[runtime] Always use SwiftCC (#13311)

Author: troughton

include/swift/Runtime/Config.h

@@ -33,18 +33,6 @@
 #endif
 #endif
 
-/// Does the current Swift platform use LLVM's intrinsic "swiftcall"
-/// calling convention for Swift functions?
-#ifndef SWIFT_USE_SWIFTCALL
-// Clang doesn't support mangling functions with the swiftcall attribute
-// on Windows and crashes during compilation: http://bugs.llvm.org/show_bug.cgi?id=32000
-#if (__has_attribute(swiftcall) || defined(__linux__)) && !defined(_WIN32)
-#define SWIFT_USE_SWIFTCALL 1
-#else
-#define SWIFT_USE_SWIFTCALL 0
-#endif
-#endif
-
 /// Does the current Swift platform allow information other than the
 /// class pointer to be stored in the isa field?  If so, when deriving
 /// the class pointer of an object, we must apply a
@@ -105,7 +93,11 @@
 #define SWIFT_CC_preserve_all  __attribute__((preserve_all))
 #define SWIFT_CC_c
 
-#if SWIFT_USE_SWIFTCALL
+// Define SWIFT_CC_swift in terms of the Swift CC for runtime functions.
+// Functions outside the stdlib or runtime that include this file may be built 
+// with a compiler that doesn't support swiftcall; don't define these macros
+// in that case so any incorrect usage is caught.
+#if __has_attribute(swiftcall)
 #define SWIFT_CC_swift __attribute__((swiftcall))
 #define SWIFT_CONTEXT __attribute__((swift_context))
 #define SWIFT_ERROR_RESULT __attribute__((swift_error_result))
@@ -117,6 +109,8 @@
 #define SWIFT_INDIRECT_RESULT
 #endif
 
+#define SWIFT_CC_SwiftCC SWIFT_CC_swift
+
 // Map a logical calling convention (e.g. RegisterPreservingCC) to LLVM calling
 // convention.
 #define SWIFT_LLVM_CC(CC) SWIFT_LLVM_CC_##CC
@@ -135,11 +129,7 @@
 
 #define SWIFT_LLVM_CC_RegisterPreservingCC llvm::CallingConv::PreserveMost
 
-#if SWIFT_USE_SWIFTCALL
 #define SWIFT_LLVM_CC_SwiftCC llvm::CallingConv::Swift
-#else
-#define SWIFT_LLVM_CC_SwiftCC llvm::CallingConv::C
-#endif
 
 // If defined, it indicates that runtime function wrappers
 // should be used on all platforms, even they do not support

lib/IRGen/GenCall.cpp

@@ -108,7 +108,7 @@ static void addInoutParameterAttributes(IRGenModule &IGM,
 
 static llvm::CallingConv::ID getFreestandingConvention(IRGenModule &IGM) {
   // TODO: use a custom CC that returns three scalars efficiently
-  return SWIFT_LLVM_CC(SwiftCC);
+  return IGM.SwiftCC;
 }
 
 /// Expand the requirements of the given abstract calling convention
@@ -146,8 +146,6 @@ static void addIndirectResultAttributes(IRGenModule &IGM,
 
 void IRGenModule::addSwiftSelfAttributes(llvm::AttributeList &attrs,
                                          unsigned argIndex) {
-  if (!UseSwiftCC)
-    return;
   llvm::AttrBuilder b;
   b.addAttribute(llvm::Attribute::SwiftSelf);
   attrs = attrs.addAttributes(this->LLVMContext,
@@ -160,7 +158,7 @@ void IRGenModule::addSwiftErrorAttributes(llvm::AttributeList &attrs,
   // We create a shadow stack location of the swifterror parameter for the
   // debugger on such platforms and so we can't mark the parameter with a
   // swifterror attribute.
-  if (!UseSwiftCC || !this->IsSwiftErrorInRegister)
+  if (!this->IsSwiftErrorInRegister)
     return;
 
   llvm::AttrBuilder b;

lib/IRGen/IRGenModule.cpp

@@ -397,8 +397,7 @@ IRGenModule::IRGenModule(IRGenerator &irgen,
   else
     RegisterPreservingCC = DefaultCC;
 
-  SwiftCC = SWIFT_LLVM_CC(SwiftCC);
-  UseSwiftCC = (SwiftCC == llvm::CallingConv::Swift);
+  SwiftCC = llvm::CallingConv::Swift;
 
   if (IRGen.Opts.DebugInfoKind > IRGenDebugInfoKind::None)
     DebugInfo = IRGenDebugInfo::createIRGenDebugInfo(IRGen.Opts, *CI, *this,

lib/IRGen/IRGenModule.h

@@ -509,7 +509,6 @@ class IRGenModule {
   llvm::CallingConv::ID DefaultCC;     /// default calling convention
   llvm::CallingConv::ID RegisterPreservingCC; /// lightweight calling convention
   llvm::CallingConv::ID SwiftCC;     /// swift calling convention
-  bool UseSwiftCC;
 
   Signature getAssociatedTypeMetadataAccessFunctionSignature();
   Signature getAssociatedTypeWitnessTableAccessFunctionSignature();

stdlib/public/runtime/HeapObject.cpp

@@ -46,6 +46,22 @@
 
 using namespace swift;
 
+// Check to make sure the runtime is being built with a compiler that
+// supports the Swift calling convention.
+//
+// If the Swift calling convention is not in use, functions such as 
+// swift_allocBox and swift_makeBoxUnique that rely on their return value 
+// being passed in a register to be compatible with Swift may miscompile on
+// some platforms and silently fail.
+#if !__has_attribute(swiftcall)
+#error "The runtime must be built with a compiler that supports swiftcall."
+#endif
+
+// Check that the user isn't manually disabling SWIFTCALL.
+#if defined(SWIFT_USE_SWIFTCALL) && !SWIFT_USE_SWIFTCALL
+#error "SWIFT_USE_SWIFTCALL=0 is not supported; swiftcall must always be used."
+#endif
+
 /// Returns true if the pointer passed to a native retain or release is valid.
 /// If false, the operation should immediately return.
 static inline bool isValidPointerForNativeRetain(const void *p) {