(OpNo, cast<Constant>(To));
> +}
> +
> +Value *ConstantOffsetExtractor::rebuildLeafWithoutConstantOffset(User *U,
> + Value
> *C) {
> + assert(U->getNumOperands() <= 2 &&
> + "We didn't trace into any operator with more than 2 operands");
> + // If U has only one operand which is the constant offset, removing the
> + // constant offset leaves U as a null value.
> + if (U->getNumOperands() == 1)
> + return Constant::getNullValue(U->getType());
> +
> + // U->getNumOperands() == 2
> + unsigned OpNo = FindFirstUse(U, C); // U->getOperand(OpNo) == C
> + assert(OpNo < 2 && "UserChain wasn't built correctly");
> + Value *TheOther = U->getOperand(1 - OpNo); // The other operand of U
> + // If U = C - X, removing C makes U = -X; otherwise U will simply be X.
> + if (!isa<SubOperator>(U) || OpNo == 1)
> + return TheOther;
> + if (isa<ConstantExpr>(U))
> + return ConstantExpr::getNeg(cast<Constant>(TheOther));
> + return BinaryOperator::CreateNeg(TheOther, "", IP);
> +}
> +
> +Value *ConstantOffsetExtractor::rebuildWithoutConstantOffset() {
> + assert(UserChain.size() > 0 && "you at least found a constant, right?");
> + // Start with the constant and go up through UserChain, each time
> building a
> + // clone of the subexpression but with the constant removed.
> + // e.g., to build a clone of (a + (b + (c + 5)) but with the 5 removed,
> we
> + // first c, then (b + c), and finally (a + (b + c)).
> + //
> + // Fast path: if the GEP index is a constant, simply returns 0.
> + if (UserChain.size() == 1)
> + return ConstantInt::get(UserChain[0]->getType(), 0);
> +
> + Value *Remainder =
> + rebuildLeafWithoutConstantOffset(UserChain[1], UserChain[0]);
> + for (size_t I = 2; I < UserChain.size(); ++I)
> + Remainder = cloneAndReplace(UserChain[I], UserChain[I - 1],
> Remainder);
> + return Remainder;
> +}
> +
> +int64_t ConstantOffsetExtractor::Extract(Value *Idx, Value *&NewIdx,
> + const DataLayout *DL,
> + Instruction *IP) {
> + ConstantOffsetExtractor Extractor(DL, IP);
> + // Find a non-zero constant offset first.
> + int64_t ConstantOffset = Extractor.find(Idx);
> + if (ConstantOffset == 0)
> + return 0;
> + // Then rebuild a new index with the constant removed.
> + NewIdx = Extractor.rebuildWithoutConstantOffset();
> + return ConstantOffset;
> +}
> +
> +int64_t ConstantOffsetExtractor::Find(Value *Idx, const DataLayout *DL) {
> + return ConstantOffsetExtractor(DL, nullptr).find(Idx);
> +}
> +
> +void ConstantOffsetExtractor::ComputeKnownBits(Value *V, APInt &KnownOne,
> + APInt &KnownZero) const {
> + IntegerType *IT = cast<IntegerType>(V->getType());
> + KnownOne = APInt(IT->getBitWidth(), 0);
> + KnownZero = APInt(IT->getBitWidth(), 0);
> + llvm::ComputeMaskedBits(V, KnownZero, KnownOne, DL, 0);
> +}
> +
> +bool ConstantOffsetExtractor::NoCommonBits(Value *LHS, Value *RHS) const {
> + assert(LHS->getType() == RHS->getType() &&
> + "LHS and RHS should have the same type");
> + APInt LHSKnownOne, LHSKnownZero, RHSKnownOne, RHSKnownZero;
> + ComputeKnownBits(LHS, LHSKnownOne, LHSKnownZero);
> + ComputeKnownBits(RHS, RHSKnownOne, RHSKnownZero);
> + return (LHSKnownZero | RHSKnownZero).isAllOnesValue();
> +}
> +
> +int64_t SeparateConstOffsetFromGEP::accumulateByteOffset(
> + GetElementPtrInst *GEP, const DataLayout *DL, bool &NeedsExtraction) {
> + NeedsExtraction = false;
> + int64_t AccumulativeByteOffset = 0;
> + gep_type_iterator GTI = gep_type_begin(*GEP);
> + for (unsigned I = 1, E = GEP->getNumOperands(); I != E; ++I, ++GTI) {
> + if (isa<SequentialType>(*GTI)) {
> + // Tries to extract a constant offset from this GEP index.
