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   assert((StackSize || !AdjustUp) && "Adjusting up by 0");
74   auto &MRI = MF.getRegInfo();
75   unsigned SPReg = MRI.createVirtualRegister(&WebAssembly::I32RegClass);
76   auto *SPSymbol = MF.createExternalSymbolName("__stack_pointer");
77   BuildMI(MBB, InsertPt, DL, TII->get(WebAssembly::CONST_I32), SPReg)
78       .addExternalSymbol(SPSymbol);
79   // This MachinePointerInfo should reference __stack_pointer as well but
80   // doesn't because MachinePointerInfo() takes a GV which we don't have for
81   // __stack_pointer. TODO: check if PseudoSourceValue::ExternalSymbolCallEntry
82   // is appropriate instead. (likewise for EmitEpologue below)
83   auto *LoadMMO = new MachineMemOperand(MachinePointerInfo(),
84                                         MachineMemOperand::MOLoad, 4, 4);
85   BuildMI(MBB, InsertPt, DL, TII->get(WebAssembly::LOAD_I32), SPReg)
86       .addImm(0) // offset
87       .addReg(SPReg) // addr
88       .addImm(2) // p2align
89       .addMemOperand(LoadMMO);
90   // Add/Subtract the frame size
91   unsigned OffsetReg = MRI.createVirtualRegister(&WebAssembly::I32RegClass);
92   BuildMI(MBB, InsertPt, DL, TII->get(WebAssembly::CONST_I32), OffsetReg)
93       .addImm(StackSize);
94   BuildMI(MBB, InsertPt, DL,
95           TII->get(AdjustUp ? WebAssembly::ADD_I32 : WebAssembly::SUB_I32),
96           WebAssembly::SP32)
97       .addReg(SPReg)
98       .addReg(OffsetReg);
99   // The SP32 register now has the new stacktop. Also write it back to memory.
100   BuildMI(MBB, InsertPt, DL, TII->get(WebAssembly::CONST_I32), OffsetReg)
101       .addExternalSymbol(SPSymbol);
102   auto *MMO = new MachineMemOperand(MachinePointerInfo(),
103                                     MachineMemOperand::MOStore, 4, 4);
104   BuildMI(MBB, InsertPt, DL, TII->get(WebAssembly::STORE_I32), WebAssembly::SP32)
105       .addImm(0)
106       .addReg(OffsetReg)
107       .addImm(2) // p2align
108       .addReg(WebAssembly::SP32)
109       .addMemOperand(MMO);
110 }
111 
112 void WebAssemblyFrameLowering::eliminateCallFramePseudoInstr(
113     MachineFunction &MF, MachineBasicBlock &MBB,
114     MachineBasicBlock::iterator I) const {
115   const auto *TII = MF.getSubtarget<WebAssemblySubtarget>().getInstrInfo();
116   DebugLoc DL = I->getDebugLoc();
117   unsigned Opc = I->getOpcode();
118   bool IsDestroy = Opc == TII->getCallFrameDestroyOpcode();
119   unsigned Amount = I->getOperand(0).getImm();
120   // TODO(dschuff): After we switch varargs to passing an explicit pointer
121   // rather than using an implicit call frame, assert here that Amount is 0
122   // and remove adjustStackPointer altogether.
123   if (Amount)
124     adjustStackPointer(Amount, IsDestroy, MF, MBB,
125                        TII, I, DL);
126   MBB.erase(I);
127 }
128 
129 void WebAssemblyFrameLowering::emitPrologue(MachineFunction &MF,
130                                             MachineBasicBlock &MBB) const {
131   // TODO: Do ".setMIFlag(MachineInstr::FrameSetup)" on emitted instructions
132   auto *MFI = MF.getFrameInfo();
133   assert(MFI->getCalleeSavedInfo().empty() &&
134          "WebAssembly should not have callee-saved registers");
135 
136   uint64_t StackSize = MFI->getStackSize();
137   if (!StackSize && !MFI->adjustsStack())
138     return;
139 
140   const auto *TII = MF.getSubtarget<WebAssemblySubtarget>().getInstrInfo();
141   auto &MRI = MF.getRegInfo();
142 
143   auto InsertPt = MBB.begin();
144   DebugLoc DL;
145 
146   unsigned SPReg = MRI.createVirtualRegister(&WebAssembly::I32RegClass);
147   auto *SPSymbol = MF.createExternalSymbolName("__stack_pointer");
148   BuildMI(MBB, InsertPt, DL, TII->get(WebAssembly::CONST_I32), SPReg)
149       .addExternalSymbol(SPSymbol);
150   // This MachinePointerInfo should reference __stack_pointer as well but
151   // doesn't because MachinePointerInfo() takes a GV which we don't have for
152   // __stack_pointer. TODO: check if PseudoSourceValue::ExternalSymbolCallEntry
153   // is appropriate instead. (likewise for EmitEpologue below)
154   auto *LoadMMO = new MachineMemOperand(MachinePointerInfo(),
155                                         MachineMemOperand::MOLoad, 4, 4);
156   // Load the SP value.
