1 //===-- WebAssemblyFrameLowering.cpp - WebAssembly Frame Lowering ----------==//
2 //
3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
4 // See https://llvm.org/LICENSE.txt for license information.
5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
6 //
7 //===----------------------------------------------------------------------===//
8 ///
9 /// \file
10 /// This file contains the WebAssembly implementation of
11 /// TargetFrameLowering class.
12 ///
13 /// On WebAssembly, there aren't a lot of things to do here. There are no
14 /// callee-saved registers to save, and no spill slots.
15 ///
16 /// The stack grows downward.
17 ///
18 //===----------------------------------------------------------------------===//
19 
20 #include "WebAssemblyFrameLowering.h"
21 #include "MCTargetDesc/WebAssemblyMCTargetDesc.h"
22 #include "WebAssembly.h"
23 #include "WebAssemblyInstrInfo.h"
24 #include "WebAssemblyMachineFunctionInfo.h"
25 #include "WebAssemblySubtarget.h"
26 #include "WebAssemblyTargetMachine.h"
27 #include "WebAssemblyUtilities.h"
28 #include "llvm/CodeGen/MachineFrameInfo.h"
29 #include "llvm/CodeGen/MachineFunction.h"
30 #include "llvm/CodeGen/MachineInstrBuilder.h"
31 #include "llvm/CodeGen/MachineModuleInfoImpls.h"
32 #include "llvm/CodeGen/MachineRegisterInfo.h"
33 #include "llvm/MC/MCAsmInfo.h"
34 #include "llvm/Support/Debug.h"
35 using namespace llvm;
36 
37 #define DEBUG_TYPE "wasm-frame-info"
38 
39 // TODO: wasm64
40 // TODO: Emit TargetOpcode::CFI_INSTRUCTION instructions
41 
42 /// We need a base pointer in the case of having items on the stack that
43 /// require stricter alignment than the stack pointer itself.  Because we need
44 /// to shift the stack pointer by some unknown amount to force the alignment,
45 /// we need to record the value of the stack pointer on entry to the function.
46 bool WebAssemblyFrameLowering::hasBP(const MachineFunction &MF) const {
47   const auto *RegInfo =
48       MF.getSubtarget<WebAssemblySubtarget>().getRegisterInfo();
49   return RegInfo->needsStackRealignment(MF);
50 }
51 
52 /// Return true if the specified function should have a dedicated frame pointer
53 /// register.
54 bool WebAssemblyFrameLowering::hasFP(const MachineFunction &MF) const {
55   const MachineFrameInfo &MFI = MF.getFrameInfo();
56 
57   // When we have var-sized objects, we move the stack pointer by an unknown
58   // amount, and need to emit a frame pointer to restore the stack to where we
59   // were on function entry.
60   // If we already need a base pointer, we use that to fix up the stack pointer.
61   // If there are no fixed-size objects, we would have no use of a frame
62   // pointer, and thus should not emit one.
63   bool HasFixedSizedObjects = MFI.getStackSize() > 0;
64   bool NeedsFixedReference = !hasBP(MF) || HasFixedSizedObjects;
65 
66   return MFI.isFrameAddressTaken() ||
67          (MFI.hasVarSizedObjects() && NeedsFixedReference) ||
68          MFI.hasStackMap() || MFI.hasPatchPoint();
69 }
70 
71 /// Under normal circumstances, when a frame pointer is not required, we reserve
72 /// argument space for call sites in the function immediately on entry to the
73 /// current function. This eliminates the need for add/sub sp brackets around
74 /// call sites. Returns true if the call frame is included as part of the stack
75 /// frame.
76 bool WebAssemblyFrameLowering::hasReservedCallFrame(
77     const MachineFunction &MF) const {
78   return !MF.getFrameInfo().hasVarSizedObjects();
79 }
80 
81 // Returns true if this function needs a local user-space stack pointer for its
82 // local frame (not for exception handling).
83 bool WebAssemblyFrameLowering::needsSPForLocalFrame(
84     const MachineFunction &MF) const {
85   auto &MFI = MF.getFrameInfo();
86   return MFI.getStackSize() || MFI.adjustsStack() || hasFP(MF);
87 }
88 
89 // In function with EH pads, we need to make a copy of the value of
90 // __stack_pointer global in SP32/64 register, in order to use it when
91 // restoring __stack_pointer after an exception is caught.
