New calling convention preserve_none (#76868)

The new experimental calling convention preserve_none is the opposite
side of existing preserve_all. It tries to preserve as few general
registers as possible. So all general registers are caller saved
registers. It can also uses more general registers to pass arguments.
This attribute doesn't impact floating-point registers. Floating-point
registers still follow the c calling convention.

Currently preserve_none is supported on X86-64 only. It changes the c
calling convention in following fields:
  
* RSP and RBP are the only preserved general registers, all other
general registers are caller saved registers.
* We can use [RDI, RSI, RDX, RCX, R8, R9, R11, R12, R13, R14, R15, RAX]
to pass arguments.

It can improve the performance of hot tailcall chain, because many
callee saved registers' save/restore instructions can be removed if the
tail functions are using preserve_none. In my experiment in protocol
buffer, the parsing functions are improved by 3% to 10%.

Author: weiguozhi

clang/include/clang-c/Index.h

@@ -2986,6 +2986,7 @@ enum CXCallingConv {
   CXCallingConv_SwiftAsync = 17,
   CXCallingConv_AArch64SVEPCS = 18,
   CXCallingConv_M68kRTD = 19,
+  CXCallingConv_PreserveNone = 20,
 
   CXCallingConv_Invalid = 100,
   CXCallingConv_Unexposed = 200

clang/include/clang/Basic/Attr.td

@@ -2959,6 +2959,12 @@ def M68kRTD: DeclOrTypeAttr {
   let Documentation = [M68kRTDDocs];
 }
 
+def PreserveNone : DeclOrTypeAttr, TargetSpecificAttr<TargetAnyX86> {
+  let Spellings = [Clang<"preserve_none">];
+  let Subjects = SubjectList<[FunctionLike]>;
+  let Documentation = [PreserveNoneDocs];
+}
+
 def Target : InheritableAttr {
   let Spellings = [GCC<"target">];
   let Args = [StringArgument<"featuresStr">];

clang/include/clang/Basic/AttrDocs.td

@@ -5506,6 +5506,23 @@ experimental at this time.
   }];
 }
 
+def PreserveNoneDocs : Documentation {
+  let Category = DocCatCallingConvs;
+  let Content = [{
+On X86-64 target, this attribute changes the calling convention of a function.
+The ``preserve_none`` calling convention tries to preserve as few general
+registers as possible. So all general registers are caller saved registers. It
+also uses more general registers to pass arguments. This attribute doesn't
+impact floating-point registers (XMMs/YMMs). Floating-point registers still
+follow the c calling convention.
+
+- Only RSP and RBP are preserved by callee.
+
+- Register RDI, RSI, RDX, RCX, R8, R9, R11, R12, R13, R14, R15 and RAX now can
+  be used to pass function arguments.
+  }];
+}
+
 def DeprecatedDocs : Documentation {
   let Category = DocCatDecl;
   let Content = [{

clang/include/clang/Basic/Specifiers.h

@@ -295,6 +295,7 @@ namespace clang {
     CC_AArch64SVEPCS, // __attribute__((aarch64_sve_pcs))
     CC_AMDGPUKernelCall, // __attribute__((amdgpu_kernel))
     CC_M68kRTD,       // __attribute__((m68k_rtd))
+    CC_PreserveNone,  // __attribute__((preserve_none))
   };
 
   /// Checks whether the given calling convention supports variadic

clang/lib/AST/ItaniumMangle.cpp

@@ -3443,6 +3443,7 @@ StringRef CXXNameMangler::getCallingConvQualifierName(CallingConv CC) {
   case CC_PreserveMost:
   case CC_PreserveAll:
   case CC_M68kRTD:
+  case CC_PreserveNone:
     // FIXME: we should be mangling all of the above.
     return "";
 

clang/lib/AST/Type.cpp

@@ -3438,6 +3438,7 @@ StringRef FunctionType::getNameForCallConv(CallingConv CC) {
   case CC_PreserveMost: return "preserve_most";
   case CC_PreserveAll: return "preserve_all";
   case CC_M68kRTD: return "m68k_rtd";
+  case CC_PreserveNone: return "preserve_none";
   }
 
   llvm_unreachable("Invalid calling convention.");
@@ -3990,6 +3991,7 @@ bool AttributedType::isCallingConv() const {
   case attr::PreserveMost:
   case attr::PreserveAll:
   case attr::M68kRTD:
+  case attr::PreserveNone:
     return true;
   }
   llvm_unreachable("invalid attr kind");

clang/lib/AST/TypePrinter.cpp

@@ -1067,6 +1067,9 @@ void TypePrinter::printFunctionAfter(const FunctionType::ExtInfo &Info,
     case CC_M68kRTD:
       OS << " __attribute__((m68k_rtd))";
       break;
+    case CC_PreserveNone:
+      OS << " __attribute__((preserve_none))";
+      break;
     }
   }
 
@@ -1911,6 +1914,9 @@ void TypePrinter::printAttributedAfter(const AttributedType *T,
   case attr::M68kRTD:
     OS << "m68k_rtd";
     break;
+  case attr::PreserveNone:
+    OS << "preserve_none";
+    break;
   case attr::NoDeref:
     OS << "noderef";
     break;

clang/lib/Basic/Targets/X86.h

@@ -777,6 +777,7 @@ class LLVM_LIBRARY_VISIBILITY X86_64TargetInfo : public X86TargetInfo {
     case CC_Win64:
     case CC_PreserveMost:
     case CC_PreserveAll:
+    case CC_PreserveNone:
     case CC_X86RegCall:
     case CC_OpenCLKernel:
       return CCCR_OK;
@@ -854,6 +855,7 @@ class LLVM_LIBRARY_VISIBILITY WindowsX86_64TargetInfo
     case CC_IntelOclBicc:
     case CC_PreserveMost:
     case CC_PreserveAll:
+    case CC_PreserveNone:
     case CC_X86_64SysV:
     case CC_Swift:
     case CC_SwiftAsync:

clang/lib/CodeGen/CGCall.cpp

@@ -73,6 +73,7 @@ unsigned CodeGenTypes::ClangCallConvToLLVMCallConv(CallingConv CC) {
   case CC_Swift: return llvm::CallingConv::Swift;
   case CC_SwiftAsync: return llvm::CallingConv::SwiftTail;
   case CC_M68kRTD: return llvm::CallingConv::M68k_RTD;
+  case CC_PreserveNone: return llvm::CallingConv::PreserveNone;
   }
 }
 
@@ -256,6 +257,9 @@ static CallingConv getCallingConventionForDecl(const ObjCMethodDecl *D,
   if (D->hasAttr<M68kRTDAttr>())
     return CC_M68kRTD;
 
+  if (D->hasAttr<PreserveNoneAttr>())
+    return CC_PreserveNone;
+
   return CC_C;
 }
 

clang/lib/CodeGen/CGDebugInfo.cpp

@@ -1450,6 +1450,8 @@ static unsigned getDwarfCC(CallingConv CC) {
     return llvm::dwarf::DW_CC_LLVM_X86RegCall;
   case CC_M68kRTD:
     return llvm::dwarf::DW_CC_LLVM_M68kRTD;
+  case CC_PreserveNone:
+    return llvm::dwarf::DW_CC_LLVM_PreserveNone;
   }
   return 0;
 }