Bug Summary

File:lib/Target/X86/X86WinAllocaExpander.cpp
Warning:line 264, column 7
Value stored to 'AmountReg' is never read

Annotated Source Code

1//===----- X86WinAllocaExpander.cpp - Expand WinAlloca pseudo instruction -===//
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 a pass that expands WinAlloca pseudo-instructions.
11//
12// It performs a conservative analysis to determine whether each allocation
13// falls within a region of the stack that is safe to use, or whether stack
14// probes must be emitted.
15//
16//===----------------------------------------------------------------------===//
17
18#include "X86.h"
19#include "X86InstrBuilder.h"
20#include "X86InstrInfo.h"
21#include "X86MachineFunctionInfo.h"
22#include "X86Subtarget.h"
23#include "llvm/ADT/PostOrderIterator.h"
24#include "llvm/CodeGen/MachineFunctionPass.h"
25#include "llvm/CodeGen/MachineInstrBuilder.h"
26#include "llvm/CodeGen/MachineRegisterInfo.h"
27#include "llvm/CodeGen/Passes.h"
28#include "llvm/IR/Function.h"
29#include "llvm/Support/raw_ostream.h"
30#include "llvm/Target/TargetInstrInfo.h"
31
32using namespace llvm;
33
34namespace {
35
36class X86WinAllocaExpander : public MachineFunctionPass {
37public:
38 X86WinAllocaExpander() : MachineFunctionPass(ID) {}
39
40 bool runOnMachineFunction(MachineFunction &MF) override;
41
42private:
43 /// Strategies for lowering a WinAlloca.
44 enum Lowering { TouchAndSub, Sub, Probe };
45
46 /// Deterministic-order map from WinAlloca instruction to desired lowering.
47 typedef MapVector<MachineInstr*, Lowering> LoweringMap;
48
49 /// Compute which lowering to use for each WinAlloca instruction.
50 void computeLowerings(MachineFunction &MF, LoweringMap& Lowerings);
51
52 /// Get the appropriate lowering based on current offset and amount.
53 Lowering getLowering(int64_t CurrentOffset, int64_t AllocaAmount);
54
55 /// Lower a WinAlloca instruction.
56 void lower(MachineInstr* MI, Lowering L);
57
58 MachineRegisterInfo *MRI;
59 const X86Subtarget *STI;
60 const TargetInstrInfo *TII;
61 const X86RegisterInfo *TRI;
62 unsigned StackPtr;
63 unsigned SlotSize;
64 int64_t StackProbeSize;
65
66 StringRef getPassName() const override { return "X86 WinAlloca Expander"; }
67 static char ID;
68};
69
70char X86WinAllocaExpander::ID = 0;
71
72} // end anonymous namespace
73
74FunctionPass *llvm::createX86WinAllocaExpander() {
75 return new X86WinAllocaExpander();
76}
77
78/// Return the allocation amount for a WinAlloca instruction, or -1 if unknown.
79static int64_t getWinAllocaAmount(MachineInstr *MI, MachineRegisterInfo *MRI) {
80 assert(MI->getOpcode() == X86::WIN_ALLOCA_32 ||((MI->getOpcode() == X86::WIN_ALLOCA_32 || MI->getOpcode
() == X86::WIN_ALLOCA_64) ? static_cast<void> (0) : __assert_fail
("MI->getOpcode() == X86::WIN_ALLOCA_32 || MI->getOpcode() == X86::WIN_ALLOCA_64"
, "/tmp/buildd/llvm-toolchain-snapshot-5.0~svn303373/lib/Target/X86/X86WinAllocaExpander.cpp"
, 81, __PRETTY_FUNCTION__))
81 MI->getOpcode() == X86::WIN_ALLOCA_64)((MI->getOpcode() == X86::WIN_ALLOCA_32 || MI->getOpcode
() == X86::WIN_ALLOCA_64) ? static_cast<void> (0) : __assert_fail
("MI->getOpcode() == X86::WIN_ALLOCA_32 || MI->getOpcode() == X86::WIN_ALLOCA_64"
, "/tmp/buildd/llvm-toolchain-snapshot-5.0~svn303373/lib/Target/X86/X86WinAllocaExpander.cpp"
, 81, __PRETTY_FUNCTION__))
;
82 assert(MI->getOperand(0).isReg())((MI->getOperand(0).isReg()) ? static_cast<void> (0)
: __assert_fail ("MI->getOperand(0).isReg()", "/tmp/buildd/llvm-toolchain-snapshot-5.0~svn303373/lib/Target/X86/X86WinAllocaExpander.cpp"
, 82, __PRETTY_FUNCTION__))
;
83
84 unsigned AmountReg = MI->getOperand(0).getReg();
85 MachineInstr *Def = MRI->getUniqueVRegDef(AmountReg);
86
87 // Look through copies.
