llvm-exegesis/lib/ParallelSnippetGenerator.cpp

//===-- ParallelSnippetGenerator.cpp ----------------------------*- C++ -*-===//
//
// 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
//
//===----------------------------------------------------------------------===//

#include "ParallelSnippetGenerator.h"

#include "BenchmarkRunner.h"
#include "MCInstrDescView.h"
#include "Target.h"

// FIXME: Load constants into registers (e.g. with fld1) to not break
// instructions like x87.

// Ideally we would like the only limitation on executing instructions to be the
// availability of the CPU resources (e.g. execution ports) needed to execute
// them, instead of the availability of their data dependencies.

// To achieve that, one approach is to generate instructions that do not have
// data dependencies between them.
//
// For some instructions, this is trivial:
//    mov rax, qword ptr [rsi]
//    mov rax, qword ptr [rsi]
//    mov rax, qword ptr [rsi]
//    mov rax, qword ptr [rsi]
// For the above snippet, haswell just renames rax four times and executes the
// four instructions two at a time on P23 and P0126.
//
// For some instructions, we just need to make sure that the source is
// different from the destination. For example, IDIV8r reads from GPR and
// writes to AX. We just need to ensure that the Var is assigned a
// register which is different from AX:
//    idiv bx
//    idiv bx
//    idiv bx
//    idiv bx
// The above snippet will be able to fully saturate the ports, while the same
// with ax would issue one uop every `latency(IDIV8r)` cycles.
//
// Some instructions make this harder because they both read and write from
// the same register:
//    inc rax
//    inc rax
//    inc rax
//    inc rax
// This has a data dependency from each instruction to the next, limit the
// number of instructions that can be issued in parallel.
// It turns out that this is not a big issue on recent Intel CPUs because they
// have heuristics to balance port pressure. In the snippet above, subsequent
// instructions will end up evenly distributed on {P0,P1,P5,P6}, but some CPUs
// might end up executing them all on P0 (just because they can), or try
// avoiding P5 because it's usually under high pressure from vector
// instructions.
// This issue is even more important for high-latency instructions because
// they increase the idle time of the CPU, e.g. :
//    imul rax, rbx
//    imul rax, rbx
//    imul rax, rbx
//    imul rax, rbx
//
// To avoid that, we do the renaming statically by generating as many
// independent exclusive assignments as possible (until all possible registers
// are exhausted) e.g.:
//    imul rax, rbx
//    imul rcx, rbx
//    imul rdx, rbx
//    imul r8,  rbx
//
// Some instruction even make the above static renaming impossible because
// they implicitly read and write from the same operand, e.g. ADC16rr reads
// and writes from EFLAGS.
// In that case we just use a greedy register assignment and hope for the
// best.

namespace llvm {
namespace exegesis {

static SmallVector<const Variable *, 8>
getVariablesWithTiedOperands(const Instruction &Instr) {
  SmallVector<const Variable *, 8> Result;
  for (const auto &Var : Instr.Variables)
    if (Var.hasTiedOperands())
      Result.push_back(&Var);
  return Result;
}

ParallelSnippetGenerator::~ParallelSnippetGenerator() = default;

void ParallelSnippetGenerator::instantiateMemoryOperands(
    const unsigned ScratchSpacePointerInReg,
    std::vector<InstructionTemplate> &Instructions) const {
  if (ScratchSpacePointerInReg == 0)
    return; // no memory operands.
  const auto &ET = State.getExegesisTarget();
  const unsigned MemStep = ET.getMaxMemoryAccessSize();
  const size_t OriginalInstructionsSize = Instructions.size();
  size_t I = 0;
  for (InstructionTemplate &IT : Instructions) {
    ET.fillMemoryOperands(IT, ScratchSpacePointerInReg, I * MemStep);
    ++I;
  }

  while (Instructions.size() < kMinNumDifferentAddresses) {
    InstructionTemplate IT = Instructions[I % OriginalInstructionsSize];
    ET.fillMemoryOperands(IT, ScratchSpacePointerInReg, I * MemStep);
    ++I;
    Instructions.push_back(std::move(IT));
  }
  assert(I * MemStep < BenchmarkRunner::ScratchSpace::kSize &&
         "not enough scratch space");
}

