Home | History | Annotate | Download | only in WebAssembly 
      1 //===-- WebAssemblyFrameLowering.cpp - WebAssembly Frame Lowering ----------==//
      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 /// \file
     11 /// \brief This file contains the WebAssembly implementation of
     12 /// TargetFrameLowering class.
     13 ///
     14 /// On WebAssembly, there aren't a lot of things to do here. There are no
     15 /// callee-saved registers to save, and no spill slots.
     16 ///
     17 /// The stack grows downward.
     18 ///
     19 //===----------------------------------------------------------------------===//
     20 
     21 #include "WebAssemblyFrameLowering.h"
     22 #include "MCTargetDesc/WebAssemblyMCTargetDesc.h"
     23 #include "WebAssemblyInstrInfo.h"
     24 #include "WebAssemblyMachineFunctionInfo.h"
     25 #include "WebAssemblySubtarget.h"
     26 #include "WebAssemblyTargetMachine.h"
     27 #include "llvm/CodeGen/MachineFrameInfo.h"
     28 #include "llvm/CodeGen/MachineFunction.h"
     29 #include "llvm/CodeGen/MachineInstrBuilder.h"
     30 #include "llvm/CodeGen/MachineModuleInfo.h"
     31 #include "llvm/CodeGen/MachineRegisterInfo.h"
     32 #include "llvm/Support/Debug.h"
     33 using namespace llvm;
     34 
     35 #define DEBUG_TYPE "wasm-frame-info"
     36 
     37 // TODO: Implement a red zone?
     38 // TODO: wasm64
     39 // TODO: Prolog/epilog should be stackified too. This pass runs after register
     40 //       stackification, so we'll have to do it manually.
     41 // TODO: Emit TargetOpcode::CFI_INSTRUCTION instructions
     42 
     43 /// Return true if the specified function should have a dedicated frame pointer
     44 /// register.
     45 bool WebAssemblyFrameLowering::hasFP(const MachineFunction &MF) const {
     46   const MachineFrameInfo *MFI = MF.getFrameInfo();
     47   const auto *RegInfo =
     48       MF.getSubtarget<WebAssemblySubtarget>().getRegisterInfo();
     49   return MFI->hasVarSizedObjects() || MFI->isFrameAddressTaken() ||
     50          MFI->hasStackMap() || MFI->hasPatchPoint() ||
     51          RegInfo->needsStackRealignment(MF);
     52 }
     53 
     54 /// Under normal circumstances, when a frame pointer is not required, we reserve
     55 /// argument space for call sites in the function immediately on entry to the
     56 /// current function. This eliminates the need for add/sub sp brackets around
     57 /// call sites. Returns true if the call frame is included as part of the stack
     58 /// frame.
     59 bool WebAssemblyFrameLowering::hasReservedCallFrame(
     60     const MachineFunction &MF) const {
     61   return !MF.getFrameInfo()->hasVarSizedObjects();
     62 }
     63 
     64 
     65 /// Adjust the stack pointer by a constant amount.
     66 static void adjustStackPointer(unsigned StackSize,
     67                                bool AdjustUp,
     68                                MachineFunction& MF,
     69                                MachineBasicBlock& MBB,
     70                                const TargetInstrInfo* TII,
     71                                MachineBasicBlock::iterator InsertPt,
     72                                const DebugLoc& DL) {
     73   auto &MRI = MF.getRegInfo();
     74   unsigned SPReg = MRI.createVirtualRegister(&WebAssembly::I32RegClass);
     75   auto *SPSymbol = MF.createExternalSymbolName("__stack_pointer");
     76   BuildMI(MBB, InsertPt, DL, TII->get(WebAssembly::CONST_I32), SPReg)
     77       .addExternalSymbol(SPSymbol);
     78   // This MachinePointerInfo should reference __stack_pointer as well but
     79   // doesn't because MachinePointerInfo() takes a GV which we don't have for
     80   // __stack_pointer. TODO: check if PseudoSourceValue::ExternalSymbolCallEntry
     81   // is appropriate instead. (likewise for EmitEpologue below)
     82   auto *LoadMMO = new MachineMemOperand(MachinePointerInfo(),
     83                                         MachineMemOperand::MOLoad, 4, 4);
     84   BuildMI(MBB, InsertPt, DL, TII->get(WebAssembly::LOAD_I32), SPReg)
     85       .addImm(0)
     86       .addReg(SPReg)
     87       .addMemOperand(LoadMMO);
     88   // Add/Subtract the frame size
     89   unsigned OffsetReg = MRI.createVirtualRegister(&WebAssembly::I32RegClass);
     90   BuildMI(MBB, InsertPt, DL, TII->get(WebAssembly::CONST_I32), OffsetReg)
     91       .addImm(StackSize);
     92   BuildMI(MBB, InsertPt, DL,
     93           TII->get(AdjustUp ? WebAssembly::ADD_I32 : WebAssembly::SUB_I32),
     94           WebAssembly::SP32)
     95       .addReg(SPReg)
     96       .addReg(OffsetReg);
     97   // The SP32 register now has the new stacktop. Also write it back to memory.
