[WinX86_64 ABI] Handle C99 _Complex as a struct

[mkuper]

Summary

MSVC does not support C99 _Complex, and apparently has no plans to support it ( https://connect.microsoft.com/VisualStudio/feedback/details/891891/c99-complex-number-support )

ICC, however, does support _Complex on windows x86_64, and treats it, for purposes of parameter passing, as equivalent to a struct containing two fields (for the real and imaginary part). This makes clang handle it the same way.

[majnemer]

How are we doing to mangle _Complex? How does _Complex interact with vectorcall?

[mkuper]

Regarding mangling, to the best of my understanding, ICC doesn't support _Complex in C++ mode, and since MSVC doesn't support it at all (only C++ std::complex), this issue never arises.
Regarding vectorcall - I'll have to double-check, but I believe the same holds as for the default x64 calling convention, that is, _Complex is treated equivalently to structs.

[majnemer]

Please add a testcase where a _Complex is returned.

[majnemer]

LGTM, please add a test case for returning _Complex sooner than later though ;)

lib/CodeGen/TargetInfo.cpp

ABIArgInfo WinX86_64ABIInfo::classify(QualType Ty, unsigned &FreeSSERegs,
   if (Ty->isMemberPointerType()) {
     // If the member pointer is represented by an LLVM int or ptr, pass it
     // directly.
     llvm::Type *LLTy = CGT.ConvertType(Ty);
     if (LLTy->isPointerTy() || LLTy->isIntegerTy())
       return ABIArgInfo::getDirect();
   }
 
-  if (RT || Ty->isMemberPointerType()) {
+  if (RT || Ty->isAnyComplexType() || Ty->isMemberPointerType()) {
     // MS x64 ABI requirement: "Any argument that doesn't fit in 8 bytes, or is
     // not 1, 2, 4, or 8 bytes, must be passed by reference."
     if (Width > 64 || !llvm::isPowerOf2_64(Width))
       return ABIArgInfo::getIndirect(0, /*ByVal=*/false);
 
     // Otherwise, coerce it to a small integer.
     return ABIArgInfo::getDirect(llvm::IntegerType::get(getVMContext(), Width));
   }

test/CodeGen/x86_64-arguments-win32.c

 // RUN: %clang_cc1 -w -triple x86_64-pc-win32 -emit-llvm -o - %s | FileCheck %s
 
 // To be ABI compatible with code generated by MSVC, there shouldn't be any
 // sign/zero extensions on types smaller than 64bit.
 
 // CHECK-LABEL: define void @f1(i8 %a)
 void f1(char a) {}
 
 // CHECK-LABEL: define void @f2(i8 %a)
 void f2(unsigned char a) {}
 
 // CHECK-LABEL: define void @f3(i16 %a)
 void f3(short a) {}
 
 // CHECK-LABEL: define void @f4(i16 %a)
 void f4(unsigned short a) {}
+
+// CHECK-LABEL: define void @f5(i64 %a.coerce)
+void f5(_Complex float a) {}
+
+// CHECK-LABEL: define void @f6({ double, double }* %a)
+void f6(_Complex double a) {}