92 bool WebAssemblyFrameLowering::needsPrologForEH(
93     const MachineFunction &MF) const {
94   auto EHType = MF.getTarget().getMCAsmInfo()->getExceptionHandlingType();
95   return EHType == ExceptionHandling::Wasm &&
96          MF.getFunction().hasPersonalityFn() && MF.getFrameInfo().hasCalls();
97 }
98 
99 /// Returns true if this function needs a local user-space stack pointer.
100 /// Unlike a machine stack pointer, the wasm user stack pointer is a global
101 /// variable, so it is loaded into a register in the prolog.
102 bool WebAssemblyFrameLowering::needsSP(const MachineFunction &MF) const {
103   return needsSPForLocalFrame(MF) || needsPrologForEH(MF);
104 }
105 
106 /// Returns true if the local user-space stack pointer needs to be written back
107 /// to __stack_pointer global by this function (this is not meaningful if
108 /// needsSP is false). If false, the stack red zone can be used and only a local
109 /// SP is needed.
110 bool WebAssemblyFrameLowering::needsSPWriteback(
111     const MachineFunction &MF) const {
112   auto &MFI = MF.getFrameInfo();
113   assert(needsSP(MF));
114   // When we don't need a local stack pointer for its local frame but only to
115   // support EH, we don't need to write SP back in the epilog, because we don't
116   // bump down the stack pointer in the prolog. We need to write SP back in the
117   // epilog only if
118   // 1. We need SP not only for EH support but also because we actually use
119   // stack or we have a frame address taken.
120   // 2. We cannot use the red zone.
121   bool CanUseRedZone = MFI.getStackSize() <= RedZoneSize && !MFI.hasCalls() &&
122                        !MF.getFunction().hasFnAttribute(Attribute::NoRedZone);
123   return needsSPForLocalFrame(MF) && !CanUseRedZone;
124 }
125 
126 unsigned WebAssemblyFrameLowering::getSPReg(const MachineFunction &MF) {
127   return MF.getSubtarget<WebAssemblySubtarget>().hasAddr64()
128              ? WebAssembly::SP64
129              : WebAssembly::SP32;
130 }
131 
132 unsigned WebAssemblyFrameLowering::getFPReg(const MachineFunction &MF) {
133   return MF.getSubtarget<WebAssemblySubtarget>().hasAddr64()
134              ? WebAssembly::FP64
135              : WebAssembly::FP32;
136 }
137 
138 unsigned
139 WebAssemblyFrameLowering::getOpcConst(const MachineFunction &MF) {
140   return MF.getSubtarget<WebAssemblySubtarget>().hasAddr64()
141              ? WebAssembly::CONST_I64
142              : WebAssembly::CONST_I32;
143 }
144 
145 unsigned WebAssemblyFrameLowering::getOpcAdd(const MachineFunction &MF) {
146   return MF.getSubtarget<WebAssemblySubtarget>().hasAddr64()
147              ? WebAssembly::ADD_I64
148              : WebAssembly::ADD_I32;
149 }
150 
151 unsigned WebAssemblyFrameLowering::getOpcSub(const MachineFunction &MF) {
152   return MF.getSubtarget<WebAssemblySubtarget>().hasAddr64()
153              ? WebAssembly::SUB_I64
154              : WebAssembly::SUB_I32;
155 }
156 
157 unsigned WebAssemblyFrameLowering::getOpcAnd(const MachineFunction &MF) {
158   return MF.getSubtarget<WebAssemblySubtarget>().hasAddr64()
159              ? WebAssembly::AND_I64
160              : WebAssembly::AND_I32;
161 }
162 
163 unsigned
164 WebAssemblyFrameLowering::getOpcGlobGet(const MachineFunction &MF) {
165   return MF.getSubtarget<WebAssemblySubtarget>().hasAddr64()
166              ? WebAssembly::GLOBAL_GET_I64
167              : WebAssembly::GLOBAL_GET_I32;
168 }
169 
170 unsigned
171 WebAssemblyFrameLowering::getOpcGlobSet(const MachineFunction &MF) {
172   return MF.getSubtarget<WebAssemblySubtarget>().hasAddr64()
173              ? WebAssembly::GLOBAL_SET_I64
174              : WebAssembly::GLOBAL_SET_I32;
175 }
176 
177 void WebAssemblyFrameLowering::writeSPToGlobal(
