[clang] Allow specifying the aapcs and aapcs-vfp for windows on arm

[compnerd]

Can you please add some more context around the motivation for this change? It seems ... unnecessary as everything is already AAPCS/AAPCS-VFP.

[mstorsjo]

In D94364#2490743, @compnerd wrote:

Can you please add some more context around the motivation for this change? It seems ... unnecessary as everything is already AAPCS/AAPCS-VFP.

Yeah it's a redundant attribute, but I don't think we'd need to warn about it, but could accept it as well. Aapcs-vfp would be a no-op while plain aapcs would pass floats in gprs.

The reason for the patch is from wine; wine uses __attribute__((pcs("aapcs-vfp"))) (within an #ifdef __GNUC__) to get the right hardfloat calling convention when building for linux with a softfloat ABI (like e.g. the android userspace). If using those headers for targeting mingw, it triggers warnings with clang - and I didn't see why one shouldn't accept these attributes even on that target. I could try making wine stop using the redundant attribute when actully building code for a mingw target too, though.

[compnerd]

I think that it makes more sense to ignore the aapcs-vfp calling convention attribute (returning CCCR_Ignore), and warn on the aapcs. The redundant calling convention attribute does nothing, and I think that simply ignoring it would solve your issue. However, changing the calling convention to aapcs would break interoperability, so warning on that seems reasonable still. I think that if Wine is willing to adjust their use of the attribute, that would really be nicer - it allows us to keep the behaviour as close to MSVC as possible.

[rnk]

I don't see how supporting this attribute would break interop with MSVC. MSVC doesn't support these attributes, only GCC and Clang do. Presumably in the wine environment, some headers using these _GNUC extensions are being included somewhere, and IMO we should accept the attributes and honor them.

clang/lib/Basic/Targets/ARM.cpp

 TargetInfo::CallingConvCheckResult
 WindowsARMTargetInfo::checkCallingConvention(CallingConv CC) const {
   switch (CC) {
   case CC_X86StdCall:
   case CC_X86ThisCall:
   case CC_X86FastCall:
   case CC_X86VectorCall:
     return CCCR_Ignore;
+  case CC_AAPCS:
+  case CC_AAPCS_VFP:
   case CC_C:
   case CC_OpenCLKernel:
   case CC_PreserveMost:
   case CC_PreserveAll:
   case CC_Swift:
     return CCCR_OK;
   default:
     return CCCR_Warning;

clang/test/Sema/callingconv-win-arm.c

+// RUN: %clang_cc1 %s -fsyntax-only -triple armv7-windows -verify
+
+// expected-no-diagnostics
+
+void __attribute__((fastcall)) foo(float *a);
+
+void __attribute__((stdcall)) bar(float *a);
+
+void __attribute__((cdecl)) ctest0();
+
+int __attribute__((pcs("aapcs"))) pcs5(void);
+int __attribute__((pcs("aapcs-vfp"))) pcs6(void);