[RISCV] Implement MC layer support for the tail pseudoinstruction

Summary:
This patch implements MC support for tail psuedo instruction.
A follow-up patch implements the codegen support as well as handling of the indirect tail pseudo instruction.

Reviewers: asb, apazos

Reviewed By: asb

Subscribers: rbar, johnrusso, simoncook, jordy.potman.lists, sabuasal, niosHD, kito-cheng, shiva0217, zzheng, edward-jones, llvm-commits

Differential Revision: https://reviews.llvm.org/D46221

llvm-svn: 332634

3 files changed, 14 insertions, 3 deletions

diff --git a/llvm/lib/Target/RISCV/AsmParser/RISCVAsmParser.cpp b/llvm/lib/Target/RISCV/AsmParser/RISCVAsmParser.cpp
index c51e4b3c606..b0f82623dd0 100644
--- a/llvm/lib/Target/RISCV/AsmParser/RISCVAsmParser.cpp
+++ b/llvm/lib/Target/RISCV/AsmParser/RISCVAsmParser.cpp
@@ -943,7 +943,8 @@ bool RISCVAsmParser::ParseInstruction(ParseInstructionInfo &Info,
     return false;
 
   // Parse first operand
-  if (parseOperand(Operands, Name == "call"))
+  bool ForceImmediate = (Name == "call" || Name == "tail");
+  if (parseOperand(Operands, ForceImmediate))
     return true;
 
   // Parse until end of statement, consuming commas between operands
diff --git a/llvm/lib/Target/RISCV/MCTargetDesc/RISCVMCCodeEmitter.cpp b/llvm/lib/Target/RISCV/MCTargetDesc/RISCVMCCodeEmitter.cpp
index 58275faa163..b10cf79dc67 100644
--- a/llvm/lib/Target/RISCV/MCTargetDesc/RISCVMCCodeEmitter.cpp
+++ b/llvm/lib/Target/RISCV/MCTargetDesc/RISCVMCCodeEmitter.cpp
@@ -97,7 +97,7 @@ void RISCVMCCodeEmitter::expandFunctionCall(const MCInst &MI, raw_ostream &OS,
                                             const MCSubtargetInfo &STI) const {
   MCInst TmpInst;
   MCOperand Func = MI.getOperand(0);
-  unsigned Ra = RISCV::X1;
+  unsigned Ra = (MI.getOpcode() == RISCV::PseudoTAIL) ? RISCV::X6 : RISCV::X1;
   uint32_t Binary;
 
   assert(Func.isExpr() && "Expected expression");
@@ -128,7 +128,8 @@ void RISCVMCCodeEmitter::encodeInstruction(const MCInst &MI, raw_ostream &OS,
   // Get byte count of instruction.
   unsigned Size = Desc.getSize();
 
-  if (MI.getOpcode() == RISCV::PseudoCALL) {
+  if (MI.getOpcode() == RISCV::PseudoCALL ||
+      MI.getOpcode() == RISCV::PseudoTAIL) {
     expandFunctionCall(MI, OS, Fixups, STI);
     MCNumEmitted += 2;
     return;
diff --git a/llvm/lib/Target/RISCV/RISCVInstrInfo.td b/llvm/lib/Target/RISCV/RISCVInstrInfo.td
index 81dcc799171..6781a62f4f4 100644
--- a/llvm/lib/Target/RISCV/RISCVInstrInfo.td
+++ b/llvm/lib/Target/RISCV/RISCVInstrInfo.td
@@ -661,6 +661,15 @@ let isBarrier = 1, isReturn = 1, isTerminator = 1 in
 def PseudoRET : Pseudo<(outs), (ins), [(RetFlag)]>,
                 PseudoInstExpansion<(JALR X0, X1, 0)>;
 
+// PseudoTAIL is a pseudo instruction similar to PseudoCALL and will eventually
+// expand to auipc and jalr while encoding.
+// Define AsmString to print "tail" when compile with -S flag.
+let isCall = 1, isTerminator = 1, isReturn = 1, isBarrier = 1, Uses = [X2],
+    hasSideEffects = 0, mayLoad = 0, mayStore = 0, isCodeGenOnly = 0 in
+def PseudoTAIL : Pseudo<(outs), (ins bare_symbol:$dst), []> {
+  let AsmString = "tail\t$dst";
+}
+
 /// Loads
 
 multiclass LdPat<PatFrag LoadOp, RVInst Inst> {