178     unsigned SrcReg, MachineFunction &MF, MachineBasicBlock &MBB,
179     MachineBasicBlock::iterator &InsertStore, const DebugLoc &DL) const {
180   const auto *TII = MF.getSubtarget<WebAssemblySubtarget>().getInstrInfo();
181 
182   const char *ES = "__stack_pointer";
183   auto *SPSymbol = MF.createExternalSymbolName(ES);
184 
185   BuildMI(MBB, InsertStore, DL, TII->get(getOpcGlobSet(MF)))
186       .addExternalSymbol(SPSymbol)
187       .addReg(SrcReg);
188 }
189 
190 MachineBasicBlock::iterator
191 WebAssemblyFrameLowering::eliminateCallFramePseudoInstr(
192     MachineFunction &MF, MachineBasicBlock &MBB,
193     MachineBasicBlock::iterator I) const {
194   assert(!I->getOperand(0).getImm() && (hasFP(MF) || hasBP(MF)) &&
195          "Call frame pseudos should only be used for dynamic stack adjustment");
196   auto &ST = MF.getSubtarget<WebAssemblySubtarget>();
197   const auto *TII = ST.getInstrInfo();
198   if (I->getOpcode() == TII->getCallFrameDestroyOpcode() &&
199       needsSPWriteback(MF)) {
200     DebugLoc DL = I->getDebugLoc();
201     writeSPToGlobal(getSPReg(MF), MF, MBB, I, DL);
202   }
203   return MBB.erase(I);
204 }
205 
206 void WebAssemblyFrameLowering::emitPrologue(MachineFunction &MF,
207                                             MachineBasicBlock &MBB) const {
208   // TODO: Do ".setMIFlag(MachineInstr::FrameSetup)" on emitted instructions
209   auto &MFI = MF.getFrameInfo();
210   assert(MFI.getCalleeSavedInfo().empty() &&
211          "WebAssembly should not have callee-saved registers");
212 
213   if (!needsSP(MF))
214     return;
215   uint64_t StackSize = MFI.getStackSize();
216 
217   auto &ST = MF.getSubtarget<WebAssemblySubtarget>();
218   const auto *TII = ST.getInstrInfo();
219   auto &MRI = MF.getRegInfo();
220 
221   auto InsertPt = MBB.begin();
222   while (InsertPt != MBB.end() &&
223          WebAssembly::isArgument(InsertPt->getOpcode()))
224     ++InsertPt;
225   DebugLoc DL;
226 
227   const TargetRegisterClass *PtrRC =
228       MRI.getTargetRegisterInfo()->getPointerRegClass(MF);
229   unsigned SPReg = getSPReg(MF);
230   if (StackSize)
231     SPReg = MRI.createVirtualRegister(PtrRC);
232 
233   const char *ES = "__stack_pointer";
234   auto *SPSymbol = MF.createExternalSymbolName(ES);
235   BuildMI(MBB, InsertPt, DL, TII->get(getOpcGlobGet(MF)), SPReg)
236       .addExternalSymbol(SPSymbol);
237 
238   bool HasBP = hasBP(MF);
239   if (HasBP) {
240     auto FI = MF.getInfo<WebAssemblyFunctionInfo>();
241     Register BasePtr = MRI.createVirtualRegister(PtrRC);
242     FI->setBasePointerVreg(BasePtr);
243     BuildMI(MBB, InsertPt, DL, TII->get(WebAssembly::COPY), BasePtr)
244         .addReg(SPReg);
245   }
246   if (StackSize) {
247     // Subtract the frame size
248     Register OffsetReg = MRI.createVirtualRegister(PtrRC);
249     BuildMI(MBB, InsertPt, DL, TII->get(getOpcConst(MF)), OffsetReg)
250         .addImm(StackSize);
251     BuildMI(MBB, InsertPt, DL, TII->get(getOpcSub(MF)), getSPReg(MF))
252         .addReg(SPReg)
253         .addReg(OffsetReg);
254   }
255   if (HasBP) {
256     Register BitmaskReg = MRI.createVirtualRegister(PtrRC);
257     Align Alignment = MFI.getMaxAlign();
258     BuildMI(MBB, InsertPt, DL, TII->get(getOpcConst(MF)), BitmaskReg)
259         .addImm((int64_t) ~(Alignment.value() - 1));
260     BuildMI(MBB, InsertPt, DL, TII->get(getOpcAnd(MF)), getSPReg(MF))
261         .addReg(getSPReg(MF))
262         .addReg(BitmaskReg);
263   }
264   if (hasFP(MF)) {
265     // Unlike most conventional targets (where FP points to the saved FP),
266     // FP points to the bottom of the fixed-size locals, so we can use positive
267     // offsets in load/store instructions.
