Do not apply calling conventions to MSVC entry points

[eandrews]

Summary

Fix link error for MSVC entry points when calling conventions are specified. MSVC entry points should have default calling convention.

[rnk]

lgtm, thanks.

[eandrews]

In D87701#2274860, @rnk wrote:

lgtm, thanks.

Thanks for taking a look!

[akhuang]

This is causing a link error in the windows chromium build:

lld-link: error: undefined symbol: _WinMain@16

referenced by D:\agent\_work\4\s\src\vctools\crt\vcstartup\src\startup\exe_common.inl:102

msvcrtd.lib(exe_winmain.obj):(int __cdecl invoke_main(void))

[akhuang]

whoops, sorry for weird formatting in the previous comment.

I'll revert for now.

[dmajor]

This broke Firefox builds too, in one of our helper binaries that uses a wWinMain, although I'm having trouble writing a minimal reproducer for it. Simply making a barebones wWinMain program doesn't hit the error.

If the patch re-lands, please cc me and I'll re-test.

[eandrews]

In D87701#2280246, @dmajor wrote:

This broke Firefox builds too, in one of our helper binaries that uses a wWinMain, although I'm having trouble writing a minimal reproducer for it. Simply making a barebones wWinMain program doesn't hit the error.

If the patch re-lands, please cc me and I'll re-test.

Will do. I think MSVC applies calling conventions to entry points when it is specified in function signature.

int WinMain(int argc) {return 1;}

Compiling this with /Gr generates symbol _WinMain@4 , meaning /Gr was ignored and fastcall was not applied to WinMain. But,

int __fastcall WinMain(int argc) {return 1;}

generates symbol @WinMain@4, meaning __fastcall calling convention was applied to WinMain.

It also looks like stdcall is default calling convention for WinMain.

Anyway, I'll upload a new patch for review once I understand the required behavior better.

clang/lib/Sema/SemaDecl.cpp

void Sema::CheckMSVCRTEntryPoint(FunctionDecl *FD) {
   // enumeration, pointer or nullptr type.
   if (FT->getReturnType()->isIntegralOrEnumerationType() ||
       FT->getReturnType()->isAnyPointerType() ||
       FT->getReturnType()->isNullPtrType())
     // DllMain is exempt because a return value of zero means it failed.
     if (FD->getName() != "DllMain")
       FD->setHasImplicitReturnZero(true);
 
+  if (FT->getCallConv() != CC_C) {
+    FT = Context.adjustFunctionType(FT, FT->getExtInfo().withCallingConv(CC_C));
+    FD->setType(QualType(FT, 0));
+  }
+
   if (!FD->isInvalidDecl() && FD->getDescribedFunctionTemplate()) {
     Diag(FD->getLocation(), diag::err_mainlike_template_decl) << FD;
     FD->setInvalidDecl();
   }
 }
 
 bool Sema::CheckForConstantInitializer(Expr *Init, QualType DclT) {
   // FIXME: Need strict checking.  In C89, we need to check for

