X86: swifttailcc pass swiftself in ECX

rdar://76230881

Author: aschwaighofer

llvm/lib/Target/X86/X86CallingConv.td

@@ -862,6 +862,10 @@ def CC_X86_32_C : CallingConv<[
   // The 'nest' parameter, if any, is passed in ECX.
   CCIfNest<CCAssignToReg<[ECX]>>,
 
+  // On swifttailcc pass swiftself in ECX.
+  CCIfCC<"CallingConv::SwiftTail",
+         CCIfSwiftSelf<CCIfType<[i32], CCAssignToReg<[ECX]>>>>,
+
   // The first 3 integer arguments, if marked 'inreg' and if the call is not
   // a vararg call, are passed in integer registers.
   CCIfNotVarArg<CCIfInReg<CCIfType<[i32], CCAssignToReg<[EAX, EDX, ECX]>>>>,

llvm/test/CodeGen/X86/swifttail-async-i386.ll

@@ -10,3 +10,13 @@ define swifttailcc void @swifttail() {
   tail call swifttailcc void @swifttail_callee()
   ret void
 }
+
+declare swifttailcc void @swiftself(i8* swiftself)
+
+define swifttailcc void @swifttail2(i8* %arg) {
+; CHECK-LABEL: swifttail2:
+; CHECK: movl {{.*}}, %ecx
+; CHECK: jmp _swiftself
+  tail call swifttailcc void @swiftself(i8* swiftself %arg)
+  ret void
+}