> + int64_t ConstantOffset =
> + ConstantOffsetExtractor::Find(GEP->getOperand(I), DL);
> + if (ConstantOffset != 0) {
> + NeedsExtraction = true;
> + // A GEP may have multiple indices. We accumulate the extracted
> + // constant offset to a byte offset, and later offset the
> remainder of
> + // the original GEP with this byte offset.
> + AccumulativeByteOffset +=
> + ConstantOffset * DL->getTypeAllocSize(GTI.getIndexedType());
> + }
> + }
> + }
> + return AccumulativeByteOffset;
> +}
> +
> +bool SeparateConstOffsetFromGEP::splitGEP(GetElementPtrInst *GEP) {
> + // Skip vector GEPs.
> + if (GEP->getType()->isVectorTy())
> + return false;
> +
> + // The backend can already nicely handle the case where all indices are
> + // constant.
> + if (GEP->hasAllConstantIndices())
> + return false;
> +
> + bool Changed = false;
> +
> + // Shortcuts integer casts. Eliminating these explicit casts can make
> + // subsequent optimizations more obvious: ConstantOffsetExtractor
> needn't
> + // trace into these casts.
> + if (GEP->isInBounds()) {
> + // Doing this to inbounds GEPs is safe because their indices are
> guaranteed
> + // to be non-negative and in bounds.
> + gep_type_iterator GTI = gep_type_begin(*GEP);
> + for (unsigned I = 1, E = GEP->getNumOperands(); I != E; ++I, ++GTI) {
> + if (isa<SequentialType>(*GTI)) {
> + if (Operator *O = dyn_cast<Operator>(GEP->getOperand(I))) {
> + if (O->getOpcode() == Instruction::SExt ||
> + O->getOpcode() == Instruction::ZExt) {
> + GEP->setOperand(I, O->getOperand(0));
> + Changed = true;
> + }
> + }
> + }
> + }
> + }
> +
> + const DataLayout *DL = &getAnalysis<DataLayoutPass>().getDataLayout();
> + bool NeedsExtraction;
> + int64_t AccumulativeByteOffset =
> + accumulateByteOffset(GEP, DL, NeedsExtraction);
> +
> + if (!NeedsExtraction)
> + return Changed;
> + // Before really splitting the GEP, check whether the backend supports
> the
> + // addressing mode we are about to produce. If no, this splitting
> probably
> + // won't be beneficial.
> + TargetTransformInfo &TTI = getAnalysis<TargetTransformInfo>();
> + if (!TTI.isLegalAddressingMode(GEP->getType()->getElementType(),
> + /*BaseGV=*/nullptr,
> AccumulativeByteOffset,
> + /*HasBaseReg=*/true, /*Scale=*/0)) {
> + return Changed;
> + }
> +
> + // Remove the constant offset in each GEP index. The resultant GEP
> computes
> + // the variadic base.
> + gep_type_iterator GTI = gep_type_begin(*GEP);
> + for (unsigned I = 1, E = GEP->getNumOperands(); I != E; ++I, ++GTI) {
> + if (isa<SequentialType>(*GTI)) {
> + Value *NewIdx = nullptr;
> + // Tries to extract a constant offset from this GEP index.