88 while (Def && Def->isCopy() && Def->getOperand(1).isReg())
89 Def = MRI->getUniqueVRegDef(Def->getOperand(1).getReg());
90
91 if (!Def ||
92 (Def->getOpcode() != X86::MOV32ri && Def->getOpcode() != X86::MOV64ri) ||
93 !Def->getOperand(1).isImm())
94 return -1;
95
96 return Def->getOperand(1).getImm();
97}
98
99X86WinAllocaExpander::Lowering
100X86WinAllocaExpander::getLowering(int64_t CurrentOffset,
101 int64_t AllocaAmount) {
102 // For a non-constant amount or a large amount, we have to probe.
103 if (AllocaAmount < 0 || AllocaAmount > StackProbeSize)
104 return Probe;
105
106 // If it fits within the safe region of the stack, just subtract.
107 if (CurrentOffset + AllocaAmount <= StackProbeSize)
108 return Sub;
109
110 // Otherwise, touch the current tip of the stack, then subtract.
111 return TouchAndSub;
112}
113
114static bool isPushPop(const MachineInstr &MI) {
115 switch (MI.getOpcode()) {
116 case X86::PUSH32i8:
117 case X86::PUSH32r:
118 case X86::PUSH32rmm:
119 case X86::PUSH32rmr:
120 case X86::PUSHi32:
121 case X86::PUSH64i8:
122 case X86::PUSH64r:
123 case X86::PUSH64rmm:
124 case X86::PUSH64rmr:
125 case X86::PUSH64i32:
126 case X86::POP32r:
127 case X86::POP64r:
128 return true;
129 default:
130 return false;
131 }
132}
133
134void X86WinAllocaExpander::computeLowerings(MachineFunction &MF,
135 LoweringMap &Lowerings) {
136 // Do a one-pass reverse post-order walk of the CFG to conservatively estimate
137 // the offset between the stack pointer and the lowest touched part of the
138 // stack, and use that to decide how to lower each WinAlloca instruction.
139
140 // Initialize OutOffset[B], the stack offset at exit from B, to something big.
141 DenseMap<MachineBasicBlock *, int64_t> OutOffset;
142 for (MachineBasicBlock &MBB : MF)
143 OutOffset[&MBB] = INT32_MAX(2147483647);
144
145 // Note: we don't know the offset at the start of the entry block since the
146 // prologue hasn't been inserted yet, and how much that will adjust the stack
147 // pointer depends on register spills, which have not been computed yet.
148
149 // Compute the reverse post-order.
150 ReversePostOrderTraversal<MachineFunction*> RPO(&MF);
151
152 for (MachineBasicBlock *MBB : RPO) {
153 int64_t Offset = -1;
154 for (MachineBasicBlock *Pred : MBB->predecessors())
155 Offset = std::max(Offset, OutOffset[Pred]);
156 if (Offset == -1) Offset = INT32_MAX(2147483647);
157
158 for (MachineInstr &MI : *MBB) {
159 if (MI.getOpcode() == X86::WIN_ALLOCA_32 ||
160 MI.getOpcode() == X86::WIN_ALLOCA_64) {
161 // A WinAlloca moves StackPtr, and potentially touches it.
162 int64_t Amount = getWinAllocaAmount(&MI, MRI);
163 Lowering L = getLowering(Offset, Amount);
164 Lowerings[&MI] = L;
165 switch (L) {
166 case Sub:
167 Offset += Amount;
168 break;
169 case TouchAndSub:
170 Offset = Amount;
171 break;
172 case Probe:
173 Offset = 0;
174 break;
175 }
176 } else if (MI.isCall() || isPushPop(MI)) {
177 // Calls, pushes and pops touch the tip of the stack.
178 Offset = 0;
179 } else if (MI.getOpcode() == X86::ADJCALLSTACKUP32 ||
180 MI.getOpcode() == X86::ADJCALLSTACKUP64) {
181 Offset -= MI.getOperand(0).getImm();
182 } else if (MI.getOpcode() == X86::ADJCALLSTACKDOWN32 ||
183 MI.getOpcode() == X86::ADJCALLSTACKDOWN64) {
184 Offset += MI.getOperand(0).getImm();
185 } else if (MI.modifiesRegister(StackPtr, TRI)) {
186 // Any other modification of SP means we've lost track of it.
