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