[llvm] [RISCV] Introduce VLOptimizer pass (PR #108640)

[Philip Reames via llvm-commits]

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+//===-------------- RISCVVLOptimizer.cpp - VL Optimizer -------------------===//
+//
+// 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 pass reduces the VL where possible at the MI level, before VSETVLI
+// instructions are inserted.
+//
+// The purpose of this optimization is to make the VL argument, for instructions
+// that have a VL argument, as small as possible. This is implemented by
+// visiting each instruction in reverse order and checking that if it has a VL
+// argument, whether the VL can be reduced.
+//
+//===---------------------------------------------------------------------===//
+
+#include "RISCV.h"
+#include "RISCVMachineFunctionInfo.h"
+#include "RISCVSubtarget.h"
+#include "llvm/ADT/SetVector.h"
+#include "llvm/CodeGen/MachineDominators.h"
+#include "llvm/CodeGen/MachineFunctionPass.h"
+#include "llvm/InitializePasses.h"
+
+#include <algorithm>
+
+using namespace llvm;
+
+#define DEBUG_TYPE "riscv-vl-optimizer"
+#define PASS_NAME "RISC-V VL Optimizer"
+
+namespace {
+
+class RISCVVLOptimizer : public MachineFunctionPass {
+  const MachineRegisterInfo *MRI;
+  const MachineDominatorTree *MDT;
+
+public:
+  static char ID;
+
+  RISCVVLOptimizer() : MachineFunctionPass(ID) {}
+
+  bool runOnMachineFunction(MachineFunction &MF) override;
+
+  void getAnalysisUsage(AnalysisUsage &AU) const override {
+    AU.setPreservesCFG();
+    AU.addRequired<MachineDominatorTreeWrapperPass>();
+    MachineFunctionPass::getAnalysisUsage(AU);
+  }
+
+  StringRef getPassName() const override { return PASS_NAME; }
+
+private:
+  bool tryReduceVL(MachineInstr &MI);
+  bool isCandidate(const MachineInstr &MI) const;
+};
+
+} // end anonymous namespace
+
+char RISCVVLOptimizer::ID = 0;
+INITIALIZE_PASS_BEGIN(RISCVVLOptimizer, DEBUG_TYPE, PASS_NAME, false, false)
+INITIALIZE_PASS_DEPENDENCY(MachineDominatorTreeWrapperPass)
+INITIALIZE_PASS_END(RISCVVLOptimizer, DEBUG_TYPE, PASS_NAME, false, false)
+
+FunctionPass *llvm::createRISCVVLOptimizerPass() {
+  return new RISCVVLOptimizer();
+}
+
+/// Return true if R is a physical or virtual vector register, false otherwise.
+static bool isVectorRegClass(Register R, const MachineRegisterInfo *MRI) {
+  if (R.isPhysical())
+    return RISCV::VRRegClass.contains(R);
+  const TargetRegisterClass *RC = MRI->getRegClass(R);
+  return RISCVRI::isVRegClass(RC->TSFlags);
+}
+
+/// Represents the EMUL and EEW of a MachineOperand.
+struct OperandInfo {
+  enum class State {
+    Unknown,
+    Known,
+  } S;
+
+  // Represent as 1,2,4,8, ... and fractional indicator. This is because
+  // EMUL can take on values that don't map to RISCVII::VLMUL values exactly.
+  // For example, a mask operand can have an EMUL less than MF8.
+  std::pair<unsigned, bool> EMUL;
+
+  unsigned Log2EEW;
+
+  OperandInfo(RISCVII::VLMUL EMUL, unsigned Log2EEW)
+      : S(State::Known), EMUL(RISCVVType::decodeVLMUL(EMUL)), Log2EEW(Log2EEW) {
+  }
+
+  OperandInfo(std::pair<unsigned, bool> EMUL, unsigned Log2EEW)
+      : S(State::Known), EMUL(EMUL), Log2EEW(Log2EEW) {}
+
+  OperandInfo(State S) : S(S) {
+    assert(S != State::Known &&
+           "This constructor may only be used to construct "
+           "an Unknown OperandInfo");
+  }
+
+  bool isUnknown() const { return S == State::Unknown; }
+  bool isKnown() const { return S == State::Known; }
+
+  static bool EMULAndEEWAreEqual(const OperandInfo &A, const OperandInfo &B) {
