[Clang][Cygwin] Cygwin x86_64 should accept __stdcall (#158385)

Cygwin should support calling convention attributes `__cdecl`,
`__stdcall`, `__thiscall`, and `__fastcall`, even though they have no
effect in x86_64, as done in MinGW.

Originally reported in
https://cygwin.com/pipermail/cygwin/2025-September/258782.html

---------

Co-authored-by: Jeremy Drake <[email protected]>

Author: kikairoya

clang/lib/Basic/Targets/X86.h

@@ -999,6 +999,29 @@ class LLVM_LIBRARY_VISIBILITY CygwinX86_64TargetInfo : public X86_64TargetInfo {
       Builder.defineMacro("_GNU_SOURCE");
   }
 
+  CallingConvCheckResult checkCallingConvention(CallingConv CC) const override {
+    switch (CC) {
+    case CC_X86StdCall:
+    case CC_X86ThisCall:
+    case CC_X86FastCall:
+      return CCCR_Ignore;
+    case CC_C:
+    case CC_X86VectorCall:
+    case CC_IntelOclBicc:
+    case CC_PreserveMost:
+    case CC_PreserveAll:
+    case CC_PreserveNone:
+    case CC_X86_64SysV:
+    case CC_Swift:
+    case CC_SwiftAsync:
+    case CC_X86RegCall:
+    case CC_DeviceKernel:
+      return CCCR_OK;
+    default:
+      return CCCR_Warning;
+    }
+  }
+
   BuiltinVaListKind getBuiltinVaListKind() const override {
     return TargetInfo::CharPtrBuiltinVaList;
   }

clang/test/CodeGen/X86/cygwin-varargs.c

@@ -1,5 +1,7 @@
 // RUN: %clang_cc1 -triple i686-windows-msvc   -emit-llvm -o - %s | FileCheck %s --check-prefix=X86
 // RUN: %clang_cc1 -triple x86_64-windows-msvc -emit-llvm -o - %s | FileCheck %s --check-prefix=X64
+// RUN: %clang_cc1 -triple x86_64-windows-gnu  -emit-llvm -o - %s | FileCheck %s --check-prefix=X64
+// RUN: %clang_cc1 -triple x86_64-cygwin       -emit-llvm -o - %s | FileCheck %s --check-prefix=X64
 // RUN: %clang_cc1 -triple i686-windows-msvc   -emit-llvm -o - %s -fdefault-calling-conv=vectorcall | FileCheck %s --check-prefix=X86-VEC
 // RUN: %clang_cc1 -triple x86_64-windows-msvc -emit-llvm -o - %s -fdefault-calling-conv=vectorcall | FileCheck %s --check-prefix=X64-VEC
 

clang/test/CodeGen/calling-conv-ignored.c

@@ -1,5 +1,7 @@
 // RUN: %clang_cc1 -triple x86_64-unknown-freebsd10.0 -emit-llvm < %s | FileCheck -check-prefix=FREEBSD %s
 // RUN: %clang_cc1 -triple x86_64-pc-win32 -emit-llvm < %s | FileCheck -check-prefix=WIN64 %s
+// RUN: %clang_cc1 -triple x86_64-mingw    -emit-llvm < %s | FileCheck -check-prefix=WIN64 %s
+// RUN: %clang_cc1 -triple x86_64-cygwin   -emit-llvm < %s | FileCheck -check-prefix=WIN64 %s
 // RUN: %clang_cc1 -triple x86_64-uefi -emit-llvm < %s | FileCheck -check-prefix=WIN64 %s
 