187 Offset = INT32_MAX(2147483647);
188 }
189 }
190
191 OutOffset[MBB] = Offset;
192 }
193}
194
195static unsigned getSubOpcode(bool Is64Bit, int64_t Amount) {
196 if (Is64Bit)
197 return isInt<8>(Amount) ? X86::SUB64ri8 : X86::SUB64ri32;
198 return isInt<8>(Amount) ? X86::SUB32ri8 : X86::SUB32ri;
199}
200
201void X86WinAllocaExpander::lower(MachineInstr* MI, Lowering L) {
202 DebugLoc DL = MI->getDebugLoc();
203 MachineBasicBlock *MBB = MI->getParent();
204 MachineBasicBlock::iterator I = *MI;
205
206 int64_t Amount = getWinAllocaAmount(MI, MRI);
207 if (Amount == 0) {
208 MI->eraseFromParent();
209 return;
210 }
211
212 bool Is64Bit = STI->is64Bit();
213 assert(SlotSize == 4 || SlotSize == 8)((SlotSize == 4 || SlotSize == 8) ? static_cast<void> (
0) : __assert_fail ("SlotSize == 4 || SlotSize == 8", "/tmp/buildd/llvm-toolchain-snapshot-5.0~svn303373/lib/Target/X86/X86WinAllocaExpander.cpp"
, 213, __PRETTY_FUNCTION__))
;
214 unsigned RegA = (SlotSize == 8) ? X86::RAX : X86::EAX;
215
216 switch (L) {
217 case TouchAndSub:
218 assert(Amount >= SlotSize)((Amount >= SlotSize) ? static_cast<void> (0) : __assert_fail
("Amount >= SlotSize", "/tmp/buildd/llvm-toolchain-snapshot-5.0~svn303373/lib/Target/X86/X86WinAllocaExpander.cpp"
, 218, __PRETTY_FUNCTION__))
;
219
220 // Use a push to touch the top of the stack.
221 BuildMI(*MBB, I, DL, TII->get(Is64Bit ? X86::PUSH64r : X86::PUSH32r))
222 .addReg(RegA, RegState::Undef);
223 Amount -= SlotSize;
224 if (!Amount)
225 break;
226
227 // Fall through to make any remaining adjustment.
228 LLVM_FALLTHROUGH[[clang::fallthrough]];
229 case Sub:
230 assert(Amount > 0)((Amount > 0) ? static_cast<void> (0) : __assert_fail
("Amount > 0", "/tmp/buildd/llvm-toolchain-snapshot-5.0~svn303373/lib/Target/X86/X86WinAllocaExpander.cpp"
, 230, __PRETTY_FUNCTION__))
;
231 if (Amount == SlotSize) {
232 // Use push to save size.
233 BuildMI(*MBB, I, DL, TII->get(Is64Bit ? X86::PUSH64r : X86::PUSH32r))
234 .addReg(RegA, RegState::Undef);
235 } else {
236 // Sub.
237 BuildMI(*MBB, I, DL, TII->get(getSubOpcode(Is64Bit, Amount)), StackPtr)
238 .addReg(StackPtr)
239 .addImm(Amount);
240 }
241 break;
242 case Probe:
243 // The probe lowering expects the amount in RAX/EAX.
244 BuildMI(*MBB, MI, DL, TII->get(TargetOpcode::COPY), RegA)
245 .addReg(MI->getOperand(0).getReg());
246
247 // Do the probe.
248 STI->getFrameLowering()->emitStackProbe(*MBB->getParent(), *MBB, MI, DL,
249 /*InPrologue=*/false);
250 break;
251 }
252
253 unsigned AmountReg = MI->getOperand(0).getReg();
254 MI->eraseFromParent();
255
256 // Delete the definition of AmountReg, possibly walking a chain of copies.
257 for (;;) {
258 if (!MRI->use_empty(AmountReg))
259 break;
260 MachineInstr *AmountDef = MRI->getUniqueVRegDef(AmountReg);
261 if (!AmountDef)
262 break;
263 if (AmountDef->isCopy() && AmountDef->getOperand(1).isReg())
264 AmountReg = AmountDef->getOperand(1).isReg();
Value stored to 'AmountReg' is never read
265 AmountDef->eraseFromParent();
266 break;
267 }
268}
269
270bool X86WinAllocaExpander::runOnMachineFunction(MachineFunction &MF) {
271 if (!MF.getInfo<X86MachineFunctionInfo>()->hasWinAlloca())
272 return false;
273
274 MRI = &MF.getRegInfo();
275 STI = &MF.getSubtarget<X86Subtarget>();
276 TII = STI->getInstrInfo();
277 TRI = STI->getRegisterInfo();
278 StackPtr = TRI->getStackRegister();
279 SlotSize = TRI->getSlotSize();
280
281 StackProbeSize = 4096;
282 if (MF.getFunction()->hasFnAttribute("stack-probe-size")) {
283 MF.getFunction()
284 ->getFnAttribute("stack-probe-size")
285 .getValueAsString()
286 .getAsInteger(0, StackProbeSize);
287 }
288
289 LoweringMap Lowerings;
290 computeLowerings(MF, Lowerings);
291 for (auto &P : Lowerings)
292 lower(P.first, P.second);
293
294 return true;
295}