[WebAssembly] Add support for -emit-relocs

This can be useful for post-link tools and for testing.  Sometimes
it can be useful to produces a regular executable but with relocations
preserved.

Differential Revision: https://reviews.llvm.org/D62378

llvm-svn: 361635

lld/test/wasm/emit-relocs.ll

@@ -0,0 +1,39 @@
+; RUN: llc -filetype=obj %s -o %t.o
+; RUN: llc -filetype=obj %p/Inputs/ret32.ll -o %t.ret32.o
+; RUN: wasm-ld --emit-relocs -o %t.wasm %t.o %t.ret32.o
+; RUN: obj2yaml %t.wasm | FileCheck %s
+
+target triple = "wasm32-unknown-unknown"
+
+declare i32 @ret32(float)
+
+define void @unused_function() {
+  ret void
+}
+
+define hidden void @_start() local_unnamed_addr #0 {
+entry:
+  call i32 @ret32(float 0.0)
+  ret void
+}
+
+; CHECK:        - Type:            CODE
+; CHECK-NEXT:     Relocations:
+; CHECK-NEXT:       - Type:            R_WASM_FUNCTION_INDEX_LEB
+; CHECK-NEXT:         Index:           1
+; CHECK-NEXT:         Offset:          0x00000009
+
+; CHECK:        - Type:            CUSTOM
+; CHECK-NEXT:     Name:            linking
+; CHECK-NEXT:     Version:         2
+; CHECK-NEXT:     SymbolTable:
+; CHECK-NEXT:       - Index:           0
+; CHECK-NEXT:         Kind:            FUNCTION
+; CHECK-NEXT:         Name:            _start
+; CHECK-NEXT:         Flags:           [  ]
+; CHECK-NEXT:         Function:        0
+; CHECK-NEXT:       - Index:           1
+; CHECK-NEXT:         Kind:            FUNCTION
+; CHECK-NEXT:         Name:            ret32
+; CHECK-NEXT:         Flags:           [ VISIBILITY_HIDDEN ]
+; CHECK-NEXT:         Function:        1

lld/wasm/Config.h

@@ -27,6 +27,7 @@ struct Configuration {
   bool CompressRelocations;
   bool Demangle;
   bool DisableVerify;
+  bool EmitRelocs;
   bool ExportAll;
   bool ExportDynamic;
   bool ExportTable;

lld/wasm/Driver.cpp

@@ -302,6 +302,7 @@ static void readConfigs(opt::InputArgList &Args) {
   Config->CompressRelocations = Args.hasArg(OPT_compress_relocations);
   Config->Demangle = Args.hasFlag(OPT_demangle, OPT_no_demangle, true);
   Config->DisableVerify = Args.hasArg(OPT_disable_verify);
+  Config->EmitRelocs = Args.hasArg(OPT_emit_relocs);
   Config->Entry = getEntry(Args);
   Config->ExportAll = Args.hasArg(OPT_export_all);
   Config->ExportDynamic = Args.hasFlag(OPT_export_dynamic,

lld/wasm/Options.td

@@ -31,6 +31,8 @@ defm demangle: B<"demangle",
     "Demangle symbol names",
     "Do not demangle symbol names">;
 
+def emit_relocs: F<"emit-relocs">, HelpText<"Generate relocations in output">;
+
 defm export_dynamic: B<"export-dynamic",
     "Put symbols in the dynamic symbol table",
     "Do not put symbols in the dynamic symbol table (default)">;

lld/wasm/SyntheticSections.h

@@ -246,7 +246,9 @@ public:
                  const std::vector<OutputSegment *> &DataSegments)
       : SyntheticSection(llvm::wasm::WASM_SEC_CUSTOM, "linking"),
         InitFunctions(InitFunctions), DataSegments(DataSegments) {}
-  bool isNeeded() const override { return Config->Relocatable; }
+  bool isNeeded() const override {
+    return Config->Relocatable || Config->EmitRelocs;
+  }
   void writeBody() override;
   void addToSymtab(Symbol *Sym);
 

lld/wasm/Writer.cpp

@@ -129,7 +129,7 @@ void Writer::createCustomSections() {
     LLVM_DEBUG(dbgs() << "createCustomSection: " << Name << "\n");
 
     OutputSection *Sec = make<CustomSection>(Name, Pair.second);
-    if (Config->Relocatable) {
+    if (Config->Relocatable || Config->EmitRelocs) {
       auto *Sym = make<OutputSectionSymbol>(Sec);
       Out.LinkingSec->addToSymtab(Sym);
       Sec->SectionSym = Sym;
@@ -330,7 +330,7 @@ void Writer::addSections() {
   createCustomSections();
 
   addSection(Out.LinkingSec);
-  if (Config->Relocatable) {
+  if (Config->EmitRelocs || Config->Relocatable) {
     createRelocSections();
   }
 
@@ -493,17 +493,17 @@ void Writer::calculateExports() {
 }
 
 void Writer::populateSymtab() {
-  if (!Config->Relocatable)
+  if (!Config->Relocatable && !Config->EmitRelocs)
     return;
 
   for (Symbol *Sym : Symtab->getSymbols())
-    if (Sym->IsUsedInRegularObj)
+    if (Sym->IsUsedInRegularObj && Sym->isLive())
       Out.LinkingSec->addToSymtab(Sym);
 
   for (ObjFile *File : Symtab->ObjectFiles) {
     LLVM_DEBUG(dbgs() << "Local symtab entries: " << File->getName() << "\n");
     for (Symbol *Sym : File->getSymbols())
-      if (Sym->isLocal() && !isa<SectionSymbol>(Sym))
+      if (Sym->isLocal() && !isa<SectionSymbol>(Sym) && Sym->isLive())
         Out.LinkingSec->addToSymtab(Sym);
   }
 }