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      1 //===-- X86VZeroUpper.cpp - AVX vzeroupper instruction inserter -----------===//
      2 //
      3 //                     The LLVM Compiler Infrastructure
      4 //
      5 // This file is distributed under the University of Illinois Open Source
      6 // License. See LICENSE.TXT for details.
      7 //
      8 //===----------------------------------------------------------------------===//
      9 //
     10 // This file defines the pass which inserts x86 AVX vzeroupper instructions
     11 // before calls to SSE encoded functions. This avoids transition latency
     12 // penalty when tranfering control between AVX encoded instructions and old
     13 // SSE encoding mode.
     14 //
     15 //===----------------------------------------------------------------------===//
     16 
     17 #define DEBUG_TYPE "x86-vzeroupper"
     18 #include "X86.h"
     19 #include "X86InstrInfo.h"
     20 #include "llvm/ADT/Statistic.h"
     21 #include "llvm/CodeGen/MachineFunctionPass.h"
     22 #include "llvm/CodeGen/MachineInstrBuilder.h"
     23 #include "llvm/CodeGen/MachineRegisterInfo.h"
     24 #include "llvm/CodeGen/Passes.h"
     25 #include "llvm/Support/Debug.h"
     26 #include "llvm/Support/raw_ostream.h"
     27 #include "llvm/Target/TargetInstrInfo.h"
     28 using namespace llvm;
     29 
     30 STATISTIC(NumVZU, "Number of vzeroupper instructions inserted");
     31 
     32 namespace {
     33   struct VZeroUpperInserter : public MachineFunctionPass {
     34     static char ID;
     35     VZeroUpperInserter() : MachineFunctionPass(ID) {}
     36 
     37     virtual bool runOnMachineFunction(MachineFunction &MF);
     38 
     39     bool processBasicBlock(MachineFunction &MF, MachineBasicBlock &MBB);
     40 
     41     virtual const char *getPassName() const { return "X86 vzeroupper inserter";}
     42 
     43   private:
     44     const TargetInstrInfo *TII; // Machine instruction info.
     45     MachineBasicBlock *MBB;     // Current basic block
     46 
     47     // Any YMM register live-in to this function?
     48     bool FnHasLiveInYmm;
     49 
     50     // BBState - Contains the state of each MBB: unknown, clean, dirty
     51     SmallVector<uint8_t, 8> BBState;
     52 
     53     // BBSolved - Keep track of all MBB which had been already analyzed
     54     // and there is no further processing required.
     55     BitVector BBSolved;
     56 
     57     // Machine Basic Blocks are classified according this pass:
     58     //
     59     //  ST_UNKNOWN - The MBB state is unknown, meaning from the entry state
     60     //    until the MBB exit there isn't a instruction using YMM to change
     61     //    the state to dirty, or one of the incoming predecessors is unknown
     62     //    and there's not a dirty predecessor between them.
     63     //
     64     //  ST_CLEAN - No YMM usage in the end of the MBB. A MBB could have
     65     //    instructions using YMM and be marked ST_CLEAN, as long as the state
     66     //    is cleaned by a vzeroupper before any call.
     67     //
     68     //  ST_DIRTY - Any MBB ending with a YMM usage not cleaned up by a
     69     //    vzeroupper instruction.
