Support.cpp

//===--------------------- Support.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
//
//===----------------------------------------------------------------------===//
/// \file
///
/// This file implements a few helper functions used by various pipeline
/// components.
///
//===----------------------------------------------------------------------===//

#include "llvm/MCA/Support.h"
#include "llvm/MC/MCSchedule.h"
#include <numeric>

namespace llvm {
namespace mca {

#define DEBUG_TYPE "llvm-mca"

ReleaseAtCycles &ReleaseAtCycles::operator+=(const ReleaseAtCycles &RHS) {
  if (Denominator == RHS.Denominator)
    Numerator += RHS.Numerator;
  else {
    // Create a common denominator for LHS and RHS by calculating the least
    // common multiple from the GCD.
    unsigned GCD = std::gcd(Denominator, RHS.Denominator);
    unsigned LCM = (Denominator * RHS.Denominator) / GCD;
    unsigned LHSNumerator = Numerator * (LCM / Denominator);
    unsigned RHSNumerator = RHS.Numerator * (LCM / RHS.Denominator);
    Numerator = LHSNumerator + RHSNumerator;
    Denominator = LCM;
  }
  return *this;
}

void computeProcResourceMasks(const MCSchedModel &SM,
                              MutableArrayRef<uint64_t> Masks) {
  unsigned ProcResourceID = 0;

  assert(Masks.size() == SM.getNumProcResourceKinds() &&
         "Invalid number of elements");
  // Resource at index 0 is the 'InvalidUnit'. Set an invalid mask for it.
  Masks[0] = 0;

  // Create a unique bitmask for every processor resource unit.
  for (unsigned I = 1, E = SM.getNumProcResourceKinds(); I < E; ++I) {
    const MCProcResourceDesc &Desc = *SM.getProcResource(I);
    if (Desc.SubUnitsIdxBegin)
      continue;
    Masks[I] = 1ULL << ProcResourceID;
    ProcResourceID++;
  }

  // Create a unique bitmask for every processor resource group.
  for (unsigned I = 1, E = SM.getNumProcResourceKinds(); I < E; ++I) {
    const MCProcResourceDesc &Desc = *SM.getProcResource(I);
    if (!Desc.SubUnitsIdxBegin)
      continue;
    Masks[I] = 1ULL << ProcResourceID;
    for (unsigned U = 0; U < Desc.NumUnits; ++U) {
      uint64_t OtherMask = Masks[Desc.SubUnitsIdxBegin[U]];
      Masks[I] |= OtherMask;
    }
    ProcResourceID++;
  }

#ifndef NDEBUG
  LLVM_DEBUG(dbgs() << "\nProcessor resource masks:"
                    << "\n");
  for (unsigned I = 0, E = SM.getNumProcResourceKinds(); I < E; ++I) {
    const MCProcResourceDesc &Desc = *SM.getProcResource(I);
    LLVM_DEBUG(dbgs() << '[' << format_decimal(I,2) << "] " << " - "
                      << format_hex(Masks[I],16) << " - "
                      << Desc.Name << '\n');
  }
#endif
}

double computeBlockRThroughput(const MCSchedModel &SM, unsigned DispatchWidth,
                               unsigned NumMicroOps,
                               ArrayRef<unsigned> ProcResourceUsage) {
  // The block throughput is bounded from above by the hardware dispatch
  // throughput. That is because the DispatchWidth is an upper bound on the
  // number of opcodes that can be part of a single dispatch group.
  double Max = static_cast<double>(NumMicroOps) / DispatchWidth;

  // The block throughput is also limited by the amount of hardware parallelism.
  // The number of available resource units affects the resource pressure
  // distribution, as well as how many blocks can be executed every cycle.
  for (unsigned I = 0, E = SM.getNumProcResourceKinds(); I < E; ++I) {
    unsigned ReleaseAtCycles = ProcResourceUsage[I];
    if (!ReleaseAtCycles)
      continue;

    const MCProcResourceDesc &MCDesc = *SM.getProcResource(I);
    double Throughput = static_cast<double>(ReleaseAtCycles) / MCDesc.NumUnits;
    Max = std::max(Max, Throughput);
  }

  // The block reciprocal throughput is computed as the MAX of:
  //  - (NumMicroOps / DispatchWidth)
  //  - (NumUnits / ReleaseAtCycles)   for every consumed processor resource.
  return Max;
}

} // namespace mca
} // namespace llvm