     98   BuildMI(MBB, InsertPt, DL, TII->get(WebAssembly::CONST_I32), OffsetReg)
     99       .addExternalSymbol(SPSymbol);
    100   auto *MMO = new MachineMemOperand(MachinePointerInfo(),
    101                                     MachineMemOperand::MOStore, 4, 4);
    102   BuildMI(MBB, InsertPt, DL, TII->get(WebAssembly::STORE_I32), WebAssembly::SP32)
    103       .addImm(0)
    104       .addReg(OffsetReg)
    105       .addReg(WebAssembly::SP32)
    106       .addMemOperand(MMO);
    107 }
    108 
    109 void WebAssemblyFrameLowering::eliminateCallFramePseudoInstr(
    110     MachineFunction &MF, MachineBasicBlock &MBB,
    111     MachineBasicBlock::iterator I) const {
    112   const auto *TII =
    113       static_cast<const WebAssemblyInstrInfo*>(MF.getSubtarget().getInstrInfo());
    114   DebugLoc DL = I->getDebugLoc();
    115   unsigned Opc = I->getOpcode();
    116   bool IsDestroy = Opc == TII->getCallFrameDestroyOpcode();
    117   unsigned Amount = I->getOperand(0).getImm();
    118   if (Amount)
    119     adjustStackPointer(Amount, IsDestroy, MF, MBB,
    120                        TII, I, DL);
    121   MBB.erase(I);
    122 }
    123 
    124 void WebAssemblyFrameLowering::emitPrologue(MachineFunction &MF,
    125                                             MachineBasicBlock &MBB) const {
    126   // TODO: Do ".setMIFlag(MachineInstr::FrameSetup)" on emitted instructions
    127   auto *MFI = MF.getFrameInfo();
    128   assert(MFI->getCalleeSavedInfo().empty() &&
    129          "WebAssembly should not have callee-saved registers");
    130   assert(!hasFP(MF) && "Functions needing frame pointers not yet supported");
    131   uint64_t StackSize = MFI->getStackSize();
    132   if (!StackSize && (!MFI->adjustsStack() || MFI->getMaxCallFrameSize() == 0))
    133     return;
    134 
    135   const auto *TII = MF.getSubtarget().getInstrInfo();
    136 
    137   auto InsertPt = MBB.begin();
    138   DebugLoc DL;
    139 
    140   adjustStackPointer(StackSize, false, MF, MBB, TII, InsertPt, DL);
    141 }
    142 
    143 void WebAssemblyFrameLowering::emitEpilogue(MachineFunction &MF,
    144                                             MachineBasicBlock &MBB) const {
    145   uint64_t StackSize = MF.getFrameInfo()->getStackSize();
    146   if (!StackSize)
    147     return;
    148   const auto *TII = MF.getSubtarget().getInstrInfo();
    149   auto &MRI = MF.getRegInfo();
    150   unsigned OffsetReg = MRI.createVirtualRegister(&WebAssembly::I32RegClass);
    151   auto InsertPt = MBB.getFirstTerminator();
    152   DebugLoc DL;
    153 
    154   if (InsertPt != MBB.end()) {
    155     DL = InsertPt->getDebugLoc();
    156   }
    157 
    158   // Restore the stack pointer. Without FP its value is just SP32 - stacksize
    159   BuildMI(MBB, InsertPt, DL, TII->get(WebAssembly::CONST_I32), OffsetReg)
    160       .addImm(StackSize);
    161   auto *SPSymbol = MF.createExternalSymbolName("__stack_pointer");
    162   BuildMI(MBB, InsertPt, DL, TII->get(WebAssembly::ADD_I32), WebAssembly::SP32)
    163       .addReg(WebAssembly::SP32)
    164       .addReg(OffsetReg);
    165   // Re-use OffsetReg to hold the address of the stacktop
    166   BuildMI(MBB, InsertPt, DL, TII->get(WebAssembly::CONST_I32), OffsetReg)
    167       .addExternalSymbol(SPSymbol);
    168   auto *MMO = new MachineMemOperand(MachinePointerInfo(),
    169                                     MachineMemOperand::MOStore, 4, 4);
    170   BuildMI(MBB, InsertPt, DL, TII->get(WebAssembly::STORE_I32), WebAssembly::SP32)
    171       .addImm(0)
    172       .addReg(OffsetReg)
    173       .addReg(WebAssembly::SP32)
    174       .addMemOperand(MMO);
    175 }
    176