     70     //
     71     //  ST_INIT - Placeholder for an empty state set
     72     //
     73     enum {
     74       ST_UNKNOWN = 0,
     75       ST_CLEAN   = 1,
     76       ST_DIRTY   = 2,
     77       ST_INIT    = 3
     78     };
     79 
     80     // computeState - Given two states, compute the resulting state, in
     81     // the following way
     82     //
     83     //  1) One dirty state yields another dirty state
     84     //  2) All states must be clean for the result to be clean
     85     //  3) If none above and one unknown, the result state is also unknown
     86     //
     87     unsigned computeState(unsigned PrevState, unsigned CurState) {
     88       if (PrevState == ST_INIT)
     89         return CurState;
     90 
     91       if (PrevState == ST_DIRTY || CurState == ST_DIRTY)
     92         return ST_DIRTY;
     93 
     94       if (PrevState == ST_CLEAN && CurState == ST_CLEAN)
     95         return ST_CLEAN;
     96 
     97       return ST_UNKNOWN;
     98     }
     99 
    100   };
    101   char VZeroUpperInserter::ID = 0;
    102 }
    103 
    104 FunctionPass *llvm::createX86IssueVZeroUpperPass() {
    105   return new VZeroUpperInserter();
    106 }
    107 
    108 static bool isYmmReg(unsigned Reg) {
    109   if (Reg >= X86::YMM0 && Reg <= X86::YMM15)
    110     return true;
    111 
    112   return false;
    113 }
    114 
    115 static bool checkFnHasLiveInYmm(MachineRegisterInfo &MRI) {
    116   for (MachineRegisterInfo::livein_iterator I = MRI.livein_begin(),
    117        E = MRI.livein_end(); I != E; ++I)
    118     if (isYmmReg(I->first))
    119       return true;
    120 
    121   return false;
    122 }
    123 
    124 static bool hasYmmReg(MachineInstr *MI) {
    125   for (int i = 0, e = MI->getNumOperands(); i != e; ++i) {
    126     const MachineOperand &MO = MI->getOperand(i);
    127     if (!MO.isReg())
    128       continue;
    129     if (MO.isDebug())
    130       continue;
    131     if (isYmmReg(MO.getReg()))
    132       return true;
    133   }
    134   return false;
    135 }
    136 
    137 /// runOnMachineFunction - Loop over all of the basic blocks, inserting
    138 /// vzero upper instructions before function calls.
    139 bool VZeroUpperInserter::runOnMachineFunction(MachineFunction &MF) {
    140   TII = MF.getTarget().getInstrInfo();
    141   MachineRegisterInfo &MRI = MF.getRegInfo();
    142   bool EverMadeChange = false;
    143 
    144   // Fast check: if the function doesn't use any ymm registers, we don't need
    145   // to insert any VZEROUPPER instructions.  This is constant-time, so it is
    146   // cheap in the common case of no ymm use.
    147   bool YMMUsed = false;
    148   const TargetRegisterClass *RC = X86::VR256RegisterClass;
    149   for (TargetRegisterClass::iterator i = RC->begin(), e = RC->end();
    150        i != e; i++) {
    151     if (MRI.isPhysRegUsed(*i)) {
    152       YMMUsed = true;
    153       break;
    154     }
    155   }
    156   if (!YMMUsed)
    157     return EverMadeChange;
    158 
    159   // Pre-compute the existence of any live-in YMM registers to this function
    160   FnHasLiveInYmm = checkFnHasLiveInYmm(MRI);
    161 
    162   assert(BBState.empty());
    163   BBState.resize(MF.getNumBlockIDs(), 0);
    164   BBSolved.resize(MF.getNumBlockIDs(), 0);
    165 
    166   // Each BB state depends on all predecessors, loop over until everything
    167   // converges.  (Once we converge, we can implicitly mark everything that is
    168   // still ST_UNKNOWN as ST_CLEAN.)
    169   while (1) {
    170     bool MadeChange = false;
    171 
    172     // Process all basic blocks.
    173     for (MachineFunction::iterator I = MF.begin(), E = MF.end(); I != E; ++I)
    174       MadeChange |= processBasicBlock(MF, *I);
    175 
    176     // If this iteration over the code changed anything, keep iterating.
    177     if (!MadeChange) break;
    178     EverMadeChange = true;
    179   }
    180 
    181   BBState.clear();
    182   BBSolved.clear();
    183   return EverMadeChange;
    184 }
    185 
    186 /// processBasicBlock - Loop over all of the instructions in the basic block,
    187 /// inserting vzero upper instructions before function calls.
