Don't use Wasm function sections for more than one function

[ncw]

Summary

Fixes Bugzilla https://bugs.llvm.org/show_bug.cgi?id=35467

If a Wasm function section is created with more than one symbol, WasmObjectWriter fails with the following message: "function sections must contain one function each".

Currently, if a C++ file contains multiple static initialisers, they'll all be put in the same function section, triggering the error.

I think this change here is safe - it seems to be a spurious optimisation. In fact, I'm not even sure how it's intended to optimise the output...?

[sunfish]

I think that was copied from LinuxTargetInfo before we figured out our current object file strategy. On native platforms, it's an icache optimization, because startup functions are all called around the same time, and then never used again. However, this doesn't really apply to WebAssembly. It is possible that WebAssembly may eventually want to put everything called during startup at the beginning, so that clever browser engines can start running code even before the whole module is downloaded, however the optimization here wouldn't be sufficient. So this change looks good.

[ncw]

Thanks for the review @sunfish (and for the other input you've given). Could you land this for me Dan, as I don't have commit access? Thanks.

lib/Basic/Targets/OSTargets.h

void getOSDefines(const LangOptions &Opts, const llvm::Triple &Triple,
     // A common platform macro.
     if (Opts.POSIXThreads)
       Builder.defineMacro("_REENTRANT");
     // Follow g++ convention and predefine _GNU_SOURCE for C++.
     if (Opts.CPlusPlus)
       Builder.defineMacro("_GNU_SOURCE");
   }
 
-  // As an optimization, group static init code together in a section.
-  const char *getStaticInitSectionSpecifier() const final {
-    return ".text.__startup";
-  }
-
 public:
   explicit WebAssemblyOSTargetInfo(const llvm::Triple &Triple,
                                    const TargetOptions &Opts)
       : OSTargetInfo<Target>(Triple, Opts) {
     this->MCountName = "__mcount";
     this->TheCXXABI.set(TargetCXXABI::WebAssembly);
   }
 };
 
 } // namespace targets
 } // namespace clang
 #endif // LLVM_CLANG_LIB_BASIC_TARGETS_OSTARGETS_H

test/CodeGenCXX/static-init-wasm.cpp

 
 // Test various aspects of static constructor calls.
 struct A {
     A();
 };
 
 A theA;
 
-// WEBASSEMBLY32: define internal void @__cxx_global_var_init() #3 section ".text.__startup" {
+// WEBASSEMBLY32: define internal void @__cxx_global_var_init() #3 {
 // WEBASSEMBLY32: call %struct.A* @_ZN1AC1Ev(%struct.A* @theA)
-// WEBASSEMBLY32: define internal void @_GLOBAL__sub_I_static_init_wasm.cpp() #3 section ".text.__startup" {
+// WEBASSEMBLY32: define internal void @_GLOBAL__sub_I_static_init_wasm.cpp() #3 {
 // WEBASSEMBLY32: call void @__cxx_global_var_init()
 //
-// WEBASSEMBLY64: define internal void @__cxx_global_var_init() #3 section ".text.__startup" {
+// WEBASSEMBLY64: define internal void @__cxx_global_var_init() #3 {
 // WEBASSEMBLY64: call %struct.A* @_ZN1AC1Ev(%struct.A* @theA)
-// WEBASSEMBLY64: define internal void @_GLOBAL__sub_I_static_init_wasm.cpp() #3 section ".text.__startup" {
+// WEBASSEMBLY64: define internal void @_GLOBAL__sub_I_static_init_wasm.cpp() #3 {
 // WEBASSEMBLY64: call void @__cxx_global_var_init()