1 //===-- NVPTXPrologEpilogPass.cpp - NVPTX prolog/epilog inserter ----------===//
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 // This file is a copy of the generic LLVM PrologEpilogInserter pass, modified
10 // to remove unneeded functionality and to handle virtual registers. Most code
11 // here is a copy of PrologEpilogInserter.cpp.
12 //
13 //===----------------------------------------------------------------------===//
14 
15 #include "NVPTX.h"
16 #include "llvm/CodeGen/MachineFrameInfo.h"
17 #include "llvm/CodeGen/MachineFunction.h"
18 #include "llvm/CodeGen/MachineFunctionPass.h"
19 #include "llvm/CodeGen/TargetFrameLowering.h"
20 #include "llvm/CodeGen/TargetRegisterInfo.h"
21 #include "llvm/CodeGen/TargetSubtargetInfo.h"
22 #include "llvm/IR/DebugInfoMetadata.h"
23 #include "llvm/Pass.h"
24 #include "llvm/Support/Debug.h"
25 #include "llvm/Support/raw_ostream.h"
26 
27 using namespace llvm;
28 
29 #define DEBUG_TYPE "nvptx-prolog-epilog"
30 
31 namespace {
32 class NVPTXPrologEpilogPass : public MachineFunctionPass {
33 public:
34   static char ID;
35   NVPTXPrologEpilogPass() : MachineFunctionPass(ID) {}
36 
37   bool runOnMachineFunction(MachineFunction &MF) override;
38 
39 private:
40   void calculateFrameObjectOffsets(MachineFunction &Fn);
41 };
42 }
43 
44 MachineFunctionPass *llvm::createNVPTXPrologEpilogPass() {
45   return new NVPTXPrologEpilogPass();
46 }
47 
48 char NVPTXPrologEpilogPass::ID = 0;
49 
50 bool NVPTXPrologEpilogPass::runOnMachineFunction(MachineFunction &MF) {
51   const TargetSubtargetInfo &STI = MF.getSubtarget();
52   const TargetFrameLowering &TFI = *STI.getFrameLowering();
53   const TargetRegisterInfo &TRI = *STI.getRegisterInfo();
54   bool Modified = false;
55 
56   calculateFrameObjectOffsets(MF);
57 
58   for (MachineBasicBlock &MBB : MF) {
59     for (MachineInstr &MI : MBB) {
60       for (unsigned i = 0, e = MI.getNumOperands(); i != e; ++i) {
61         if (!MI.getOperand(i).isFI())
62           continue;
63 
64         // Frame indices in debug values are encoded in a target independent
65         // way with simply the frame index and offset rather than any
66         // target-specific addressing mode.
67         if (MI.isDebugValue()) {
68           assert(i == 0 && "Frame indices can only appear as the first "
69                            "operand of a DBG_VALUE machine instruction");
70           Register Reg;
71           int64_t Offset =
72               TFI.getFrameIndexReference(MF, MI.getOperand(0).getIndex(), Reg)
73                   .getFixed();
74           MI.getOperand(0).ChangeToRegister(Reg, /*isDef=*/false);
75           MI.getOperand(0).setIsDebug();
76           auto *DIExpr = DIExpression::prepend(
77               MI.getDebugExpression(), DIExpression::ApplyOffset, Offset);
78           MI.getDebugExpressionOp().setMetadata(DIExpr);
79           continue;
80         }
81 
82         TRI.eliminateFrameIndex(MI, 0, i, nullptr);
83         Modified = true;
84       }
85     }
86   }
87 
88   // Add function prolog/epilog
89   TFI.emitPrologue(MF, MF.front());
90 
91   for (MachineFunction::iterator I = MF.begin(), E = MF.end(); I != E; ++I) {
92     // If last instruction is a return instruction, add an epilogue
93     if (I->isReturnBlock())
94       TFI.emitEpilogue(MF, *I);
95   }
96 
97   return Modified;
98 }
99 
100 /// AdjustStackOffset - Helper function used to adjust the stack frame offset.
101 static inline void AdjustStackOffset(MachineFrameInfo &MFI, int FrameIdx,
102                                      bool StackGrowsDown, int64_t &Offset,
103                                      Align &MaxAlign) {
104   // If the stack grows down, add the object size to find the lowest address.
105   if (StackGrowsDown)
106     Offset += MFI.getObjectSize(FrameIdx);
107 
108   Align Alignment = MFI.getObjectAlign(FrameIdx);
109 
110   // If the alignment of this object is greater than that of the stack, then
111   // increase the stack alignment to match.
112   MaxAlign = std::max(MaxAlign, Alignment);
113 
114   // Adjust to alignment boundary.
115   Offset = alignTo(Offset, Alignment);
116 
117   if (StackGrowsDown) {
118     LLVM_DEBUG(dbgs() << "alloc FI(" << FrameIdx << ") at SP[" << -Offset
119                       << "]\n");
120     MFI.setObjectOffset(FrameIdx, -Offset); // Set the computed offset
121   } else {
122     LLVM_DEBUG(dbgs() << "alloc FI(" << FrameIdx << ") at SP[" << Offset
123                       << "]\n");
124     MFI.setObjectOffset(FrameIdx, Offset);
125     Offset += MFI.getObjectSize(FrameIdx);
126   }
127 }
128 
129 void
130 NVPTXPrologEpilogPass::calculateFrameObjectOffsets(MachineFunction &Fn) {
131   const TargetFrameLowering &TFI = *Fn.getSubtarget().getFrameLowering();
132   const TargetRegisterInfo *RegInfo = Fn.getSubtarget().getRegisterInfo();
133 
134   bool StackGrowsDown =
135     TFI.getStackGrowthDirection() == TargetFrameLowering::StackGrowsDown;
136 
137   // Loop over all of the stack objects, assigning sequential addresses...