> + int64_t ConstantOffset =
> + ConstantOffsetExtractor::Extract(GEP->getOperand(I), NewIdx,
> DL, GEP);
> + if (ConstantOffset != 0) {
> + assert(NewIdx && "ConstantOffset != 0 implies NewIdx is set");
> + GEP->setOperand(I, NewIdx);
> + // Clear the inbounds attribute because the new index may be
> off-bound.
> + // e.g.,
> + //
> + // b = add i64 a, 5
> + // addr = gep inbounds float* p, i64 b
> + //
> + // is transformed to:
> + //
> + // addr2 = gep float* p, i64 a
> + // addr = gep float* addr2, i64 5
> + //
> + // If a is -4, although the old index b is in bounds, the new
> index a is
> + // off-bound. http://llvm.org/docs/LangRef.html#id181 says "if
> the
> + // inbounds keyword is not present, the offsets are added to the
> base
> + // address with silently-wrapping two's complement arithmetic".
> + // Therefore, the final code will be a semantically equivalent.
> + //
> + // TODO(jingyue): do some range analysis to keep as many inbounds
> as
> + // possible. GEPs with inbounds are more friendly to alias
> analysis.
> + GEP->setIsInBounds(false);
> + Changed = true;
> + }
> + }
> + }
> +
> + // Offsets the base with the accumulative byte offset.
> + //
> + // %gep ; the base
> + // ... %gep ...
> + //
> + // => add the offset
> + //
> + // %gep2 ; clone of %gep
> + // %0 = ptrtoint %gep2
> + // %1 = add %0, <offset>
> + // %new.gep = inttoptr %1
> + // %gep ; will be removed
> + // ... %gep ...
> + //
> + // => replace all uses of %gep with %new.gep and remove %gep
> + //
> + // %gep2 ; clone of %gep
> + // %0 = ptrtoint %gep2
> + // %1 = add %0, <offset>
> + // %new.gep = inttoptr %1
> + // ... %new.gep ...
> + //
> + // TODO(jingyue): Emit a GEP instead of an "uglygep"
> + // (http://llvm.org/docs/GetElementPtr.html#what-s-an-uglygep) to make
> the IR
> + // prettier and more alias analysis friendly. One caveat: if the
> original GEP
> + // ends with a StructType, we need to split the GEP at the last
> + // SequentialType. For instance, consider the following IR:
> + //
> + // %struct.S = type { float, double }
> + // @array = global [1024 x %struct.S]
> + // %p = getelementptr %array, 0, %i + 5, 1
> + //
> + // To separate the constant 5 from %p, we would need to split %p at the
> last
> + // array index so that we have:
> + //
> + // %addr = gep %array, 0, %i
> + // %p = gep %addr, 5, 1
> + Instruction *NewGEP = GEP->clone();
> + NewGEP->insertBefore(GEP);
> + Type *IntPtrTy = DL->getIntPtrType(GEP->getType());
> + Value *Addr = new PtrToIntInst(NewGEP, IntPtrTy, "", GEP);
> + Addr = BinaryOperator::CreateAdd(
> + Addr, ConstantInt::get(IntPtrTy, AccumulativeByteOffset, true), "",
> GEP);
> + Addr = new IntToPtrInst(Addr, GEP->getType(), "", GEP);
> +
> + GEP->replaceAllUsesWith(Addr);
> + GEP->eraseFromParent();
> +
> + return true;
> +}
> +
> +bool SeparateConstOffsetFromGEP::runOnFunction(Function &F) {
> + if (DisableSeparateConstOffsetFromGEP)
> + return false;
> +
> + bool Changed = false;
> + for (Function::iterator B = F.begin(), BE = F.end(); B != BE; ++B) {
> + for (BasicBlock::iterator I = B->begin(), IE = B->end(); I != IE; ) {
> + if (GetElementPtrInst *GEP = dyn_cast<GetElementPtrInst>(I++)) {
> + Changed |= splitGEP(GEP);
> + }
> + // No need to split GEP ConstantExprs because all its indices are
> constant
> + // already.
