1 //===-- RISCVTargetObjectFile.cpp - RISCV Object Info -----------------===//
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 #include "RISCVTargetObjectFile.h"
10 #include "RISCVTargetMachine.h"
11 #include "llvm/BinaryFormat/ELF.h"
12 #include "llvm/MC/MCContext.h"
13 #include "llvm/MC/MCSectionELF.h"
14 
15 using namespace llvm;
16 
17 void RISCVELFTargetObjectFile::Initialize(MCContext &Ctx,
18                                           const TargetMachine &TM) {
19   TargetLoweringObjectFileELF::Initialize(Ctx, TM);
20   InitializeELF(TM.Options.UseInitArray);
21 
22   SmallDataSection = getContext().getELFSection(
23       ".sdata", ELF::SHT_PROGBITS, ELF::SHF_WRITE | ELF::SHF_ALLOC);
24   SmallBSSSection = getContext().getELFSection(".sbss", ELF::SHT_NOBITS,
25                                                ELF::SHF_WRITE | ELF::SHF_ALLOC);
26 }
27 
28 // A address must be loaded from a small section if its size is less than the
29 // small section size threshold. Data in this section could be addressed by
30 // using gp_rel operator.
31 bool RISCVELFTargetObjectFile::isInSmallSection(uint64_t Size) const {
32   // gcc has traditionally not treated zero-sized objects as small data, so this
33   // is effectively part of the ABI.
34   return Size > 0 && Size <= SSThreshold;
35 }
36 
37 // Return true if this global address should be placed into small data/bss
38 // section.
39 bool RISCVELFTargetObjectFile::isGlobalInSmallSection(
40     const GlobalObject *GO, const TargetMachine &TM) const {
41   // Only global variables, not functions.
42   const GlobalVariable *GVA = dyn_cast<GlobalVariable>(GO);
43   if (!GVA)
44     return false;
45 
46   // If the variable has an explicit section, it is placed in that section.
47   if (GVA->hasSection()) {
48     StringRef Section = GVA->getSection();
49 
50     // Explicitly placing any variable in the small data section overrides
51     // the global -G value.
52     if (Section == ".sdata" || Section == ".sbss")
53       return true;
54 
55     // Otherwise reject putting the variable to small section if it has an
56     // explicit section name.
57     return false;
58   }
59 
60   if (((GVA->hasExternalLinkage() && GVA->isDeclaration()) ||
61        GVA->hasCommonLinkage()))
62     return false;
63 
64   Type *Ty = GVA->getValueType();
65   // It is possible that the type of the global is unsized, i.e. a declaration
66   // of a extern struct. In this case don't presume it is in the small data
67   // section. This happens e.g. when building the FreeBSD kernel.
68   if (!Ty->isSized())
69     return false;
70 
71   return isInSmallSection(
72       GVA->getParent()->getDataLayout().getTypeAllocSize(Ty));
73 }
74 
75 MCSection *RISCVELFTargetObjectFile::SelectSectionForGlobal(
76     const GlobalObject *GO, SectionKind Kind, const TargetMachine &TM) const {
77   // Handle Small Section classification here.
78   if (Kind.isBSS() && isGlobalInSmallSection(GO, TM))
79     return SmallBSSSection;
80   if (Kind.isData() && isGlobalInSmallSection(GO, TM))
81     return SmallDataSection;
82 
83   // Otherwise, we work the same as ELF.
84   return TargetLoweringObjectFileELF::SelectSectionForGlobal(GO, Kind, TM);
85 }
86 
87 void RISCVELFTargetObjectFile::getModuleMetadata(Module &M) {
88   SmallVector<Module::ModuleFlagEntry, 8> ModuleFlags;
89   M.getModuleFlagsMetadata(ModuleFlags);
90 
91   for (const auto &MFE : ModuleFlags) {
92     StringRef Key = MFE.Key->getString();
93     if (Key == "SmallDataLimit") {
94       SSThreshold = mdconst::extract<ConstantInt>(MFE.Val)->getZExtValue();
95       break;
96     }
97   }
98 }
99 
100 /// Return true if this constant should be placed into small data section.
101 bool RISCVELFTargetObjectFile::isConstantInSmallSection(
102     const DataLayout &DL, const Constant *CN) const {
103   return isInSmallSection(DL.getTypeAllocSize(CN->getType()));
104 }
105 
106 MCSection *RISCVELFTargetObjectFile::getSectionForConstant(
107     const DataLayout &DL, SectionKind Kind, const Constant *C,
108     unsigned &Align) const {
109   if (isConstantInSmallSection(DL, C))
110     return SmallDataSection;
111 
112   // Otherwise, we work the same as ELF.
113   return TargetLoweringObjectFileELF::getSectionForConstant(DL, Kind, C, Align);
114 }
115