+    assert(A.isKnown() && B.isKnown() && "Both operands must be known");
+    return A.Log2EEW == B.Log2EEW && A.EMUL.first == B.EMUL.first &&
+           A.EMUL.second == B.EMUL.second;
+  }
+
+  void print(raw_ostream &OS) const {
+    if (isUnknown()) {
+      OS << "Unknown";
+      return;
+    }
+    OS << "EMUL: ";
+    if (EMUL.second)
+      OS << "m";
+    OS << "f" << EMUL.first;
+    OS << ", EEW: " << (1 << Log2EEW);
+  }
+};
+
+static raw_ostream &operator<<(raw_ostream &OS, const OperandInfo &OI) {
+  OI.print(OS);
+  return OS;
+}
+
+namespace llvm {
+namespace RISCVVType {
+/// Return the RISCVII::VLMUL that is two times VLMul.
+/// Precondition: VLMul is not LMUL_RESERVED or LMUL_8.
+static RISCVII::VLMUL twoTimesVLMUL(RISCVII::VLMUL VLMul) {
+  switch (VLMul) {
+  case RISCVII::VLMUL::LMUL_F8:
+    return RISCVII::VLMUL::LMUL_F4;
+  case RISCVII::VLMUL::LMUL_F4:
+    return RISCVII::VLMUL::LMUL_F2;
+  case RISCVII::VLMUL::LMUL_F2:
+    return RISCVII::VLMUL::LMUL_1;
+  case RISCVII::VLMUL::LMUL_1:
+    return RISCVII::VLMUL::LMUL_2;
+  case RISCVII::VLMUL::LMUL_2:
+    return RISCVII::VLMUL::LMUL_4;
+  case RISCVII::VLMUL::LMUL_4:
+    return RISCVII::VLMUL::LMUL_8;
+  case RISCVII::VLMUL::LMUL_8:
+  default:
+    llvm_unreachable("Could not multiply VLMul by 2");
+  }
+}
+
+/// Return the RISCVII::VLMUL that is VLMul / 2.
+/// Precondition: VLMul is not LMUL_RESERVED or LMUL_MF8.
+static RISCVII::VLMUL halfVLMUL(RISCVII::VLMUL VLMul) {
+  switch (VLMul) {
+  case RISCVII::VLMUL::LMUL_F4:
+    return RISCVII::VLMUL::LMUL_F8;
+  case RISCVII::VLMUL::LMUL_F2:
+    return RISCVII::VLMUL::LMUL_F4;
+  case RISCVII::VLMUL::LMUL_1:
+    return RISCVII::VLMUL::LMUL_F2;
+  case RISCVII::VLMUL::LMUL_2:
+    return RISCVII::VLMUL::LMUL_1;
+  case RISCVII::VLMUL::LMUL_4:
+    return RISCVII::VLMUL::LMUL_2;
+  case RISCVII::VLMUL::LMUL_8:
+    return RISCVII::VLMUL::LMUL_4;
+  case RISCVII::VLMUL::LMUL_F8:
+  default:
+    llvm_unreachable("Could not divide VLMul by 2");
+  }
+}
+
+/// Return EMUL = (EEW / SEW) * LMUL where EEW comes from Log2EEW and LMUL and
+/// SEW are from the TSFlags of MI.
+static std::pair<unsigned, bool>
+getEMULEqualsEEWDivSEWTimesLMUL(unsigned Log2EEW, const MachineInstr &MI) {
+  RISCVII::VLMUL MIVLMUL = RISCVII::getLMul(MI.getDesc().TSFlags);
+  auto [MILMUL, MILMULIsFractional] = RISCVVType::decodeVLMUL(MIVLMUL);
+  unsigned MILog2SEW =
+      MI.getOperand(RISCVII::getSEWOpNum(MI.getDesc())).getImm();
+  unsigned MISEW = 1 << MILog2SEW;
+
+  unsigned EEW = 1 << Log2EEW;
+  // Calculate (EEW/SEW)*LMUL preserving fractions less than 1. Use GCD
+  // to put fraction in simplest form.
+  unsigned Num = EEW, Denom = MISEW;
+  int GCD = MILMULIsFractional ? std::gcd(Num, Denom * MILMUL)
+                               : std::gcd(Num * MILMUL, Denom);
+  Num = MILMULIsFractional ? Num / GCD : Num * MILMUL / GCD;
+  Denom = MILMULIsFractional ? Denom * MILMUL / GCD : Denom / GCD;
+  return std::make_pair(Num > Denom ? Num : Denom, Denom > Num);
+}
+} // end namespace RISCVVType
+} // end namespace llvm
+
+static bool isOpN(const MachineOperand &MO, unsigned OpN) {
+  const MachineInstr &MI = *MO.getParent();
+  bool HasPassthru = RISCVII::isFirstDefTiedToFirstUse(MI.getDesc());
+
+  if (HasPassthru)
----------------
preames wrote:

This silent off by one on the indexing is confusing to say the least.  Given this is only used in a handful of places, I'd prefer to see this inlined and specialized for each user.

https://github.com/llvm/llvm-project/pull/108640