> + }
> + }
> + return Changed;
> +}
>
> Added:
> llvm/trunk/test/Transforms/SeparateConstOffsetFromGEP/NVPTX/lit.local.cfg
> URL:
> http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Transforms/SeparateConstOffsetFromGEP/NVPTX/lit.local.cfg?rev=207783&view=auto
>
> ==============================================================================
> ---
> llvm/trunk/test/Transforms/SeparateConstOffsetFromGEP/NVPTX/lit.local.cfg
> (added)
> +++
> llvm/trunk/test/Transforms/SeparateConstOffsetFromGEP/NVPTX/lit.local.cfg
> Thu May 1 13:38:36 2014
> @@ -0,0 +1,4 @@
> +targets = set(config.root.targets_to_build.split())
> +if not 'NVPTX' in targets:
> + config.unsupported = True
> +
>
> Added:
> llvm/trunk/test/Transforms/SeparateConstOffsetFromGEP/NVPTX/split-gep-and-gvn.ll
> URL:
> http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Transforms/SeparateConstOffsetFromGEP/NVPTX/split-gep-and-gvn.ll?rev=207783&view=auto
>
> ==============================================================================
> ---
> llvm/trunk/test/Transforms/SeparateConstOffsetFromGEP/NVPTX/split-gep-and-gvn.ll
> (added)
> +++
> llvm/trunk/test/Transforms/SeparateConstOffsetFromGEP/NVPTX/split-gep-and-gvn.ll
> Thu May 1 13:38:36 2014
> @@ -0,0 +1,60 @@
> +; RUN: llc < %s -march=nvptx -mcpu=sm_20 | FileCheck %s --check-prefix=PTX
> +; RUN: llc < %s -march=nvptx64 -mcpu=sm_20 | FileCheck %s
> --check-prefix=PTX
> +; RUN: opt < %s -S -separate-const-offset-from-gep -gvn -dce | FileCheck
> %s --check-prefix=IR
> +
> +; Verifies the SeparateConstOffsetFromGEP pass.
> +; The following code computes
> +; *output = array[x][y] + array[x][y+1] + array[x+1][y] + array[x+1][y+1]
> +;
> +; We expect SeparateConstOffsetFromGEP to transform it to
> +;
> +; float *base = &a[x][y];
> +; *output = base[0] + base[1] + base[32] + base[33];
> +;
> +; so the backend can emit PTX that uses fewer virtual registers.
> +
> +target datalayout = "e-i64:64-v16:16-v32:32-n16:32:64"
> +target triple = "nvptx64-unknown-unknown"
> +
> + at array = internal addrspace(3) constant [32 x [32 x float]]
> zeroinitializer, align 4
> +
> +define void @sum_of_array(i32 %x, i32 %y, float* nocapture %output) {
> +.preheader:
> + %0 = zext i32 %y to i64
> + %1 = zext i32 %x to i64
> + %2 = getelementptr inbounds [32 x [32 x float]] addrspace(3)* @array,
> i64 0, i64 %1, i64 %0
> + %3 = addrspacecast float addrspace(3)* %2 to float*
> + %4 = load float* %3, align 4
> + %5 = fadd float %4, 0.000000e+00
> + %6 = add i32 %y, 1
> + %7 = zext i32 %6 to i64
> + %8 = getelementptr inbounds [32 x [32 x float]] addrspace(3)* @array,
> i64 0, i64 %1, i64 %7
> + %9 = addrspacecast float addrspace(3)* %8 to float*
> + %10 = load float* %9, align 4
> + %11 = fadd float %5, %10
> + %12 = add i32 %x, 1
> + %13 = zext i32 %12 to i64
> + %14 = getelementptr inbounds [32 x [32 x float]] addrspace(3)* @array,
