1 files changed, 209 insertions, 0 deletions
diff --git a/contrib/llvm-project/llvm/lib/Target/X86/X86PadShortFunction.cpp b/contrib/llvm-project/llvm/lib/Target/X86/X86PadShortFunction.cpp
new file mode 100644
index 000000000000..af974c805c36
--- /dev/null
+++ b/contrib/llvm-project/llvm/lib/Target/X86/X86PadShortFunction.cpp
@@ -0,0 +1,209 @@
+//===-------- X86PadShortFunction.cpp - pad short functions -----------===//
+//
+// 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 defines the pass which will pad short functions to prevent
+// a stall if a function returns before the return address is ready. This
+// is needed for some Intel Atom processors.
+//
+//===----------------------------------------------------------------------===//
+
+
+#include "X86.h"
+#include "X86InstrInfo.h"
+#include "X86Subtarget.h"
+#include "llvm/ADT/Statistic.h"
+#include "llvm/CodeGen/MachineFunctionPass.h"
+#include "llvm/CodeGen/MachineInstrBuilder.h"
+#include "llvm/CodeGen/Passes.h"
+#include "llvm/CodeGen/TargetSchedule.h"
+#include "llvm/IR/Function.h"
+#include "llvm/Support/Debug.h"
+#include "llvm/Support/raw_ostream.h"
+
+using namespace llvm;
+
+#define DEBUG_TYPE "x86-pad-short-functions"
+
+STATISTIC(NumBBsPadded, "Number of basic blocks padded");
+
+namespace {
+  struct VisitedBBInfo {
+    // HasReturn - Whether the BB contains a return instruction
+    bool HasReturn;
+
+    // Cycles - Number of cycles until return if HasReturn is true, otherwise
+    // number of cycles until end of the BB
+    unsigned int Cycles;
+
+    VisitedBBInfo() : HasReturn(false), Cycles(0) {}
+    VisitedBBInfo(bool HasReturn, unsigned int Cycles)
+      : HasReturn(HasReturn), Cycles(Cycles) {}
+  };
+
+  struct PadShortFunc : public MachineFunctionPass {
+    static char ID;
+    PadShortFunc() : MachineFunctionPass(ID)
+                   , Threshold(4) {}
+
+    bool runOnMachineFunction(MachineFunction &MF) override;
+
+    MachineFunctionProperties getRequiredProperties() const override {
+      return MachineFunctionProperties().set(
+          MachineFunctionProperties::Property::NoVRegs);
+    }
+
+    StringRef getPassName() const override {
+      return "X86 Atom pad short functions";
+    }
+
+  private:
+    void findReturns(MachineBasicBlock *MBB,
+                     unsigned int Cycles = 0);
+
+    bool cyclesUntilReturn(MachineBasicBlock *MBB,
+                           unsigned int &Cycles);
+
+    void addPadding(MachineBasicBlock *MBB,
+                    MachineBasicBlock::iterator &MBBI,
+                    unsigned int NOOPsToAdd);
+
+    const unsigned int Threshold;
+
+    // ReturnBBs - Maps basic blocks that return to the minimum number of
+    // cycles until the return, starting from the entry block.
+    DenseMap<MachineBasicBlock*, unsigned int> ReturnBBs;
+
+    // VisitedBBs - Cache of previously visited BBs.
+    DenseMap<MachineBasicBlock*, VisitedBBInfo> VisitedBBs;
+
+    TargetSchedModel TSM;
+  };
+
+  char PadShortFunc::ID = 0;
+}
+
+FunctionPass *llvm::createX86PadShortFunctions() {
+  return new PadShortFunc();
+}
+
+/// runOnMachineFunction - Loop over all of the basic blocks, inserting
+/// NOOP instructions before early exits.