    188 bool VZeroUpperInserter::processBasicBlock(MachineFunction &MF,
    189                                            MachineBasicBlock &BB) {
    190   bool Changed = false;
    191   unsigned BBNum = BB.getNumber();
    192   MBB = &BB;
    193 
    194   // Don't process already solved BBs
    195   if (BBSolved[BBNum])
    196     return false; // No changes
    197 
    198   // Check the state of all predecessors
    199   unsigned EntryState = ST_INIT;
    200   for (MachineBasicBlock::const_pred_iterator PI = BB.pred_begin(),
    201        PE = BB.pred_end(); PI != PE; ++PI) {
    202     EntryState = computeState(EntryState, BBState[(*PI)->getNumber()]);
    203     if (EntryState == ST_DIRTY)
    204       break;
    205   }
    206 
    207 
    208   // The entry MBB for the function may set the inital state to dirty if
    209   // the function receives any YMM incoming arguments
    210   if (MBB == MF.begin()) {
    211     EntryState = ST_CLEAN;
    212     if (FnHasLiveInYmm)
    213       EntryState = ST_DIRTY;
    214   }
    215 
    216   // The current state is initialized according to the predecessors
    217   unsigned CurState = EntryState;
    218   bool BBHasCall = false;
    219 
    220   for (MachineBasicBlock::iterator I = BB.begin(); I != BB.end(); ++I) {
    221     MachineInstr *MI = I;
    222     DebugLoc dl = I->getDebugLoc();
    223     bool isControlFlow = MI->isCall() || MI->isReturn();
    224 
    225     // Shortcut: don't need to check regular instructions in dirty state.
    226     if (!isControlFlow && CurState == ST_DIRTY)
    227       continue;
    228 
    229     if (hasYmmReg(MI)) {
    230       // We found a ymm-using instruction; this could be an AVX instruction,
    231       // or it could be control flow.
    232       CurState = ST_DIRTY;
    233       continue;
    234     }
    235 
    236     // Check for control-flow out of the current function (which might
    237     // indirectly execute SSE instructions).
    238     if (!isControlFlow)
    239       continue;
    240 
    241     BBHasCall = true;
    242 
    243     // The VZEROUPPER instruction resets the upper 128 bits of all Intel AVX
    244     // registers. This instruction has zero latency. In addition, the processor
    245     // changes back to Clean state, after which execution of Intel SSE
    246     // instructions or Intel AVX instructions has no transition penalty. Add
    247     // the VZEROUPPER instruction before any function call/return that might
    248     // execute SSE code.
    249     // FIXME: In some cases, we may want to move the VZEROUPPER into a
    250     // predecessor block.
    251     if (CurState == ST_DIRTY) {
    252       // Only insert the VZEROUPPER in case the entry state isn't unknown.
    253       // When unknown, only compute the information within the block to have
    254       // it available in the exit if possible, but don't change the block.
    255       if (EntryState != ST_UNKNOWN) {
    256         BuildMI(*MBB, I, dl, TII->get(X86::VZEROUPPER));
    257         ++NumVZU;
    258       }
    259 
    260       // After the inserted VZEROUPPER the state becomes clean again, but
    261       // other YMM may appear before other subsequent calls or even before
    262       // the end of the BB.
    263       CurState = ST_CLEAN;
    264     }
    265   }
    266 
    267   DEBUG(dbgs() << "MBB #" << BBNum
    268                << ", current state: " << CurState << '\n');
    269 
    270   // A BB can only be considered solved when we both have done all the
    271   // necessary transformations, and have computed the exit state.  This happens
    272   // in two cases:
    273   //  1) We know the entry state: this immediately implies the exit state and
    274   //     all the necessary transformations.
    275   //  2) There are no calls, and and a non-call instruction marks this block:
    276   //     no transformations are necessary, and we know the exit state.
    277   if (EntryState != ST_UNKNOWN || (!BBHasCall && CurState != ST_UNKNOWN))
    278     BBSolved[BBNum] = true;
    279 
    280   if (CurState != BBState[BBNum])
    281     Changed = true;
    282 
    283   BBState[BBNum] = CurState;
    284   return Changed;
    285 }
    286