138   MachineFrameInfo &MFI = Fn.getFrameInfo();
139 
140   // Start at the beginning of the local area.
141   // The Offset is the distance from the stack top in the direction
142   // of stack growth -- so it's always nonnegative.
143   int LocalAreaOffset = TFI.getOffsetOfLocalArea();
144   if (StackGrowsDown)
145     LocalAreaOffset = -LocalAreaOffset;
146   assert(LocalAreaOffset >= 0
147          && "Local area offset should be in direction of stack growth");
148   int64_t Offset = LocalAreaOffset;
149 
150   // If there are fixed sized objects that are preallocated in the local area,
151   // non-fixed objects can't be allocated right at the start of local area.
152   // We currently don't support filling in holes in between fixed sized
153   // objects, so we adjust 'Offset' to point to the end of last fixed sized
154   // preallocated object.
155   for (int i = MFI.getObjectIndexBegin(); i != 0; ++i) {
156     int64_t FixedOff;
157     if (StackGrowsDown) {
158       // The maximum distance from the stack pointer is at lower address of
159       // the object -- which is given by offset. For down growing stack
160       // the offset is negative, so we negate the offset to get the distance.
161       FixedOff = -MFI.getObjectOffset(i);
162     } else {
163       // The maximum distance from the start pointer is at the upper
164       // address of the object.
165       FixedOff = MFI.getObjectOffset(i) + MFI.getObjectSize(i);
166     }
167     if (FixedOff > Offset) Offset = FixedOff;
168   }
169 
170   // NOTE: We do not have a call stack
171 
172   Align MaxAlign = MFI.getMaxAlign();
173 
174   // No scavenger
175 
176   // FIXME: Once this is working, then enable flag will change to a target
177   // check for whether the frame is large enough to want to use virtual
178   // frame index registers. Functions which don't want/need this optimization
179   // will continue to use the existing code path.
180   if (MFI.getUseLocalStackAllocationBlock()) {
181     Align Alignment = MFI.getLocalFrameMaxAlign();
182 
183     // Adjust to alignment boundary.
184     Offset = alignTo(Offset, Alignment);
185 
186     LLVM_DEBUG(dbgs() << "Local frame base offset: " << Offset << "\n");
187 
188     // Resolve offsets for objects in the local block.
189     for (unsigned i = 0, e = MFI.getLocalFrameObjectCount(); i != e; ++i) {
190       std::pair<int, int64_t> Entry = MFI.getLocalFrameObjectMap(i);
191       int64_t FIOffset = (StackGrowsDown ? -Offset : Offset) + Entry.second;
192       LLVM_DEBUG(dbgs() << "alloc FI(" << Entry.first << ") at SP[" << FIOffset
193                         << "]\n");
194       MFI.setObjectOffset(Entry.first, FIOffset);
195     }
196     // Allocate the local block
197     Offset += MFI.getLocalFrameSize();
198 
199     MaxAlign = std::max(Alignment, MaxAlign);
200   }
201 
202   // No stack protector
203 
204   // Then assign frame offsets to stack objects that are not used to spill
205   // callee saved registers.
206   for (unsigned i = 0, e = MFI.getObjectIndexEnd(); i != e; ++i) {
207     if (MFI.isObjectPreAllocated(i) &&
208         MFI.getUseLocalStackAllocationBlock())
209       continue;
210     if (MFI.isDeadObjectIndex(i))
211       continue;
212 
213     AdjustStackOffset(MFI, i, StackGrowsDown, Offset, MaxAlign);
214   }
215 
216   // No scavenger
217 
218   if (!TFI.targetHandlesStackFrameRounding()) {
219     // If we have reserved argument space for call sites in the function
220     // immediately on entry to the current function, count it as part of the
221     // overall stack size.
222     if (MFI.adjustsStack() && TFI.hasReservedCallFrame(Fn))
223       Offset += MFI.getMaxCallFrameSize();
224 
225     // Round up the size to a multiple of the alignment.  If the function has
226     // any calls or alloca's, align to the target's StackAlignment value to
227     // ensure that the callee's frame or the alloca data is suitably aligned;
228     // otherwise, for leaf functions, align to the TransientStackAlignment
229     // value.
230     Align StackAlign;
231     if (MFI.adjustsStack() || MFI.hasVarSizedObjects() ||
232         (RegInfo->needsStackRealignment(Fn) && MFI.getObjectIndexEnd() != 0))
233       StackAlign = TFI.getStackAlign();
234     else
235       StackAlign = TFI.getTransientStackAlign();
236 
237     // If the frame pointer is eliminated, all frame offsets will be relative to
238     // SP not FP. Align to MaxAlign so this works.
239     Offset = alignTo(Offset, std::max(StackAlign, MaxAlign));
240   }
241 
242   // Update frame info to pretend that this is part of the stack...
243   int64_t StackSize = Offset - LocalAreaOffset;
244   MFI.setStackSize(StackSize);
245 }
246