1 //===- LocalStackSlotAllocation.cpp - Pre-allocate locals to stack slots --===//
3 // The LLVM Compiler Infrastructure
5 // This file is distributed under the University of Illinois Open Source
6 // License. See LICENSE.TXT for details.
8 //===----------------------------------------------------------------------===//
10 // This pass assigns local frame indices to stack slots relative to one another
11 // and allocates additional base registers to access them when the target
12 // estimates the are likely to be out of range of stack pointer and frame
13 // pointer relative addressing.
15 //===----------------------------------------------------------------------===//
17 #define DEBUG_TYPE "localstackalloc"
18 #include "llvm/Constants.h"
19 #include "llvm/DerivedTypes.h"
20 #include "llvm/Instructions.h"
21 #include "llvm/Intrinsics.h"
22 #include "llvm/LLVMContext.h"
23 #include "llvm/Module.h"
24 #include "llvm/Pass.h"
25 #include "llvm/ADT/SmallSet.h"
26 #include "llvm/ADT/Statistic.h"
27 #include "llvm/CodeGen/MachineFrameInfo.h"
28 #include "llvm/CodeGen/MachineFunction.h"
29 #include "llvm/CodeGen/MachineFunctionPass.h"
30 #include "llvm/CodeGen/MachineRegisterInfo.h"
31 #include "llvm/CodeGen/Passes.h"
32 #include "llvm/Support/Debug.h"
33 #include "llvm/Support/ErrorHandling.h"
34 #include "llvm/Support/raw_ostream.h"
35 #include "llvm/Target/TargetRegisterInfo.h"
36 #include "llvm/Target/TargetFrameInfo.h"
40 STATISTIC(NumAllocations, "Number of frame indices allocated into local block");
41 STATISTIC(NumBaseRegisters, "Number of virtual frame base registers allocated");
42 STATISTIC(NumReplacements, "Number of frame indices references replaced");
45 class LocalStackSlotPass: public MachineFunctionPass {
46 void calculateFrameObjectOffsets(MachineFunction &Fn);
48 void insertFrameReferenceRegisters(MachineFunction &Fn);
50 static char ID; // Pass identification, replacement for typeid
51 explicit LocalStackSlotPass() : MachineFunctionPass(ID) { }
52 bool runOnMachineFunction(MachineFunction &MF);
54 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
56 MachineFunctionPass::getAnalysisUsage(AU);
58 const char *getPassName() const {
59 return "Local Stack Slot Allocation";
64 } // end anonymous namespace
66 char LocalStackSlotPass::ID = 0;
68 FunctionPass *llvm::createLocalStackSlotAllocationPass() {
69 return new LocalStackSlotPass();
72 bool LocalStackSlotPass::runOnMachineFunction(MachineFunction &MF) {
73 // Lay out the local blob.
74 calculateFrameObjectOffsets(MF);
76 // Insert virtual base registers to resolve frame index references.
77 insertFrameReferenceRegisters(MF);
81 /// AdjustStackOffset - Helper function used to adjust the stack frame offset.
83 AdjustStackOffset(MachineFrameInfo *MFI, int FrameIdx, int64_t &Offset,
85 unsigned Align = MFI->getObjectAlignment(FrameIdx);
87 // If the alignment of this object is greater than that of the stack, then
88 // increase the stack alignment to match.
89 MaxAlign = std::max(MaxAlign, Align);
91 // Adjust to alignment boundary.
92 Offset = (Offset + Align - 1) / Align * Align;
94 DEBUG(dbgs() << "Allocate FI(" << FrameIdx << ") to local offset "
96 MFI->mapLocalFrameObject(FrameIdx, Offset);
97 Offset += MFI->getObjectSize(FrameIdx);
102 /// calculateFrameObjectOffsets - Calculate actual frame offsets for all of the
103 /// abstract stack objects.
105 void LocalStackSlotPass::calculateFrameObjectOffsets(MachineFunction &Fn) {
106 // Loop over all of the stack objects, assigning sequential addresses...
