[WebAssembly] Make clang emit correct va_arg code for structs

Summary:
In the WebAssembly backend, when lowering variadic function calls, non-single
member aggregate type arguments are always passed by pointer.

However, when emitting va_arg code in clang, the arguments are instead read as
if they are passed directly. This results in the pointer being read as the
actual structure.

Fixes https://github.com/emscripten-core/emscripten/issues/9042.

Reviewers: tlively, sbc100, kripken, aheejin, dschuff

Reviewed By: dschuff

Subscribers: dschuff, jgravelle-google, sunfish, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D66168

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@368750 91177308-0d34-0410-b5e6-96231b3b80d8

lib/CodeGen/TargetInfo.cpp

@@ -833,10 +833,12 @@ ABIArgInfo WebAssemblyABIInfo::classifyReturnType(QualType RetTy) const {
 
 Address WebAssemblyABIInfo::EmitVAArg(CodeGenFunction &CGF, Address VAListAddr,
                                       QualType Ty) const {
-  return emitVoidPtrVAArg(CGF, VAListAddr, Ty, /*IsIndirect=*/ false,
+  bool IsIndirect =
+      isAggregateTypeForABI(Ty) && !isSingleElementStruct(Ty, getContext());
+  return emitVoidPtrVAArg(CGF, VAListAddr, Ty, IsIndirect,
                           getContext().getTypeInfoInChars(Ty),
                           CharUnits::fromQuantity(4),
-                          /*AllowHigherAlign=*/ true);
+                          /*AllowHigherAlign=*/true);
 }
 
 //===----------------------------------------------------------------------===//

test/CodeGen/wasm-varargs.c

@@ -87,12 +87,13 @@ struct S test_struct(char *fmt, ...) {
 // CHECK:   [[VA1:%[^,=]+]] = bitcast i8** [[VA]] to i8*
 // CHECK:   call void @llvm.va_start(i8* [[VA1]])
 // CHECK:   [[ARGP_CUR:%[^,=]+]] = load i8*, i8** [[VA]], align 4
-// CHECK:   [[ARGP_NEXT:%[^,=]+]] = getelementptr inbounds i8, i8* [[ARGP_CUR]], i32 12
+// CHECK:   [[ARGP_NEXT:%[^,=]+]] = getelementptr inbounds i8, i8* [[ARGP_CUR]], i32 4
 // CHECK:   store i8* [[ARGP_NEXT]], i8** [[VA]], align 4
-// CHECK:   [[R3:%[^,=]+]] = bitcast i8* [[ARGP_CUR]] to [[STRUCT_S]]*
-// CHECK:   [[R4:%[^,=]+]] = bitcast [[STRUCT_S]]* [[AGG_RESULT]] to i8*
-// CHECK:   [[R5:%[^,=]+]] = bitcast [[STRUCT_S]]* [[R3]] to i8*
-// CHECK:   call void @llvm.memcpy.p0i8.p0i8.i32(i8* align 4 [[R4]], i8* align 4 [[R5]], i32 12, i1 false)
+// CHECK:   [[R3:%[^,=]+]] = bitcast i8* [[ARGP_CUR]] to [[STRUCT_S]]**
+// CHECK:   [[R4:%[^,=]+]] = load [[STRUCT_S]]*, [[STRUCT_S]]** %0, align 4
+// CHECK:   [[R5:%[^,=]+]] = bitcast [[STRUCT_S]]* [[AGG_RESULT]] to i8*
+// CHECK:   [[R6:%[^,=]+]] = bitcast [[STRUCT_S]]* [[R4]] to i8*
+// CHECK:   call void @llvm.memcpy.p0i8.p0i8.i32(i8* align 4 [[R5]], i8* align 4 [[R6]], i32 12, i1 false)
 // CHECK:   [[VA2:%[^,=]+]] = bitcast i8** [[VA]] to i8*
 // CHECK:   call void @llvm.va_end(i8* [[VA2]])
 // CHECK:   ret void