Add return_reg encodings for RISC-V.

filetests/isa/riscv/encoding.cton

@@ -15,5 +15,7 @@ ebb0(v1: i32, v2: i32):
     ; check: [R#10c]
     ; sameln: $v12 = imul
 
-    return
+    return_reg v1
+    ; check: [Iret#19]
+    ; sameln: return_reg
 }

lib/cretonne/meta/isa/riscv/encodings.py

@@ -4,7 +4,7 @@ RISC-V Encodings.
 from __future__ import absolute_import
 from base import instructions as base
 from .defs import RV32, RV64
-from .recipes import OPIMM, OPIMM32, OP, OP32, R, Rshamt, I
+from .recipes import OPIMM, OPIMM32, OP, OP32, JALR, R, Rshamt, I, Iret
 from .settings import use_m
 
 # Basic arithmetic binary instructions are encoded in an R-type instruction.
@@ -52,3 +52,12 @@ for inst,           inst_imm,      f3,    f7 in [
 RV32.enc(base.imul.i32, R, OP(0b000, 0b0000001), isap=use_m)
 RV64.enc(base.imul.i64, R, OP(0b000, 0b0000001), isap=use_m)
 RV64.enc(base.imul.i32, R, OP32(0b000, 0b0000001), isap=use_m)
+
+# Control flow.
+
+# Returns are a special case of JALR.
+# Note: Return stack predictors will only recognize this as a return when the
+# return address is provided in `x1`. We may want a special encoding to enforce
+# that.
+RV32.enc(base.return_reg.i32, Iret, JALR())
+RV64.enc(base.return_reg.i64, Iret, JALR())

lib/cretonne/meta/isa/riscv/recipes.py

@@ -11,7 +11,7 @@ instruction formats described in the reference:
 from __future__ import absolute_import
 from cdsl.isa import EncRecipe
 from cdsl.predicates import IsSignedInt
-from base.formats import Binary, BinaryImm
+from base.formats import Binary, BinaryImm, ReturnReg
 from .registers import GPR
 
 # The low 7 bits of a RISC-V instruction is the base opcode. All 32-bit
@@ -40,6 +40,12 @@ def BRANCH(funct3):
     return 0b11000 | (funct3 << 5)
 
 
+def JALR(funct3=0):
+    # type: (int) -> int
+    assert funct3 <= 0b111
+    return 0b11001 | (funct3 << 5)
+
+
 def OPIMM(funct3, funct7=0):
     # type: (int, int) -> int
     assert funct3 <= 0b111
@@ -76,3 +82,8 @@ Rshamt = EncRecipe('Rshamt', BinaryImm, ins=GPR, outs=GPR)
 I = EncRecipe(
         'I', BinaryImm, ins=GPR, outs=GPR,
         instp=IsSignedInt(BinaryImm.imm, 12))
+
+# I-type encoding for `jalr` as a return instruction. We won't use the
+# immediate offset.
+# The variable return values are not encoded.
+Iret = EncRecipe('Iret', ReturnReg, ins=GPR, outs=())

lib/reader/src/parser.rs

@@ -1327,9 +1327,12 @@ impl<'a> Parser<'a> {
                 }
             }
             InstructionFormat::ReturnReg => {
-                let raddr = try!(self.match_value("expected SSA value return addr operand"));
-                try!(self.match_token(Token::Comma, "expected ',' between operands"));
-                let args = try!(self.parse_value_list());
+                let raddr = try!(self.match_value("expected SSA value return address operand"));
+                let args = if self.optional(Token::Comma) {
+                    try!(self.parse_value_list())
+                } else {
+                    VariableArgs::new()
+                };
                 InstructionData::ReturnReg {
                     opcode: opcode,
                     ty: VOID,