[WebAssembly] Implement --print-gc-sections for synthetic functions

Enables cleaning up confusion between which name variables are mangled
and which are unmangled, and --print-gc-sections then excersises and
tests that.

Differential Revision: https://reviews.llvm.org/D44440

llvm-svn: 330449

lld/test/wasm/undefined-weak-call.ll

@@ -1,5 +1,6 @@
 ; RUN: llc -filetype=obj %s -o %t.o
-; RUN: wasm-ld --check-signatures --no-entry %t.o -o %t.wasm
+; RUN: wasm-ld --check-signatures --no-entry --print-gc-sections %t.o \
+; RUN:     -o %t.wasm 2>&1 | FileCheck -check-prefix=CHECK-GC %s
 ; RUN: obj2yaml %t.wasm | FileCheck %s
 
 ; Check that calling an undefined weak function generates an appropriate stub
@@ -12,6 +13,8 @@ declare extern_weak void @weakFunc2()         ; same signature
 declare extern_weak void @weakFunc3(i32 %arg) ; different
 declare extern_weak void @weakFunc4()         ; should be GC'd as not called
 
+; CHECK-GC: removing unused section {{.*}}:(weakFunc4)
+
 define i32 @callWeakFuncs() {
   call void @weakFunc1()
   call void @weakFunc2()

lld/wasm/Driver.cpp

@@ -240,10 +240,11 @@ static void handleWeakUndefines() {
     // Add a synthetic dummy for weak undefined functions.  These dummies will
     // be GC'd if not used as the target of any "call" instructions.
     Optional<std::string> SymName = demangleItanium(Sym->getName());
-    StringRef StubName =
+    StringRef DebugName =
         Saver.save("undefined function " +
                    (SymName ? StringRef(*SymName) : Sym->getName()));
-    SyntheticFunction *Func = make<SyntheticFunction>(Sig, StubName);
+    SyntheticFunction *Func =
+        make<SyntheticFunction>(Sig, Sym->getName(), DebugName);
     Func->setBody(UnreachableFn);
     // Ensure it compares equal to the null pointer, and so that table relocs
     // don't pull in the stub body (only call-operand relocs should do that).

lld/wasm/InputChunks.h

@@ -57,6 +57,7 @@ public:
   ArrayRef<WasmRelocation> getRelocations() const { return Relocations; }
 
   virtual StringRef getName() const = 0;
+  virtual StringRef getDebugName() const = 0;
   virtual uint32_t getComdat() const = 0;
   StringRef getComdatName() const;
 
@@ -99,6 +100,7 @@ public:
 
   uint32_t getAlignment() const { return Segment.Data.Alignment; }
   StringRef getName() const override { return Segment.Data.Name; }
+  StringRef getDebugName() const override { return StringRef(); }
   uint32_t getComdat() const override { return Segment.Data.Comdat; }
 
   const OutputSegment *OutputSeg = nullptr;
@@ -125,7 +127,8 @@ public:
            C->kind() == InputChunk::SyntheticFunction;
   }
 
-  StringRef getName() const override { return Function->Name; }
+  StringRef getName() const override { return Function->SymbolName; }
+  StringRef getDebugName() const override { return Function->DebugName; }
   uint32_t getComdat() const override { return Function->Comdat; }
   uint32_t getFunctionIndex() const { return FunctionIndex.getValue(); }
   bool hasFunctionIndex() const { return FunctionIndex.hasValue(); }
@@ -152,8 +155,9 @@ protected:
 
 class SyntheticFunction : public InputFunction {
 public:
-  SyntheticFunction(const WasmSignature &S, StringRef Name)
-      : InputFunction(S, nullptr, nullptr), Name(Name) {
+  SyntheticFunction(const WasmSignature &S, StringRef Name,
+                    StringRef DebugName = {})
+      : InputFunction(S, nullptr, nullptr), Name(Name), DebugName(DebugName) {
     SectionKind = InputChunk::SyntheticFunction;
   }
 
@@ -162,6 +166,7 @@ public:
   }
 
   StringRef getName() const override { return Name; }
+  StringRef getDebugName() const override { return DebugName; }
   uint32_t getComdat() const override { return UINT32_MAX; }
 
   void setBody(ArrayRef<uint8_t> Body_) { Body = Body_; }
@@ -170,6 +175,7 @@ protected:
   ArrayRef<uint8_t> data() const override { return Body; }
 
   StringRef Name;
+  StringRef DebugName;
   ArrayRef<uint8_t> Body;
 };
 
@@ -182,6 +188,7 @@ public:
   }
 
   StringRef getName() const override { return Section.Name; }
+  StringRef getDebugName() const override { return StringRef(); }
   uint32_t getComdat() const override { return UINT32_MAX; }
 
 protected:

lld/wasm/MarkLive.cpp

@@ -105,5 +105,8 @@ void lld::wasm::markLive() {
         if (!C->Live)
           message("removing unused section " + toString(C));
     }
+    for (InputChunk *C : Symtab->SyntheticFunctions)
+      if (!C->Live)
+        message("removing unused section " + toString(C));
   }
 }

lld/wasm/Writer.cpp

@@ -534,7 +534,7 @@ void Writer::createLinkingSection() {
 void Writer::createNameSection() {
   unsigned NumNames = NumImportedFunctions;
   for (const InputFunction *F : InputFunctions)
-    if (!F->getName().empty())
+    if (!F->getName().empty() || !F->getDebugName().empty())
       ++NumNames;
 
   if (NumNames == 0)
@@ -558,8 +558,12 @@ void Writer::createNameSection() {
   for (const InputFunction *F : InputFunctions) {
     if (!F->getName().empty()) {
       writeUleb128(Sub.OS, F->getFunctionIndex(), "func index");
-      Optional<std::string> Name = demangleItanium(F->getName());
-      writeStr(Sub.OS, Name ? StringRef(*Name) : F->getName(), "symbol name");
+      if (!F->getDebugName().empty()) {
+        writeStr(Sub.OS, F->getDebugName(), "symbol name");
+      } else {
+        Optional<std::string> Name = demangleItanium(F->getName());
+        writeStr(Sub.OS, Name ? StringRef(*Name) : F->getName(), "symbol name");
+      }
     }
   }