 struct foo {

clang/test/CodeGen/ms_abi.c

@@ -1,7 +1,11 @@
 // RUN: %clang_cc1 -triple x86_64-pc-win32 -emit-llvm  -target-cpu skylake-avx512 < %s | FileCheck %s --check-prefixes=CHECK,AVX
+// RUN: %clang_cc1 -triple x86_64-mingw    -emit-llvm  -target-cpu skylake-avx512 < %s | FileCheck %s --check-prefixes=CHECK,AVX
+// RUN: %clang_cc1 -triple x86_64-cygwin   -emit-llvm  -target-cpu skylake-avx512 < %s | FileCheck %s --check-prefixes=CHECK,AVX
 // RUN: %clang_cc1 -triple x86_64-linux -emit-llvm  -target-cpu skylake-avx512 < %s | FileCheck %s --check-prefixes=CHECK,AVX
 // RUN: %clang_cc1 -triple x86_64-uefi -emit-llvm  -target-cpu skylake-avx512 < %s | FileCheck %s --check-prefixes=CHECK,AVX
 // RUN: %clang_cc1 -triple x86_64-pc-win32 -emit-llvm < %s | FileCheck %s --check-prefixes=CHECK,NOAVX
+// RUN: %clang_cc1 -triple x86_64-mingw    -emit-llvm < %s | FileCheck %s --check-prefixes=CHECK,NOAVX
+// RUN: %clang_cc1 -triple x86_64-cygwin   -emit-llvm < %s | FileCheck %s --check-prefixes=CHECK,NOAVX
 // RUN: %clang_cc1 -triple x86_64-linux -emit-llvm < %s | FileCheck %s --check-prefixes=CHECK,NOAVX
 // RUN: %clang_cc1 -triple x86_64-uefi -emit-llvm < %s | FileCheck %s --check-prefixes=CHECK,NOAVX
 

clang/test/CodeGen/sysv_abi.c

@@ -1,5 +1,7 @@
 // RUN: %clang_cc1 -triple x86_64-pc-linux-gnu -o - -emit-llvm -debug-info-kind=limited %s | FileCheck %s --check-prefix=LINUX
-// RUN: %clang_cc1 -triple x86_64-unknown-windows-msvc -o - -emit-llvm -debug-info-kind=limited %s | FileCheck %s --check-prefix=WINDOWS
+// RUN: %clang_cc1 -triple x86_64-unknown-windows-msvc   -o - -emit-llvm -debug-info-kind=limited %s | FileCheck %s --check-prefix=WINDOWS
+// RUN: %clang_cc1 -triple x86_64-unknown-windows-gnu    -o - -emit-llvm -debug-info-kind=limited %s | FileCheck %s --check-prefix=WINDOWS
+// RUN: %clang_cc1 -triple x86_64-unknown-windows-cygnus -o - -emit-llvm -debug-info-kind=limited %s | FileCheck %s --check-prefix=WINDOWS
 // RUN: %clang_cc1 -triple i386-pc-linux-gnu -o - -emit-llvm -debug-info-kind=limited %s | FileCheck %s --check-prefix=LINUX32
 // RUN: %clang_cc1 -triple armv7--linux-gnueabihf -o - -emit-llvm -debug-info-kind=limited %s | FileCheck %s --check-prefix=ARM
 
@@ -77,7 +79,7 @@ __attribute__((intel_ocl_bicc)) int add_inteloclbicc(int a, int b) {
 }
 #endif
 
-#ifdef _WIN64
+#if defined(_WIN64) || defined(__CYGWIN__)
 // WINDOWS: !DISubprogram({{.*}}"add_sysvabi", {{.*}}type: ![[FTY:[0-9]+]]
 // WINDOWS: ![[FTY]] = !DISubroutineType({{.*}}cc: DW_CC_LLVM_X86_64SysV,
 __attribute__((sysv_abi)) int add_sysvabi(int a, int b) {

clang/test/DebugInfo/Generic/cc.c

@@ -1,4 +1,6 @@
-// RUN: %clang_cc1 -triple x86_64-windows -fms-compatibility -fsyntax-only -verify %s
+// RUN: %clang_cc1 -triple x86_64-windows -fsyntax-only -verify %s
+// RUN: %clang_cc1 -triple x86_64-mingw   -fsyntax-only -verify %s
+// RUN: %clang_cc1 -triple x86_64-cygwin  -fsyntax-only -verify %s
 
 int __cdecl cdecl(int a, int b, int c, int d) { // expected-no-diagnostics
   return a + b + c + d;

clang/test/Parser/x64-windows-calling-convention-handling.c

@@ -1,4 +1,6 @@
-// RUN: %clang_cc1 %s -Wmicrosoft -verify -fms-compatibility -triple x86_64-pc-win32
+// RUN: %clang_cc1 %s -Wmicrosoft -verify -triple x86_64-pc-win32
+// RUN: %clang_cc1 %s -Wmicrosoft -verify -triple x86_64-w64-mingw32
+// RUN: %clang_cc1 %s -Wmicrosoft -verify -triple x86_64-pc-cygwin
 
 // None of these should warn. stdcall is treated as equivalent to cdecl on
 // x64.