1 //===- ErlangGCPrinter.cpp - Erlang/OTP frametable emitter ----------------===//
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 implements the compiler plugin that is used in order to emit
10 // garbage collection information in a convenient layout for parsing and
11 // loading in the Erlang/OTP runtime.
12 //
13 //===----------------------------------------------------------------------===//
14 
15 #include "llvm/BinaryFormat/ELF.h"
16 #include "llvm/CodeGen/AsmPrinter.h"
17 #include "llvm/CodeGen/BuiltinGCs.h"
18 #include "llvm/CodeGen/GCMetadata.h"
19 #include "llvm/CodeGen/GCMetadataPrinter.h"
20 #include "llvm/CodeGen/GCStrategy.h"
21 #include "llvm/IR/DataLayout.h"
22 #include "llvm/IR/Function.h"
23 #include "llvm/IR/Module.h"
24 #include "llvm/MC/MCContext.h"
25 #include "llvm/MC/MCSectionELF.h"
26 #include "llvm/MC/MCStreamer.h"
27 #include "llvm/MC/MCSymbol.h"
28 #include "llvm/Target/TargetLoweringObjectFile.h"
29 
30 using namespace llvm;
31 
32 namespace {
33 
34 class ErlangGCPrinter : public GCMetadataPrinter {
35 public:
36   void finishAssembly(Module &M, GCModuleInfo &Info, AsmPrinter &AP) override;
37 };
38 
39 } // end anonymous namespace
40 
41 static GCMetadataPrinterRegistry::Add<ErlangGCPrinter>
42     X("erlang", "erlang-compatible garbage collector");
43 
44 void ErlangGCPrinter::finishAssembly(Module &M, GCModuleInfo &Info,
45                                      AsmPrinter &AP) {
46   MCStreamer &OS = *AP.OutStreamer;
47   unsigned IntPtrSize = M.getDataLayout().getPointerSize();
48 
49   // Put this in a custom .note section.
50   OS.SwitchSection(
51       AP.getObjFileLowering().getContext().getELFSection(".note.gc",
52                                                          ELF::SHT_PROGBITS, 0));
53 
54   // For each function...
55   for (GCModuleInfo::FuncInfoVec::iterator FI = Info.funcinfo_begin(),
56                                            IE = Info.funcinfo_end();
57        FI != IE; ++FI) {
58     GCFunctionInfo &MD = **FI;
59     if (MD.getStrategy().getName() != getStrategy().getName())
60       // this function is managed by some other GC
61       continue;
62     /** A compact GC layout. Emit this data structure:
63      *
64      * struct {
65      *   int16_t PointCount;
66      *   void *SafePointAddress[PointCount];
67      *   int16_t StackFrameSize; (in words)
68      *   int16_t StackArity;
69      *   int16_t LiveCount;
70      *   int16_t LiveOffsets[LiveCount];
71      * } __gcmap_<FUNCTIONNAME>;
72      **/
73 
74     // Align to address width.
75     AP.emitAlignment(IntPtrSize == 4 ? Align(4) : Align(8));
76 
77     // Emit PointCount.
78     OS.AddComment("safe point count");
79     AP.emitInt16(MD.size());
80 
81     // And each safe point...
82     for (GCFunctionInfo::iterator PI = MD.begin(), PE = MD.end(); PI != PE;
83          ++PI) {
84       // Emit the address of the safe point.
85       OS.AddComment("safe point address");
86       MCSymbol *Label = PI->Label;
87       AP.emitLabelPlusOffset(Label /*Hi*/, 0 /*Offset*/, 4 /*Size*/);
88     }
89 
90     // Stack information never change in safe points! Only print info from the
91     // first call-site.
92     GCFunctionInfo::iterator PI = MD.begin();
93 
94     // Emit the stack frame size.
95     OS.AddComment("stack frame size (in words)");
96     AP.emitInt16(MD.getFrameSize() / IntPtrSize);
97 
98     // Emit stack arity, i.e. the number of stacked arguments.
99     unsigned RegisteredArgs = IntPtrSize == 4 ? 5 : 6;
100     unsigned StackArity = MD.getFunction().arg_size() > RegisteredArgs
101                               ? MD.getFunction().arg_size() - RegisteredArgs
102                               : 0;
103     OS.AddComment("stack arity");
104     AP.emitInt16(StackArity);
105 
106     // Emit the number of live roots in the function.
107     OS.AddComment("live root count");
108     AP.emitInt16(MD.live_size(PI));
109 
110     // And for each live root...
111     for (GCFunctionInfo::live_iterator LI = MD.live_begin(PI),
112                                        LE = MD.live_end(PI);
113          LI != LE; ++LI) {
114       // Emit live root's offset within the stack frame.
115       OS.AddComment("stack index (offset / wordsize)");
116       AP.emitInt16(LI->StackOffset / IntPtrSize);
117     }
118   }
119 }
120 
121 void llvm::linkErlangGCPrinter() {}
122