+bool PadShortFunc::runOnMachineFunction(MachineFunction &MF) {
+  if (skipFunction(MF.getFunction()))
+    return false;
+
+  if (MF.getFunction().hasOptSize())
+    return false;
+
+  if (!MF.getSubtarget<X86Subtarget>().padShortFunctions())
+    return false;
+
+  TSM.init(&MF.getSubtarget());
+
+  // Search through basic blocks and mark the ones that have early returns
+  ReturnBBs.clear();
+  VisitedBBs.clear();
+  findReturns(&MF.front());
+
+  bool MadeChange = false;
+
+  // Pad the identified basic blocks with NOOPs
+  for (DenseMap<MachineBasicBlock*, unsigned int>::iterator I = ReturnBBs.begin();
+       I != ReturnBBs.end(); ++I) {
+    MachineBasicBlock *MBB = I->first;
+    unsigned Cycles = I->second;
+
+    if (Cycles < Threshold) {
+      // BB ends in a return. Skip over any DBG_VALUE instructions
+      // trailing the terminator.
+      assert(MBB->size() > 0 &&
+             "Basic block should contain at least a RET but is empty");
+      MachineBasicBlock::iterator ReturnLoc = --MBB->end();
+
+      while (ReturnLoc->isDebugInstr())
+        --ReturnLoc;
+      assert(ReturnLoc->isReturn() && !ReturnLoc->isCall() &&
+             "Basic block does not end with RET");
+
+      addPadding(MBB, ReturnLoc, Threshold - Cycles);
+      NumBBsPadded++;
+      MadeChange = true;
+    }
+  }
+
+  return MadeChange;
+}
+
+/// findReturn - Starting at MBB, follow control flow and add all
+/// basic blocks that contain a return to ReturnBBs.
+void PadShortFunc::findReturns(MachineBasicBlock *MBB, unsigned int Cycles) {
+  // If this BB has a return, note how many cycles it takes to get there.
+  bool hasReturn = cyclesUntilReturn(MBB, Cycles);
+  if (Cycles >= Threshold)
+    return;
+
+  if (hasReturn) {
+    ReturnBBs[MBB] = std::max(ReturnBBs[MBB], Cycles);
+    return;
+  }
+
+  // Follow branches in BB and look for returns
+  for (MachineBasicBlock::succ_iterator I = MBB->succ_begin();
+       I != MBB->succ_end(); ++I) {
+    if (*I == MBB)
+      continue;
+    findReturns(*I, Cycles);
+  }
+}
+
+/// cyclesUntilReturn - return true if the MBB has a return instruction,
+/// and return false otherwise.
+/// Cycles will be incremented by the number of cycles taken to reach the
+/// return or the end of the BB, whichever occurs first.
+bool PadShortFunc::cyclesUntilReturn(MachineBasicBlock *MBB,
+                                     unsigned int &Cycles) {
+  // Return cached result if BB was previously visited
+  DenseMap<MachineBasicBlock*, VisitedBBInfo>::iterator it
+    = VisitedBBs.find(MBB);
+  if (it != VisitedBBs.end()) {
+    VisitedBBInfo BBInfo = it->second;
+    Cycles += BBInfo.Cycles;
+    return BBInfo.HasReturn;
+  }
+
+  unsigned int CyclesToEnd = 0;
+
+  for (MachineInstr &MI : *MBB) {
+    // Mark basic blocks with a return instruction. Calls to other
+    // functions do not count because the called function will be padded,
+    // if necessary.
+    if (MI.isReturn() && !MI.isCall()) {
+      VisitedBBs[MBB] = VisitedBBInfo(true, CyclesToEnd);
+      Cycles += CyclesToEnd;
+      return true;
+    }
+
+    CyclesToEnd += TSM.computeInstrLatency(&MI);
+  }
+
+  VisitedBBs[MBB] = VisitedBBInfo(false, CyclesToEnd);
+  Cycles += CyclesToEnd;
+  return false;
+}
+
+/// addPadding - Add the given number of NOOP instructions to the function
+/// just prior to the return at MBBI
+void PadShortFunc::addPadding(MachineBasicBlock *MBB,
+                              MachineBasicBlock::iterator &MBBI,
+                              unsigned int NOOPsToAdd) {
+  DebugLoc DL = MBBI->getDebugLoc();
+  unsigned IssueWidth = TSM.getIssueWidth();
+
+  for (unsigned i = 0, e = IssueWidth * NOOPsToAdd; i != e; ++i)
+    BuildMI(*MBB, MBBI, DL, TSM.getInstrInfo()->get(X86::NOOP));
+}