107 MachineFrameInfo *MFI = Fn.getFrameInfo();
109 unsigned MaxAlign = 0;
111 // Make sure that the stack protector comes before the local variables on the
113 SmallSet<int, 16> LargeStackObjs;
114 if (MFI->getStackProtectorIndex() >= 0) {
115 AdjustStackOffset(MFI, MFI->getStackProtectorIndex(), Offset, MaxAlign);
117 // Assign large stack objects first.
118 for (unsigned i = 0, e = MFI->getObjectIndexEnd(); i != e; ++i) {
119 if (MFI->isDeadObjectIndex(i))
121 if (MFI->getStackProtectorIndex() == (int)i)
123 if (!MFI->MayNeedStackProtector(i))
126 AdjustStackOffset(MFI, i, Offset, MaxAlign);
127 LargeStackObjs.insert(i);
131 // Then assign frame offsets to stack objects that are not used to spill
132 // callee saved registers.
133 for (unsigned i = 0, e = MFI->getObjectIndexEnd(); i != e; ++i) {
134 if (MFI->isDeadObjectIndex(i))
136 if (MFI->getStackProtectorIndex() == (int)i)
138 if (LargeStackObjs.count(i))
141 AdjustStackOffset(MFI, i, Offset, MaxAlign);
144 // Remember how big this blob of stack space is
145 MFI->setLocalFrameSize(Offset);
146 MFI->setLocalFrameMaxAlign(MaxAlign);
149 void LocalStackSlotPass::insertFrameReferenceRegisters(MachineFunction &Fn) {
150 // Scan the function's instructions looking for frame index references.
151 // For each, ask the target if it wants a virtual base register for it
152 // based on what we can tell it about where the local will end up in the
153 // stack frame. If it wants one, re-use a suitable one we've previously
154 // allocated, or if there isn't one that fits the bill, allocate a new one
155 // and ask the target to create a defining instruction for it.
157 MachineFrameInfo *MFI = Fn.getFrameInfo();
158 const TargetRegisterInfo *TRI = Fn.getTarget().getRegisterInfo();
160 for (MachineFunction::iterator BB = Fn.begin(),
161 E = Fn.end(); BB != E; ++BB) {
162 for (MachineBasicBlock::iterator I = BB->begin(); I != BB->end(); ++I) {
163 MachineInstr *MI = I;
164 // Debug value instructions can't be out of range, so they don't need
166 // FIXME: When we extend this stuff to handle functions with both
167 // VLAs and dynamic realignment, we should update the debug values
168 // to reference the new base pointer when possible.
169 if (MI->isDebugValue())
172 // For now, allocate the base register(s) within the basic block
173 // where they're used, and don't try to keep them around outside
174 // of that. It may be beneficial to try sharing them more broadly
175 // than that, but the increased register pressure makes that a
176 // tricky thing to balance. Investigate if re-materializing these
178 for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) {
179 // Consider replacing all frame index operands that reference
180 // an object allocated in the local block.
181 if (MI->getOperand(i).isFI()) {
182 int FrameIdx = MI->getOperand(i).getIndex();
183 // Don't try this with values not in the local block.
184 if (!MFI->isObjectPreAllocated(FrameIdx))
187 DEBUG(dbgs() << "Considering: " << *MI);
188 if (TRI->needsFrameBaseReg(MI, i)) {
189 DEBUG(dbgs() << " Replacing FI in: " << *MI);
190 // FIXME: Make sure any new base reg is aligned reasonably. TBD
191 // what "reasonably" really means. Conservatively, can just
192 // use the alignment of the local block.
194 // If we have a suitable base register available, use it; otherwise
196 // FIXME: For the moment, just always create a new one.
198 const TargetRegisterClass *RC = TRI->getPointerRegClass();
199 unsigned BaseReg = Fn.getRegInfo().createVirtualRegister(RC);
201 // Tell the target to insert the instruction to initialize
202 // the base register.
203 TRI->materializeFrameBaseRegister(I, BaseReg, FrameIdx);
205 // Modify the instruction to use the new base register rather
206 // than the frame index operand.
207 TRI->resolveFrameIndex(I, BaseReg, 0);