[WebAssembly] Initial implementation of PIC code generation
This change implements lowering of references global symbols in PIC
mode.
This change implements lowering of global references in PIC mode using a
new @GOT reference type. @GOT references can be used with function or
data symbol names combined with the get_global instruction. In this case
the linker will insert the wasm global that stores the address of the
symbol (either in memory for data symbols or in the wasm table for
function symbols).
For now I'm continuing to use the R_WASM_GLOBAL_INDEX_LEB relocation
type for this type of reference which means that this relocation type
can refer to either a global or a function or data symbol. We could
choose to introduce specific relocation types for GOT entries in the
future. See the current dynamic linking proposal:
https://github.com/WebAssembly/tool-conventions/blob/master/DynamicLinking.md
Differential Revision: https://reviews.llvm.org/D54647
llvm-svn: 357022
diff --git a/lld/wasm/Driver.cpp b/lld/wasm/Driver.cpp
index f9dfd5c..8f8d00d 100644
--- a/lld/wasm/Driver.cpp
+++ b/lld/wasm/Driver.cpp
@@ -519,6 +519,7 @@
}
if (Config->Shared) {
+ Config->ImportMemory = true;
Config->ExportDynamic = true;
Config->AllowUndefined = true;
}
diff --git a/lld/wasm/InputChunks.cpp b/lld/wasm/InputChunks.cpp
index 238c111..5d6dfa0 100644
--- a/lld/wasm/InputChunks.cpp
+++ b/lld/wasm/InputChunks.cpp
@@ -74,11 +74,14 @@
if (BytesRead && BytesRead != 5)
warn("expected LEB at relocation site be 5-byte padded");
- uint32_t ExpectedValue = File->calcExpectedValue(Rel);
- if (ExpectedValue != ExistingValue)
- warn("unexpected existing value for " + reloctTypeToString(Rel.Type) +
- ": existing=" + Twine(ExistingValue) +
- " expected=" + Twine(ExpectedValue));
+
+ if (Rel.Type != R_WASM_GLOBAL_INDEX_LEB) {
+ uint32_t ExpectedValue = File->calcExpectedValue(Rel);
+ if (ExpectedValue != ExistingValue)
+ warn("unexpected existing value for " + reloctTypeToString(Rel.Type) +
+ ": existing=" + Twine(ExistingValue) +
+ " expected=" + Twine(ExpectedValue));
+ }
}
}
diff --git a/lld/wasm/InputFiles.cpp b/lld/wasm/InputFiles.cpp
index b43cab7..2e49696 100644
--- a/lld/wasm/InputFiles.cpp
+++ b/lld/wasm/InputFiles.cpp
@@ -152,9 +152,12 @@
return TypeMap[Reloc.Index];
case R_WASM_FUNCTION_INDEX_LEB:
return getFunctionSymbol(Reloc.Index)->getFunctionIndex();
- case R_WASM_GLOBAL_INDEX_LEB:
- return getGlobalSymbol(Reloc.Index)->getGlobalIndex();
- case R_WASM_EVENT_INDEX_LEB:
+ case R_WASM_GLOBAL_INDEX_LEB: {
+ const Symbol* Sym = Symbols[Reloc.Index];
+ if (auto GS = dyn_cast<GlobalSymbol>(Sym))
+ return GS->getGlobalIndex();
+ return Sym->getGOTIndex();
+ } case R_WASM_EVENT_INDEX_LEB:
return getEventSymbol(Reloc.Index)->getEventIndex();
case R_WASM_FUNCTION_OFFSET_I32:
if (auto *Sym = dyn_cast<DefinedFunction>(getFunctionSymbol(Reloc.Index))) {
diff --git a/lld/wasm/Symbols.cpp b/lld/wasm/Symbols.cpp
index f8ea471..248581c 100644
--- a/lld/wasm/Symbols.cpp
+++ b/lld/wasm/Symbols.cpp
@@ -93,6 +93,12 @@
OutputSymbolIndex = Index;
}
+void Symbol::setGOTIndex(uint32_t Index) {
+ LLVM_DEBUG(dbgs() << "setGOTIndex " << Name << " -> " << Index << "\n");
+ assert(GOTIndex == INVALID_INDEX);
+ GOTIndex = Index;
+}
+
bool Symbol::isWeak() const {
return (Flags & WASM_SYMBOL_BINDING_MASK) == WASM_SYMBOL_BINDING_WEAK;
}
diff --git a/lld/wasm/Symbols.h b/lld/wasm/Symbols.h
index b59e8bb..a2ecef1 100644
--- a/lld/wasm/Symbols.h
+++ b/lld/wasm/Symbols.h
@@ -113,6 +113,16 @@
const WasmSignature* getSignature() const;
+ bool isInGOT() const { return GOTIndex != INVALID_INDEX; }
+
+ uint32_t getGOTIndex() const {
+ assert(GOTIndex != INVALID_INDEX);
+ return GOTIndex;
+ }
+
+ void setGOTIndex(uint32_t Index);
+ bool hasGOTIndex() const { return GOTIndex != INVALID_INDEX; }
+
protected:
Symbol(StringRef Name, Kind K, uint32_t Flags, InputFile *F)
: IsUsedInRegularObj(false), ForceExport(false), Traced(false),
@@ -124,6 +134,7 @@
uint32_t Flags;
InputFile *File;
uint32_t OutputSymbolIndex = INVALID_INDEX;
+ uint32_t GOTIndex = INVALID_INDEX;
bool Referenced;
};
@@ -249,21 +260,6 @@
static bool classof(const Symbol *S) {
return S->kind() == UndefinedDataKind;
}
-
- // Undefined data symbols are imported as wasm globals so also have a global
- // index.