> i64 0, i64 %13, i64 %0
> + %15 = addrspacecast float addrspace(3)* %14 to float*
> + %16 = load float* %15, align 4
> + %17 = fadd float %11, %16
> + %18 = getelementptr inbounds [32 x [32 x float]] addrspace(3)* @array,
> i64 0, i64 %13, i64 %7
> + %19 = addrspacecast float addrspace(3)* %18 to float*
> + %20 = load float* %19, align 4
> + %21 = fadd float %17, %20
> + store float %21, float* %output, align 4
> + ret void
> +}
> +
> +; PTX-LABEL: sum_of_array(
> +; PTX: ld.shared.f32 {{%f[0-9]+}}, {{\[}}[[BASE_REG:%(rl|r)[0-9]+]]{{\]}}
> +; PTX: ld.shared.f32 {{%f[0-9]+}}, {{\[}}[[BASE_REG]]+4{{\]}}
> +; PTX: ld.shared.f32 {{%f[0-9]+}}, {{\[}}[[BASE_REG]]+128{{\]}}
> +; PTX: ld.shared.f32 {{%f[0-9]+}}, {{\[}}[[BASE_REG]]+132{{\]}}
> +
> +; IR-LABEL: @sum_of_array(
> +; IR: [[BASE_PTR:%[0-9]+]] = getelementptr inbounds [32 x [32 x float]]
> addrspace(3)* @array, i64 0, i32 %x, i32 %y
> +; IR: [[BASE_INT:%[0-9]+]] = ptrtoint float addrspace(3)* [[BASE_PTR]] to
> i64
> +; IR: %5 = add i64 [[BASE_INT]], 4
> +; IR: %10 = add i64 [[BASE_INT]], 128
> +; IR: %15 = add i64 [[BASE_INT]], 132
>
> Added:
> llvm/trunk/test/Transforms/SeparateConstOffsetFromGEP/NVPTX/split-gep.ll
> URL:
> http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Transforms/SeparateConstOffsetFromGEP/NVPTX/split-gep.ll?rev=207783&view=auto
>
> ==============================================================================
> ---
> llvm/trunk/test/Transforms/SeparateConstOffsetFromGEP/NVPTX/split-gep.ll
> (added)
> +++
> llvm/trunk/test/Transforms/SeparateConstOffsetFromGEP/NVPTX/split-gep.ll
> Thu May 1 13:38:36 2014
> @@ -0,0 +1,101 @@
> +; RUN: opt < %s -separate-const-offset-from-gep -dce -S | FileCheck %s
> +
> +; Several unit tests for -separate-const-offset-from-gep. The
> transformation
> +; heavily relies on TargetTransformInfo, so we put these tests under
> +; target-specific folders.
> +
> +target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
> +; target triple is necessary; otherwise TargetTransformInfo rejects any
> +; addressing mode.
> +target triple = "nvptx64-unknown-unknown"
> +
> +%struct.S = type { float, double }
> +
> + at struct_array = global [1024 x %struct.S] zeroinitializer, align 16
> + at float_2d_array = global [32 x [32 x float]] zeroinitializer, align 4
> +
> +; We should not extract any struct field indices, because fields in a
> struct
> +; may have different types.
> +define double* @struct(i32 %i) {
> +entry:
> + %add = add nsw i32 %i, 5
> + %idxprom = sext i32 %add to i64
> + %p = getelementptr inbounds [1024 x %struct.S]* @struct_array, i64 0,
> i64 %idxprom, i32 1
> + ret double* %p
> +}
> +; CHECK-LABEL: @struct
> +; CHECK: getelementptr [1024 x %struct.S]* @struct_array, i64 0, i32 %i,
> i32 1
> +
> +; We should be able to trace into sext/zext if it's directly used as a GEP
> +; index.
