[lld][WebAssembly] Allow ctors functions that return values

[sbc100]

Summary

Some projects use the constructor attribute on functions that also
return values. In this case we just ignore them.

The error was reported in the libgpg-error project that marks
gpg_err_init with the __constructor__ attribute.

[sunfish]

This change looks fine to me, though if you wanted to minimize the amount of code being emitted by the linker, another option would be to look into whether LLVM's FixFunctionBitcasts could handle this -- there should be a bitcast in the llvm.global_ctors array.

[sbc100]

In D81962#2097082, @sunfish wrote:

This change looks fine to me, though if you wanted to minimize the amount of code being emitted by the linker, another option would be to look into whether LLVM's FixFunctionBitcasts could handle this -- there should be a bitcast in the llvm.global_ctors array.

Interesting.. currently FixFunctionBitcasts only considered casts that are arguments to call instructions. Other kinds of casts such as ones that are used in stores we assume instead that that reverse cast will be applied when they are used.

Do you think its better to modify FixFunctionBitcasts to add this special case? (assuming I can).

[sbc100]

ping..

Having looked at FixFunctionBitcasts it this fix is more appropriate. FixFunctionBitcasts is designed for callsites only (at least today).

lld/test/wasm/ctor_return_value.s

+# RUN: llvm-mc -filetype=obj -triple=wasm32-unknown-unknown -o %t.o %s
+# RUN: wasm-ld %t.o -o %t.wasm
+# RUN: obj2yaml %t.wasm | FileCheck %s
+
+  .globl  myctor
+myctor:
+  .functype myctor () -> (i32)
+  i32.const 1
+
+  end_function
+
+  .globl  _start
+_start:
+  .functype _start () -> ()
+  call __wasm_call_ctors
+  end_function
+
+  .section  .init_array.100,"",@
+  .p2align  2
+  .int32  myctor
+  .int32  myctor
+  .int32  myctor
+
+.type __wasm_call_ctors,@function
+
+#      CHECK:   - Type:            CODE
+# CHECK-NEXT:     Functions:
+# CHECK-NEXT:       - Index:           0
+# CHECK-NEXT:         Locals:          []
+# CHECK-NEXT:         Body:            10011A10011A10011A0B
+# CHECK-NEXT:       - Index:           1
+# CHECK-NEXT:         Locals:          []
+# CHECK-NEXT:         Body:            41010B
+# CHECK-NEXT:       - Index:           2
+# CHECK-NEXT:         Locals:          []
+# CHECK-NEXT:         Body:            1080808080000B
+# CHECK-NEXT:   - Type:            CUSTOM
+# CHECK-NEXT:     Name:            name
+# CHECK-NEXT:     FunctionNames:
+# CHECK-NEXT:       - Index:           0
+# CHECK-NEXT:         Name:            __wasm_call_ctors
+# CHECK-NEXT:       - Index:           1
+# CHECK-NEXT:         Name:            myctor
+# CHECK-NEXT:       - Index:           2
+# CHECK-NEXT:         Name:            _start
+# CHECK-NEXT: ...

lld/wasm/Writer.cpp

if (config->isPic) {
       writeUleb128(os, WasmSym::applyRelocs->getFunctionIndex(),
                    "function index");
     }
 
     // Call constructors
     for (const WasmInitEntry &f : initFunctions) {
       writeU8(os, WASM_OPCODE_CALL, "CALL");
       writeUleb128(os, f.sym->getFunctionIndex(), "function index");
+      for (size_t i = 0; i < f.sym->signature->Returns.size(); i++) {
+        writeU8(os, WASM_OPCODE_DROP, "DROP");
+      }
     }
     writeU8(os, WASM_OPCODE_END, "END");
   }
 
   createFunction(WasmSym::callCtors, bodyContent);
 }
 
 void Writer::createInitTLSFunction() {
void Writer::calculateInitFunctions() {
   for (ObjFile *file : symtab->objectFiles) {
     const WasmLinkingData &l = file->getWasmObj()->linkingData();
     for (const WasmInitFunc &f : l.InitFunctions) {
       FunctionSymbol *sym = file->getFunctionSymbol(f.Symbol);
       // comdat exclusions can cause init functions be discarded.
       if (sym->isDiscarded())
         continue;
       assert(sym->isLive());
-      if (*sym->signature != WasmSignature{{}, {}})
-        error("invalid signature for init func: " + toString(*sym));
+      if (sym->signature->Params.size() != 0)
+        error("constructor functions cannot take arguments: " + toString(*sym));
       LLVM_DEBUG(dbgs() << "initFunctions: " << toString(*sym) << "\n");
       initFunctions.emplace_back(WasmInitEntry{sym, f.Priority});
     }
   }
 
   // Sort in order of priority (lowest first) so that they are called
   // in the correct order.
   llvm::stable_sort(initFunctions,