static std::vector<InstructionTemplate> generateSnippetUsingStaticRenaming(
    const LLVMState &State, const InstructionTemplate &IT,
    const ArrayRef<const Variable *> TiedVariables,
    const BitVector &ForbiddenRegisters) {
  std::vector<InstructionTemplate> Instructions;
  // Assign registers to variables in a round-robin manner. This is simple but
  // ensures that the most register-constrained variable does not get starved.
  std::vector<BitVector> PossibleRegsForVar;
  for (const Variable *Var : TiedVariables) {
    assert(Var);
    const Operand &Op = IT.getInstr().getPrimaryOperand(*Var);
    assert(Op.isReg());
    BitVector PossibleRegs = Op.getRegisterAliasing().sourceBits();
    remove(PossibleRegs, ForbiddenRegisters);
    PossibleRegsForVar.push_back(std::move(PossibleRegs));
  }
  SmallVector<int, 2> Iterators(TiedVariables.size(), 0);
  while (true) {
    InstructionTemplate TmpIT = IT;
    // Find a possible register for each variable in turn, marking the
    // register as taken.
    for (size_t VarId = 0; VarId < TiedVariables.size(); ++VarId) {
      const int NextPossibleReg =
          PossibleRegsForVar[VarId].find_next(Iterators[VarId]);
      if (NextPossibleReg <= 0) {
        return Instructions;
      }
      TmpIT.getValueFor(*TiedVariables[VarId]) =
          MCOperand::createReg(NextPossibleReg);
      // Bump iterator.
      Iterators[VarId] = NextPossibleReg;
      // Prevent other variables from using the register.
      for (BitVector &OtherPossibleRegs : PossibleRegsForVar) {
        OtherPossibleRegs.reset(NextPossibleReg);
      }
    }
    Instructions.push_back(std::move(TmpIT));
  }
}

Expected<std::vector<CodeTemplate>> ParallelSnippetGenerator::generateCodeTemplates(
    const Instruction &Instr, const BitVector &ForbiddenRegisters) const {
  CodeTemplate CT;
  CT.ScratchSpacePointerInReg =
      Instr.hasMemoryOperands()
          ? State.getExegesisTarget().getScratchMemoryRegister(
                State.getTargetMachine().getTargetTriple())
          : 0;
  const AliasingConfigurations SelfAliasing(Instr, Instr);
  InstructionTemplate IT(&Instr);
  if (SelfAliasing.empty()) {
    CT.Info = "instruction is parallel, repeating a random one.";
    CT.Instructions.push_back(std::move(IT));
    instantiateMemoryOperands(CT.ScratchSpacePointerInReg, CT.Instructions);
    return getSingleton(std::move(CT));
  }
  if (SelfAliasing.hasImplicitAliasing()) {
    CT.Info = "instruction is serial, repeating a random one.";
    CT.Instructions.push_back(std::move(IT));
    instantiateMemoryOperands(CT.ScratchSpacePointerInReg, CT.Instructions);
    return getSingleton(std::move(CT));
  }
  const auto TiedVariables = getVariablesWithTiedOperands(Instr);
  if (!TiedVariables.empty()) {
    CT.Info = "instruction has tied variables, using static renaming.";
    CT.Instructions = generateSnippetUsingStaticRenaming(
        State, IT, TiedVariables, ForbiddenRegisters);
    instantiateMemoryOperands(CT.ScratchSpacePointerInReg, CT.Instructions);
    return getSingleton(std::move(CT));
  }
  // No tied variables, we pick random values for defs.
  BitVector Defs(State.getRegInfo().getNumRegs());
  for (const auto &Op : Instr.Operands) {
    if (Op.isReg() && Op.isExplicit() && Op.isDef() && !Op.isMemory()) {
      auto PossibleRegisters = Op.getRegisterAliasing().sourceBits();
      // Do not use forbidden registers.
      remove(PossibleRegisters, ForbiddenRegisters);
      assert(PossibleRegisters.any() && "No register left to choose from");
      const auto RandomReg = randomBit(PossibleRegisters);
      Defs.set(RandomReg);
      IT.getValueFor(Op) = MCOperand::createReg(RandomReg);
    }
  }
  // And pick random use values that are not reserved and don't alias with defs.
  const auto DefAliases = getAliasedBits(State.getRegInfo(), Defs);
  for (const auto &Op : Instr.Operands) {
    if (Op.isReg() && Op.isExplicit() && Op.isUse() && !Op.isMemory()) {
      auto PossibleRegisters = Op.getRegisterAliasing().sourceBits();
      remove(PossibleRegisters, ForbiddenRegisters);
      remove(PossibleRegisters, DefAliases);
      assert(PossibleRegisters.any() && "No register left to choose from");
      const auto RandomReg = randomBit(PossibleRegisters);
      IT.getValueFor(Op) = MCOperand::createReg(RandomReg);
    }
  }
  CT.Info =
      "instruction has no tied variables picking Uses different from defs";
  CT.Instructions.push_back(std::move(IT));
  instantiateMemoryOperands(CT.ScratchSpacePointerInReg, CT.Instructions);
  return getSingleton(std::move(CT));
}

constexpr const size_t ParallelSnippetGenerator::kMinNumDifferentAddresses;

} // namespace exegesis
} // namespace llvm