> +define float* @sext_zext(i32 %i, i32 %j) {
> +entry:
> + %i1 = add i32 %i, 1
> + %j2 = add i32 %j, 2
> + %i1.ext = sext i32 %i1 to i64
> + %j2.ext = zext i32 %j2 to i64
> + %p = getelementptr inbounds [32 x [32 x float]]* @float_2d_array, i64
> 0, i64 %i1.ext, i64 %j2.ext
> + ret float* %p
> +}
> +; CHECK-LABEL: @sext_zext
> +; CHECK: getelementptr [32 x [32 x float]]* @float_2d_array, i64 0, i32
> %i, i32 %j
> +; CHECK: add i64 %{{[0-9]+}}, 136
> +
> +; We should be able to trace into sext/zext if it can be distributed to
> both
> +; operands, e.g., sext (add nsw a, b) == add nsw (sext a), (sext b)
> +define float* @ext_add_no_overflow(i64 %a, i32 %b, i64 %c, i32 %d) {
> + %b1 = add nsw i32 %b, 1
> + %b2 = sext i32 %b1 to i64
> + %i = add i64 %a, %b2
> + %d1 = add nuw i32 %d, 1
> + %d2 = zext i32 %d1 to i64
> + %j = add i64 %c, %d2
> + %p = getelementptr inbounds [32 x [32 x float]]* @float_2d_array, i64
> 0, i64 %i, i64 %j
> + ret float* %p
> +}
> +; CHECK-LABEL: @ext_add_no_overflow
> +; CHECK: [[BASE_PTR:%[0-9]+]] = getelementptr [32 x [32 x float]]*
> @float_2d_array, i64 0, i64 %{{[0-9]+}}, i64 %{{[0-9]+}}
> +; CHECK: [[BASE_INT:%[0-9]+]] = ptrtoint float* [[BASE_PTR]] to i64
> +; CHECK: add i64 [[BASE_INT]], 132
> +
> +; We should treat "or" with no common bits (%k) as "add", and leave "or"
> with
> +; potentially common bits (%l) as is.
> +define float* @or(i64 %i) {
> +entry:
> + %j = shl i64 %i, 2
> + %k = or i64 %j, 3 ; no common bits
> + %l = or i64 %j, 4 ; potentially common bits
> + %p = getelementptr inbounds [32 x [32 x float]]* @float_2d_array, i64
> 0, i64 %k, i64 %l
> + ret float* %p
> +}
> +; CHECK-LABEL: @or
> +; CHECK: getelementptr [32 x [32 x float]]* @float_2d_array, i64 0, i64
> %j, i64 %l
> +; CHECK: add i64 %{{[0-9]+}}, 384
> +
> +; The subexpression (b + 5) is used in both "i = a + (b + 5)" and "*out =
> b +
> +; 5". When extracting the constant offset 5, make sure "*out = b + 5"
> isn't
> +; affected.
> +define float* @expr(i64 %a, i64 %b, i64* %out) {
> +entry:
> + %b5 = add i64 %b, 5
> + %i = add i64 %b5, %a
> + %p = getelementptr inbounds [32 x [32 x float]]* @float_2d_array, i64
> 0, i64 %i, i64 0
> + store i64 %b5, i64* %out
> + ret float* %p
> +}
> +; CHECK-LABEL: @expr
> +; CHECK: getelementptr [32 x [32 x float]]* @float_2d_array, i64 0, i64
> %0, i64 0
> +; CHECK: add i64 %{{[0-9]+}}, 640
> +; CHECK: store i64 %b5, i64* %out
> +
> +; Verifies we handle "sub" correctly.
> +define float* @sub(i64 %i, i64 %j) {
> + %i2 = sub i64 %i, 5 ; i - 5
> + %j2 = sub i64 5, %j ; 5 - i
> + %p = getelementptr inbounds [32 x [32 x float]]* @float_2d_array, i64
> 0, i64 %i2, i64 %j2
> + ret float* %p
> +}
> +; CHECK-LABEL: @sub
> +; CHECK: %[[j2:[0-9]+]] = sub i64 0, %j
> +; CHECK: getelementptr [32 x [32 x float]]* @float_2d_array, i64 0, i64
> %i, i64 %[[j2]]
> +; CHECK: add i64 %{{[0-9]+}}, -620
>
>
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