157   BuildMI(MBB, InsertPt, DL, TII->get(WebAssembly::LOAD_I32),
158           StackSize ? SPReg : (unsigned)WebAssembly::SP32)
159       .addImm(0)     // offset
160       .addReg(SPReg) // addr
161       .addImm(2)     // p2align
162       .addMemOperand(LoadMMO);
163 
164   unsigned OffsetReg = 0;
165   if (StackSize) {
166     // Subtract the frame size
167     OffsetReg = MRI.createVirtualRegister(&WebAssembly::I32RegClass);
168     BuildMI(MBB, InsertPt, DL, TII->get(WebAssembly::CONST_I32), OffsetReg)
169         .addImm(StackSize);
170     BuildMI(MBB, InsertPt, DL,
171             TII->get(WebAssembly::SUB_I32),
172             WebAssembly::SP32)
173         .addReg(SPReg)
174         .addReg(OffsetReg);
175   }
176   if (hasFP(MF)) {
177     // Unlike most conventional targets (where FP points to the saved FP),
178     // FP points to the bottom of the fixed-size locals, so we can use positive
179     // offsets in load/store instructions.
180     BuildMI(MBB, InsertPt, DL, TII->get(WebAssembly::COPY_LOCAL_I32),
181             WebAssembly::FP32)
182         .addReg(WebAssembly::SP32);
183   }
184   if (StackSize) {
185     assert(OffsetReg);
186   // The SP32 register now has the new stacktop. Also write it back to memory.
187   BuildMI(MBB, InsertPt, DL, TII->get(WebAssembly::CONST_I32), OffsetReg)
188       .addExternalSymbol(SPSymbol);
189   auto *MMO = new MachineMemOperand(MachinePointerInfo(),
190                                     MachineMemOperand::MOStore, 4, 4);
191   BuildMI(MBB, InsertPt, DL, TII->get(WebAssembly::STORE_I32), WebAssembly::SP32)
192       .addImm(0)
193       .addReg(OffsetReg)
194       .addImm(2) // p2align
195       .addReg(WebAssembly::SP32)
196       .addMemOperand(MMO);
197   }
198 }
199 
200 void WebAssemblyFrameLowering::emitEpilogue(MachineFunction &MF,
201                                             MachineBasicBlock &MBB) const {
202   auto *MFI = MF.getFrameInfo();
203   uint64_t StackSize = MFI->getStackSize();
204   if (!StackSize && !MFI->adjustsStack())
205     return;
206   const auto *TII = MF.getSubtarget<WebAssemblySubtarget>().getInstrInfo();
207   auto &MRI = MF.getRegInfo();
208   unsigned OffsetReg = MRI.createVirtualRegister(&WebAssembly::I32RegClass);
209   auto InsertPt = MBB.getFirstTerminator();
210   DebugLoc DL;
211 
212   if (InsertPt != MBB.end()) {
213     DL = InsertPt->getDebugLoc();
214   }
215 
216   // Restore the stack pointer. If we had fixed-size locals, add the offset
217   // subtracted in the prolog.
218   if (StackSize) {
219     BuildMI(MBB, InsertPt, DL, TII->get(WebAssembly::CONST_I32), OffsetReg)
220         .addImm(StackSize);
221     BuildMI(MBB, InsertPt, DL, TII->get(WebAssembly::ADD_I32), WebAssembly::SP32)
222         .addReg(hasFP(MF) ? WebAssembly::FP32 : WebAssembly::SP32)
223         .addReg(OffsetReg);
224   }
225 
226   auto *SPSymbol = MF.createExternalSymbolName("__stack_pointer");
227   // Re-use OffsetReg to hold the address of the stacktop
228   BuildMI(MBB, InsertPt, DL, TII->get(WebAssembly::CONST_I32), OffsetReg)
229       .addExternalSymbol(SPSymbol);
230   auto *MMO = new MachineMemOperand(MachinePointerInfo(),
231                                     MachineMemOperand::MOStore, 4, 4);
232   BuildMI(MBB, InsertPt, DL, TII->get(WebAssembly::STORE_I32), WebAssembly::SP32)
233       .addImm(0)
234       .addReg(OffsetReg)
235       .addImm(2) // p2align
236       .addReg((!StackSize && hasFP(MF)) ? WebAssembly::FP32 : WebAssembly::SP32)
237       .addMemOperand(MMO);
238 }
239