AArch64PromoteConstant.cpp | Explore in Territory

//==- AArch64PromoteConstant.cpp - Promote constant to global for AArch64 --==//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//
//
// This file implements the AArch64PromoteConstant pass which promotes constants
// to global variables when this is likely to be more efficient. Currently only
// types related to constant vector (i.e., constant vector, array of constant
// vectors, constant structure with a constant vector field, etc.) are promoted
// to global variables. Constant vectors are likely to be lowered in target
// constant pool during instruction selection already; therefore, the access
// will remain the same (memory load), but the structure types are not split
// into different constant pool accesses for each field. A bonus side effect is
// that created globals may be merged by the global merge pass.
//
// FIXME: This pass may be useful for other targets too.
//===----------------------------------------------------------------------===//

#include "AArch64.h"
#include "llvm/ADT/DenseMap.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/IR/BasicBlock.h"
#include "llvm/IR/Constant.h"
#include "llvm/IR/Constants.h"
#include "llvm/IR/Dominators.h"
#include "llvm/IR/Function.h"
#include "llvm/IR/GlobalValue.h"
#include "llvm/IR/GlobalVariable.h"
#include "llvm/IR/IRBuilder.h"
#include "llvm/IR/InlineAsm.h"
#include "llvm/IR/InstIterator.h"
#include "llvm/IR/Instruction.h"
#include "llvm/IR/Instructions.h"
#include "llvm/IR/IntrinsicInst.h"
#include "llvm/IR/Module.h"
#include "llvm/IR/Type.h"
#include "llvm/InitializePasses.h"
#include "llvm/Pass.h"
#include "llvm/Support/Casting.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/raw_ostream.h"
#include <algorithm>
#include <cassert>
#include <utility>

usingnamespacellvm;

#define DEBUG_TYPE …

// Stress testing mode - disable heuristics.
static cl::opt<bool> Stress("aarch64-stress-promote-const", cl::Hidden,
                            cl::desc("Promote all vector constants"));

STATISTIC(NumPromoted, "Number of promoted constants");
STATISTIC(NumPromotedUses, "Number of promoted constants uses");

//===----------------------------------------------------------------------===//
//                       AArch64PromoteConstant
//===----------------------------------------------------------------------===//

namespace {

/// Promotes interesting constant into global variables.
/// The motivating example is:
/// static const uint16_t TableA[32] = {
///   41944, 40330, 38837, 37450, 36158, 34953, 33826, 32768,
///   31776, 30841, 29960, 29128, 28340, 27595, 26887, 26215,
///   25576, 24967, 24386, 23832, 23302, 22796, 22311, 21846,
///   21400, 20972, 20561, 20165, 19785, 19419, 19066, 18725,
/// };
///
/// uint8x16x4_t LoadStatic(void) {
///   uint8x16x4_t ret;
///   ret.val[0] = vld1q_u16(TableA +  0);
///   ret.val[1] = vld1q_u16(TableA +  8);
///   ret.val[2] = vld1q_u16(TableA + 16);
///   ret.val[3] = vld1q_u16(TableA + 24);
///   return ret;
/// }
///
/// The constants in this example are folded into the uses. Thus, 4 different
/// constants are created.
///
/// As their type is vector the cheapest way to create them is to load them
/// for the memory.
///
/// Therefore the final assembly final has 4 different loads. With this pass
/// enabled, only one load is issued for the constants.
class AArch64PromoteConstant : public ModulePass { … };

} // end anonymous namespace

char AArch64PromoteConstant::ID = …;

INITIALIZE_PASS_BEGIN(AArch64PromoteConstant, "aarch64-promote-const",
                      "AArch64 Promote Constant Pass", false, false)
INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
INITIALIZE_PASS_END(AArch64PromoteConstant, "aarch64-promote-const",
                    "AArch64 Promote Constant Pass", false, false)

ModulePass *llvm::createAArch64PromoteConstantPass() { … }

/// Check if the given type uses a vector type.
static bool isConstantUsingVectorTy(const Type *CstTy) { … }

// Returns true if \p C contains only ConstantData leafs and no global values,
// block addresses or constant expressions. Traverses ConstantAggregates.
static bool containsOnlyConstantData(const Constant *C) { … }

/// Check if the given use (Instruction + OpIdx) of Cst should be converted into
/// a load of a global variable initialized with Cst.
/// A use should be converted if it is legal to do so.
/// For instance, it is not legal to turn the mask operand of a shuffle vector
/// into a load of a global variable.
static bool shouldConvertUse(const Constant *Cst, const Instruction *Instr,
                             unsigned OpIdx) { … }

/// Check if the given Cst should be converted into
/// a load of a global variable initialized with Cst.
/// A constant should be converted if it is likely that the materialization of
/// the constant will be tricky. Thus, we give up on zero or undef values.
///
/// \todo Currently, accept only vector related types.
/// Also we give up on all simple vector type to keep the existing
/// behavior. Otherwise, we should push here all the check of the lowering of
/// BUILD_VECTOR. By giving up, we lose the potential benefit of merging
/// constant via global merge and the fact that the same constant is stored
/// only once with this method (versus, as many function that uses the constant
/// for the regular approach, even for float).
/// Again, the simplest solution would be to promote every
/// constant and rematerialize them when they are actually cheap to create.
static bool shouldConvertImpl(const Constant *Cst) { … }

static bool
shouldConvert(Constant &C,
              AArch64PromoteConstant::PromotionCacheTy &PromotionCache) { … }

Instruction *AArch64PromoteConstant::findInsertionPoint(Instruction &User,
                                                        unsigned OpNo) { … }

bool AArch64PromoteConstant::isDominated(Instruction *NewPt, Instruction *User,
                                         unsigned OpNo,
                                         InsertionPoints &InsertPts) { … }

bool AArch64PromoteConstant::tryAndMerge(Instruction *NewPt, Instruction *User,
                                         unsigned OpNo,
                                         InsertionPoints &InsertPts) { … }

void AArch64PromoteConstant::computeInsertionPoint(
    Instruction *User, unsigned OpNo, InsertionPoints &InsertPts) { … }

static void ensurePromotedGV(Function &F, Constant &C,
                             AArch64PromoteConstant::PromotedConstant &PC) { … }

void AArch64PromoteConstant::insertDefinitions(Function &F,
                                               GlobalVariable &PromotedGV,
                                               InsertionPoints &InsertPts) { … }

void AArch64PromoteConstant::promoteConstants(
    Function &F, SmallVectorImpl<UpdateRecord> &Updates,
    PromotionCacheTy &PromotionCache) { … }

bool AArch64PromoteConstant::runOnFunction(Function &F,
                                           PromotionCacheTy &PromotionCache) { … }
llvm/llvm/lib/Target/AArch64/AArch64PromoteConstant.cpp