268     BuildMI(MBB, InsertPt, DL, TII->get(WebAssembly::COPY), getFPReg(MF))
269         .addReg(getSPReg(MF));
270   }
271   if (StackSize && needsSPWriteback(MF)) {
272     writeSPToGlobal(getSPReg(MF), MF, MBB, InsertPt, DL);
273   }
274 }
275 
276 void WebAssemblyFrameLowering::emitEpilogue(MachineFunction &MF,
277                                             MachineBasicBlock &MBB) const {
278   uint64_t StackSize = MF.getFrameInfo().getStackSize();
279   if (!needsSP(MF) || !needsSPWriteback(MF))
280     return;
281   auto &ST = MF.getSubtarget<WebAssemblySubtarget>();
282   const auto *TII = ST.getInstrInfo();
283   auto &MRI = MF.getRegInfo();
284   auto InsertPt = MBB.getFirstTerminator();
285   DebugLoc DL;
286 
287   if (InsertPt != MBB.end())
288     DL = InsertPt->getDebugLoc();
289 
290   // Restore the stack pointer. If we had fixed-size locals, add the offset
291   // subtracted in the prolog.
292   unsigned SPReg = 0;
293   unsigned SPFPReg = hasFP(MF) ? getFPReg(MF) : getSPReg(MF);
294   if (hasBP(MF)) {
295     auto FI = MF.getInfo<WebAssemblyFunctionInfo>();
296     SPReg = FI->getBasePointerVreg();
297   } else if (StackSize) {
298     const TargetRegisterClass *PtrRC =
299         MRI.getTargetRegisterInfo()->getPointerRegClass(MF);
300     Register OffsetReg = MRI.createVirtualRegister(PtrRC);
301     BuildMI(MBB, InsertPt, DL, TII->get(getOpcConst(MF)), OffsetReg)
302         .addImm(StackSize);
303     // In the epilog we don't need to write the result back to the SP32/64
304     // physreg because it won't be used again. We can use a stackified register
305     // instead.
306     SPReg = MRI.createVirtualRegister(PtrRC);
307     BuildMI(MBB, InsertPt, DL, TII->get(getOpcAdd(MF)), SPReg)
308         .addReg(SPFPReg)
309         .addReg(OffsetReg);
310   } else {
311     SPReg = SPFPReg;
312   }
313 
314   writeSPToGlobal(SPReg, MF, MBB, InsertPt, DL);
315 }
316 
317 TargetFrameLowering::DwarfFrameBase
318 WebAssemblyFrameLowering::getDwarfFrameBase(const MachineFunction &MF) const {
319   DwarfFrameBase Loc;
320   Loc.Kind = DwarfFrameBase::WasmFrameBase;
321   const WebAssemblyFunctionInfo &MFI = *MF.getInfo<WebAssemblyFunctionInfo>();
322   if (needsSP(MF) && MFI.isFrameBaseVirtual()) {
323     unsigned LocalNum = MFI.getFrameBaseLocal();
324     Loc.Location.WasmLoc = {WebAssembly::TI_LOCAL, LocalNum};
325   } else {
326     // TODO: This should work on a breakpoint at a function with no frame,
327     // but probably won't work for traversing up the stack.
328     Loc.Location.WasmLoc = {WebAssembly::TI_GLOBAL_RELOC, 0};
329   }
330   return Loc;
331 }
332