def FeatureLeaForSP : SubtargetFeature<"lea-sp", "UseLeaForSP", "true",
"Use LEA for adjusting the stack pointer">;
def FeatureSlowDivide : SubtargetFeature<"idiv-to-divb",
- "HasSlowDivide", "true",
- "Use small divide for positive values less than 256">;
+ "HasSlowDivide", "true",
+ "Use small divide for positive values less than 256">;
+def FeaturePadShortFunctions : SubtargetFeature<"pad-short-functions",
+ "PadShortFunctions", "true",
+ "Pad short functions">;
//===----------------------------------------------------------------------===//
// X86 processors supported.
FeatureSlowBTMem]>;
def : AtomProc<"atom", [ProcIntelAtom, FeatureSSSE3, FeatureCMPXCHG16B,
FeatureMOVBE, FeatureSlowBTMem, FeatureLeaForSP,
- FeatureSlowDivide]>;
+ FeatureSlowDivide, FeaturePadShortFunctions]>;
// "Arrandale" along with corei3 and corei5
def : Proc<"corei7", [FeatureSSE42, FeatureCMPXCHG16B,
FeatureSlowBTMem, FeatureFastUAMem,
--- /dev/null
+//===-------- X86PadShortFunction.cpp - pad short functions -----------===//
+//
+// The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+//
+// 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 <algorithm>
+
+#define DEBUG_TYPE "x86-pad-short-functions"
+#include "X86.h"
+#include "X86InstrInfo.h"
+#include "llvm/ADT/Statistic.h"
+#include "llvm/CodeGen/MachineFunctionPass.h"
+#include "llvm/CodeGen/MachineInstrBuilder.h"
+#include "llvm/CodeGen/MachineRegisterInfo.h"
+#include "llvm/CodeGen/Passes.h"
+#include "llvm/IR/Function.h"
+#include "llvm/Support/Debug.h"
+#include "llvm/Support/raw_ostream.h"
+#include "llvm/Target/TargetInstrInfo.h"
+
+using namespace llvm;
+
+STATISTIC(NumBBsPadded, "Number of basic blocks padded");
+
+namespace {
+ struct PadShortFunc : public MachineFunctionPass {
+ static char ID;
+ PadShortFunc() : MachineFunctionPass(ID)
+ , Threshold(4), TM(0), TII(0) {}
+
+ virtual bool runOnMachineFunction(MachineFunction &MF);
+
+ virtual const char *getPassName() const {
+ return "X86 Atom pad short functions";
+ }
+
+ private:
+ void findReturns(MachineBasicBlock *MBB,
+ unsigned int Cycles = 0);
+
+ bool cyclesUntilReturn(MachineBasicBlock *MBB,
+ unsigned int &Cycles,
+ MachineBasicBlock::iterator *Location = 0);
+
+ void addPadding(MachineBasicBlock *MBB,
+ MachineBasicBlock::iterator &MBBI,
+ unsigned int NOOPsToAdd);
+
+ const unsigned int Threshold;
+ DenseMap<MachineBasicBlock*, unsigned int> ReturnBBs;
+
+ const TargetMachine *TM;
+ const TargetInstrInfo *TII;
+ };
+
+ 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) {
+ bool OptForSize = MF.getFunction()->getAttributes().
+ hasAttribute(AttributeSet::FunctionIndex, Attribute::OptimizeForSize);
+
+ if (OptForSize)
+ return false;
+
+ TM = &MF.getTarget();
+ TII = TM->getInstrInfo();
+
+ // Search through basic blocks and mark the ones that have early returns
+ ReturnBBs.clear();
+ findReturns(MF.begin());
+
+ bool MadeChange = false;
+
+ MachineBasicBlock::iterator ReturnLoc;
+ MachineBasicBlock *MBB;
+ unsigned int Cycles = 0;
+ unsigned int BBCycles;
+
+ // Pad the identified basic blocks with NOOPs
+ for (DenseMap<MachineBasicBlock*, unsigned int>::iterator I = ReturnBBs.begin();
+ I != ReturnBBs.end(); ++I) {
+ MBB = I->first;
+ Cycles = I->second;
+
+ if (Cycles < Threshold) {
+ if (!cyclesUntilReturn(MBB, BBCycles, &ReturnLoc))
+ continue;
+
+ 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) {
+ findReturns(*I, Cycles);
+ }
+}
+
+/// cyclesUntilReturn - if the MBB has a return instruction, set Location
+/// to the instruction and return true. 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,
+ MachineBasicBlock::iterator *Location) {
+ for (MachineBasicBlock::iterator MBBI = MBB->begin();
+ MBBI != MBB->end(); ++MBBI) {
+ MachineInstr *MI = MBBI;
+ // 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()) {
+ if (Location)
+ *Location = MBBI;
+ return true;
+ }
+
+ Cycles += TII->getInstrLatency(TM->getInstrItineraryData(), MI);
+ }
+
+ 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();
+
+ while (NOOPsToAdd-- > 0) {
+ BuildMI(*MBB, MBBI, DL, TII->get(X86::NOOP));
+ BuildMI(*MBB, MBBI, DL, TII->get(X86::NOOP));
+ }
+}
projects / oota-llvm.git / commitdiff