[IR] Define 'swiftcorocc' calling convention.

The 'swiftcorocc' calling convention is a variant of 'swiftcc', but
additionally allows the 'swiftcorocc' function to have popless returns.

"popless" returns don't fully restore the stack, thereby allowing the
caller to access some stack allocations made in the 'swiftcorocc'
callee.

Calls to these functions don't restore SP (but do restore FP).

So the most important characteristic of a 'swiftcorocc' call is that it
forces the caller function to access its stack through FP, like it does
with e.g., variable-size allocas.

This patch only implements the 'swiftcorocc' keyword and CallingConv,
but doesn't implement its support on any target yet.

Author: ahmedbougacha

llvm/include/llvm/AsmParser/LLToken.h

@@ -161,6 +161,7 @@ enum Kind {
   kw_anyregcc,
   kw_swiftcc,
   kw_swifttailcc,
+  kw_swiftcorocc,
   kw_preserve_mostcc,
   kw_preserve_allcc,
   kw_preserve_nonecc,

llvm/include/llvm/IR/CallingConv.h

@@ -270,6 +270,10 @@ namespace CallingConv {
     /// Preserve X1-X15, X19-X29, SP, Z0-Z31, P0-P15.
     AArch64_SME_ABI_Support_Routines_PreserveMost_From_X1 = 111,
 
+    /// This follows the Swift calling convention in how arguments are passed
+    /// but doesn't clean up the stack on a return.
+    SwiftCoro = 124,
+
     /// The highest possible ID. Must be some 2^k - 1.
     MaxID = 1023
   };

llvm/lib/AsmParser/LLLexer.cpp

@@ -619,6 +619,7 @@ lltok::Kind LLLexer::LexIdentifier() {
   KEYWORD(x86_regcallcc);
   KEYWORD(swiftcc);
   KEYWORD(swifttailcc);
+  KEYWORD(swiftcorocc);
   KEYWORD(anyregcc);
   KEYWORD(preserve_mostcc);
   KEYWORD(preserve_allcc);

llvm/lib/AsmParser/LLParser.cpp

@@ -2187,6 +2187,7 @@ void LLParser::parseOptionalDLLStorageClass(unsigned &Res) {
 ///   ::= 'ghccc'
 ///   ::= 'swiftcc'
 ///   ::= 'swifttailcc'
+///   ::= 'swiftcorocc'
 ///   ::= 'x86_intrcc'
 ///   ::= 'hhvmcc'
 ///   ::= 'hhvm_ccc'
@@ -2252,6 +2253,7 @@ bool LLParser::parseOptionalCallingConv(unsigned &CC) {
   case lltok::kw_ghccc:          CC = CallingConv::GHC; break;
   case lltok::kw_swiftcc:        CC = CallingConv::Swift; break;
   case lltok::kw_swifttailcc:    CC = CallingConv::SwiftTail; break;
+  case lltok::kw_swiftcorocc:    CC = CallingConv::SwiftCoro; break;
   case lltok::kw_x86_intrcc:     CC = CallingConv::X86_INTR; break;
   case lltok::kw_hhvmcc:
     CC = CallingConv::DUMMY_HHVM;

llvm/lib/IR/AsmWriter.cpp

@@ -343,6 +343,7 @@ static void PrintCallingConv(unsigned cc, raw_ostream &Out) {
   case CallingConv::SPIR_KERNEL:   Out << "spir_kernel"; break;
   case CallingConv::Swift:         Out << "swiftcc"; break;
   case CallingConv::SwiftTail:     Out << "swifttailcc"; break;
+  case CallingConv::SwiftCoro:     Out << "swiftcorocc"; break;
   case CallingConv::X86_INTR:      Out << "x86_intrcc"; break;
   case CallingConv::DUMMY_HHVM:
     Out << "hhvmcc";

llvm/test/Bitcode/compatibility.ll

@@ -516,6 +516,10 @@ declare cc96 void @f.cc96()
 ; CHECK: declare amdgpu_es void @f.cc96()
 declare amdgpu_es void @f.amdgpu_es()
 ; CHECK: declare amdgpu_es void @f.amdgpu_es()
++declare cc124 void @f.cc124()
++; CHECK: declare swiftcorocc void @f.cc124()
++declare swiftcorocc void @f.swiftcorocc()
++; CHECK: declare swiftcorocc void @f.swiftcorocc()
 declare cc1023 void @f.cc1023()
 ; CHECK: declare cc1023 void @f.cc1023()