1 //===- TargetFrameLoweringImpl.cpp - Implement target frame interface ------==//
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 // Implements the layout of a stack frame on the target machine.
11 //
12 //===----------------------------------------------------------------------===//
13 
14 #include "llvm/ADT/BitVector.h"
15 #include "llvm/CodeGen/MachineFrameInfo.h"
16 #include "llvm/CodeGen/MachineFunction.h"
17 #include "llvm/CodeGen/MachineRegisterInfo.h"
18 #include "llvm/CodeGen/TargetFrameLowering.h"
19 #include "llvm/CodeGen/TargetRegisterInfo.h"
20 #include "llvm/CodeGen/TargetSubtargetInfo.h"
21 #include "llvm/IR/Attributes.h"
22 #include "llvm/IR/CallingConv.h"
23 #include "llvm/IR/Function.h"
24 #include "llvm/MC/MCRegisterInfo.h"
25 #include "llvm/Support/Compiler.h"
26 #include "llvm/Target/TargetMachine.h"
27 #include "llvm/Target/TargetOptions.h"
28 
29 using namespace llvm;
30 
31 TargetFrameLowering::~TargetFrameLowering() = default;
32 
33 /// The default implementation just looks at attribute "no-frame-pointer-elim".
34 bool TargetFrameLowering::noFramePointerElim(const MachineFunction &MF) const {
35   auto Attr = MF.getFunction().getFnAttribute("no-frame-pointer-elim");
36   return Attr.getValueAsString() == "true";
37 }
38 
39 bool TargetFrameLowering::enableCalleeSaveSkip(const MachineFunction &MF) const {
40   assert(MF.getFunction().hasFnAttribute(Attribute::NoReturn) &&
41          MF.getFunction().hasFnAttribute(Attribute::NoUnwind) &&
42          !MF.getFunction().hasFnAttribute(Attribute::UWTable));
43   return false;
44 }
45 
46 /// Returns the displacement from the frame register to the stack
47 /// frame of the specified index, along with the frame register used
48 /// (in output arg FrameReg). This is the default implementation which
49 /// is overridden for some targets.
50 int TargetFrameLowering::getFrameIndexReference(const MachineFunction &MF,
51                                              int FI, unsigned &FrameReg) const {
52   const MachineFrameInfo &MFI = MF.getFrameInfo();
53   const TargetRegisterInfo *RI = MF.getSubtarget().getRegisterInfo();
54 
55   // By default, assume all frame indices are referenced via whatever
56   // getFrameRegister() says. The target can override this if it's doing
57   // something different.
58   FrameReg = RI->getFrameRegister(MF);
59 
60   return MFI.getObjectOffset(FI) + MFI.getStackSize() -
61          getOffsetOfLocalArea() + MFI.getOffsetAdjustment();
62 }
63 
64 bool TargetFrameLowering::needsFrameIndexResolution(
65     const MachineFunction &MF) const {
66   return MF.getFrameInfo().hasStackObjects();
67 }
68 
69 void TargetFrameLowering::determineCalleeSaves(MachineFunction &MF,
70                                                BitVector &SavedRegs,
71                                                RegScavenger *RS) const {
72   const TargetRegisterInfo &TRI = *MF.getSubtarget().getRegisterInfo();
73 
74   // Resize before the early returns. Some backends expect that
75   // SavedRegs.size() == TRI.getNumRegs() after this call even if there are no
76   // saved registers.
77   SavedRegs.resize(TRI.getNumRegs());
78 
79   // When interprocedural register allocation is enabled caller saved registers
80   // are preferred over callee saved registers.
81   if (MF.getTarget().Options.EnableIPRA && isSafeForNoCSROpt(MF.getFunction()))
82     return;
83 
84   // Get the callee saved register list...
85   const MCPhysReg *CSRegs = MF.getRegInfo().getCalleeSavedRegs();
86 
87   // Early exit if there are no callee saved registers.
88   if (!CSRegs || CSRegs[0] == 0)
89     return;
90 
91   // In Naked functions we aren't going to save any registers.
92   if (MF.getFunction().hasFnAttribute(Attribute::Naked))
93     return;
94 
95   // Noreturn+nounwind functions never restore CSR, so no saves are needed.
96   // Purely noreturn functions may still return through throws, so those must
97   // save CSR for caller exception handlers.
98   //
99   // If the function uses longjmp to break out of its current path of
100   // execution we do not need the CSR spills either: setjmp stores all CSRs
101   // it was called with into the jmp_buf, which longjmp then restores.
102   if (MF.getFunction().hasFnAttribute(Attribute::NoReturn) &&
103         MF.getFunction().hasFnAttribute(Attribute::NoUnwind) &&
104         !MF.getFunction().hasFnAttribute(Attribute::UWTable) &&
105         enableCalleeSaveSkip(MF))
106     return;
107 
108   // Functions which call __builtin_unwind_init get all their registers saved.
109   bool CallsUnwindInit = MF.callsUnwindInit();
110   const MachineRegisterInfo &MRI = MF.getRegInfo();
111   for (unsigned i = 0; CSRegs[i]; ++i) {
112     unsigned Reg = CSRegs[i];
113     if (CallsUnwindInit || MRI.isPhysRegModified(Reg))
114       SavedRegs.set(Reg);
115   }
116 }
117 
118 unsigned TargetFrameLowering::getStackAlignmentSkew(
119     const MachineFunction &MF) const {
120   // When HHVM function is called, the stack is skewed as the return address
121   // is removed from the stack before we enter the function.
122   if (LLVM_UNLIKELY(MF.getFunction().getCallingConv() == CallingConv::HHVM))
123     return MF.getTarget().getAllocaPointerSize();
124 
125   return 0;
126 }
127 
128 int TargetFrameLowering::getInitialCFAOffset(const MachineFunction &MF) const {
129   llvm_unreachable("getInitialCFAOffset() not implemented!");
130 }
131 
132 unsigned TargetFrameLowering::getInitialCFARegister(const MachineFunction &MF)
133     const {
134   llvm_unreachable("getInitialCFARegister() not implemented!");
135 }