- uint32_t getGlobalIndex() const {
- assert(GlobalIndex != INVALID_INDEX);
- return GlobalIndex;
- }
- void setGlobalIndex(uint32_t Index) {
- assert(GlobalIndex == INVALID_INDEX);
- GlobalIndex = Index;
- }
- bool hasGlobalIndex() const { return GlobalIndex != INVALID_INDEX; }
-
-protected:
- uint32_t GlobalIndex = INVALID_INDEX;
};
class GlobalSymbol : public Symbol {
diff --git a/lld/wasm/Writer.cpp b/lld/wasm/Writer.cpp
index 5f12ce7..480257f 100644
--- a/lld/wasm/Writer.cpp
+++ b/lld/wasm/Writer.cpp
@@ -118,6 +118,7 @@
std::vector<const WasmSignature *> Types;
DenseMap<WasmSignature, int32_t> TypeIndices;
std::vector<const Symbol *> ImportedSymbols;
+ std::vector<const Symbol *> GOTSymbols;
unsigned NumImportedFunctions = 0;
unsigned NumImportedGlobals = 0;
unsigned NumImportedEvents = 0;
@@ -147,7 +148,7 @@
} // anonymous namespace
void Writer::createImportSection() {
- uint32_t NumImports = ImportedSymbols.size();
+ uint32_t NumImports = ImportedSymbols.size() + GOTSymbols.size();
if (Config->ImportMemory)
++NumImports;
if (Config->ImportTable)
@@ -204,9 +205,6 @@
if (auto *FunctionSym = dyn_cast<FunctionSymbol>(Sym)) {
Import.Kind = WASM_EXTERNAL_FUNCTION;
Import.SigIndex = lookupType(*FunctionSym->Signature);
- } else if (auto *DataSym = dyn_cast<UndefinedData>(Sym)) {
- Import.Kind = WASM_EXTERNAL_GLOBAL;
- Import.Global = {WASM_TYPE_I32, true};
} else if (auto *GlobalSym = dyn_cast<GlobalSymbol>(Sym)) {
Import.Kind = WASM_EXTERNAL_GLOBAL;
Import.Global = *GlobalSym->getGlobalType();
@@ -218,6 +216,18 @@
}
writeImport(OS, Import);
}
+
+ for (const Symbol *Sym : GOTSymbols) {
+ WasmImport Import;
+ Import.Kind = WASM_EXTERNAL_GLOBAL;
+ Import.Global = {WASM_TYPE_I32, true};
+ if (isa<DataSymbol>(Sym))
+ Import.Module = "GOT.mem";
+ else
+ Import.Module = "GOT.func";
+ Import.Field = Sym->getName();
+ writeImport(OS, Import);
+ }
}
void Writer::createTypeSection() {
@@ -957,9 +967,9 @@
continue;
if (!Sym->IsUsedInRegularObj)
continue;
- // In relocatable output we don't generate imports for data symbols.
- // These live only in the symbol table.
- if (Config->Relocatable && isa<DataSymbol>(Sym))
+ // We don't generate imports for data symbols. They however can be imported
+ // as GOT entries.
+ if (isa<DataSymbol>(Sym))
continue;
LLVM_DEBUG(dbgs() << "import: " << Sym->getName() << "\n");
@@ -968,8 +978,6 @@
F->setFunctionIndex(NumImportedFunctions++);
else if (auto *G = dyn_cast<GlobalSymbol>(Sym))
G->setGlobalIndex(NumImportedGlobals++);
- else if (auto *D = dyn_cast<UndefinedData>(Sym))
- D->setGlobalIndex(NumImportedGlobals++);
else
cast<EventSymbol>(Sym)->setEventIndex(NumImportedEvents++);
}
@@ -1145,6 +1153,13 @@
DataSym->getName());
break;
}
+ case R_WASM_GLOBAL_INDEX_LEB: {
+ auto* Sym = File->getSymbols()[Reloc.Index];
+ if (!isa<GlobalSymbol>(Sym) && !Sym->isInGOT()) {
+ Sym->setGOTIndex(NumImportedGlobals++);
+ GOTSymbols.push_back(Sym);
+ }
+ }
}
}
}