clang/test/CodeGenCXX/default_calling_conv.cpp

-// RUN: %clang_cc1 -triple i386-unknown-linux-gnu -fdefault-calling-conv=cdecl -emit-llvm -o - %s | FileCheck %s --check-prefix=CDECL --check-prefix=ALL
-// RUN: %clang_cc1 -triple i786-unknown-linux-gnu -target-feature +sse4.2 -fdefault-calling-conv=fastcall -emit-llvm -o - %s | FileCheck %s --check-prefix=FASTCALL --check-prefix=ALL
-// RUN: %clang_cc1 -triple i486-unknown-linux-gnu -fdefault-calling-conv=stdcall -emit-llvm -o - %s | FileCheck %s --check-prefix=STDCALL --check-prefix=ALL
-// RUN: %clang_cc1 -triple i486-unknown-linux-gnu -mrtd -emit-llvm -o - %s | FileCheck %s --check-prefix=STDCALL --check-prefix=ALL
-// RUN: %clang_cc1 -triple i986-unknown-linux-gnu -fdefault-calling-conv=vectorcall -emit-llvm -o - %s | FileCheck %s --check-prefix=VECTORCALL --check-prefix=ALL
-// RUN: %clang_cc1 -triple i986-unknown-linux-gnu -fdefault-calling-conv=regcall -emit-llvm -o - %s | FileCheck %s --check-prefix=REGCALL --check-prefix=ALL
-
+// RUN: %clang_cc1 -triple i386-unknown-linux-gnu -fdefault-calling-conv=cdecl -emit-llvm -o - %s -DMAIN | FileCheck %s --check-prefix=CDECL --check-prefix=ALL
+// RUN: %clang_cc1 -triple i786-unknown-linux-gnu -target-feature +sse4.2 -fdefault-calling-conv=fastcall -emit-llvm -o - %s -DMAIN | FileCheck %s --check-prefix=FASTCALL --check-prefix=ALL
+// RUN: %clang_cc1 -triple i486-unknown-linux-gnu -fdefault-calling-conv=stdcall -emit-llvm -o - %s -DMAIN | FileCheck %s --check-prefix=STDCALL --check-prefix=ALL
+// RUN: %clang_cc1 -triple i486-unknown-linux-gnu -mrtd -emit-llvm -o - %s -DMAIN | FileCheck %s --check-prefix=STDCALL --check-prefix=ALL
+// RUN: %clang_cc1 -triple i986-unknown-linux-gnu -fdefault-calling-conv=vectorcall -emit-llvm -o - %s -DMAIN | FileCheck %s --check-prefix=VECTORCALL --check-prefix=ALL
+// RUN: %clang_cc1 -triple i986-unknown-linux-gnu -fdefault-calling-conv=regcall -emit-llvm -o - %s -DMAIN | FileCheck %s --check-prefix=REGCALL --check-prefix=ALL
+// RUN: %clang_cc1 -triple i386-pc-win32  -target-feature +sse4.2 -fdefault-calling-conv=fastcall -emit-llvm -o - %s -DWMAIN | FileCheck %s  --check-prefix=WMAIN
+// RUN: %clang_cc1 -triple i386-pc-win32  -target-feature +sse4.2 -fdefault-calling-conv=fastcall -emit-llvm -o - %s -DWINMAIN | FileCheck %s  --check-prefix=WINMAIN
+// RUN: %clang_cc1 -triple i386-pc-win32  -target-feature +sse4.2 -fdefault-calling-conv=fastcall -emit-llvm -o - %s -DWWINMAIN | FileCheck %s  --check-prefix=WWINMAIN
+// RUN: %clang_cc1 -triple i386-pc-win32  -target-feature +sse4.2 -fdefault-calling-conv=fastcall -emit-llvm -o - %s -DDLLMAIN | FileCheck %s  --check-prefix=DLLMAIN
+//
 // CDECL: define void @_Z5test1v
 // FASTCALL: define x86_fastcallcc void @_Z5test1v
 // STDCALL: define x86_stdcallcc void @_Z5test1v
 // VECTORCALL: define x86_vectorcallcc void @_Z5test1v
 // REGCALL: define x86_regcallcc void @_Z17__regcall3__test1v
 void test1() {}
 
 // fastcall, stdcall, vectorcall and regcall do not support variadic functions.
public:
   void test_member() {}
 };
 
 void test() {
   A a;
   a.test_member();
 }
 
+#ifdef MAIN
 // ALL: define i32 @main
 int main() {
   return 1;
 }
+#endif // main
+
+#ifdef WMAIN
+// WMAIN: define dso_local i32 @wmain
+int wmain() {
+  return 1;
+}
+#endif // wmain
+
+#ifdef WINMAIN
+// WINMAIN: define dso_local i32 @WinMain
+int WinMain() {
+  return 1;
+}
+#endif // WinMain
+
+#ifdef WWINMAIN
+// WWINMAIN: define dso_local i32 @wWinMain
+int wWinMain() {
+  return 1;
+}
+#endif // wWinMain
+
+#ifdef DLLMAIN
+// DLLMAIN: define dso_local i32 @DllMain
+int DllMain() {
+  return 1;
+}
+#endif // DllMain