Implement binary emission of RISC-V return instructions.

The return address is now always supplied in %x1, so the return address
predictor will recognize the jalr as a return and not some indirect
branch.

cranelift/filetests/isa/riscv/binary32.cton

@@ -2,10 +2,10 @@
 test binemit
 isa riscv
 
-function RV32I() {
+function RV32I(i32 link [%x1]) -> i32 link [%x1] {
     fn0 = function foo()
 
-ebb0:
+ebb0(v9999: i32):
     [-,%x10]            v1 = iconst.i32 1
     [-,%x21]            v2 = iconst.i32 2
 
@@ -83,7 +83,10 @@ ebb0:
     call fn0()                                  ; bin: Call(fn0) 000000ef
 
     brz v1, ebb3
-    fallthrough ebb1
+    brnz v1, ebb1
+
+    ; jalr %x0, %x1, 0
+    return v9999                        ; bin: 00008067
 
 ebb1:
     ; beq 0x000

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

@@ -110,7 +110,7 @@ Iicmp = EncRecipe(
 # 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', MultiAry, size=4, ins=GPR, outs=())
+Iret = EncRecipe('Iret', MultiAry, size=4, ins=(), outs=())
 
 # U-type instructions have a 20-bit immediate that targets bits 12-31.
 U = EncRecipe(

lib/cretonne/src/isa/riscv/binemit.rs

@@ -172,8 +172,14 @@ fn recipe_iicmp<CS: CodeSink + ?Sized>(func: &Function, inst: Inst, sink: &mut C
     }
 }
 
-fn recipe_iret<CS: CodeSink + ?Sized>(_func: &Function, _inst: Inst, _sink: &mut CS) {
-    unimplemented!()
+fn recipe_iret<CS: CodeSink + ?Sized>(func: &Function, inst: Inst, sink: &mut CS) {
+    // Return instructions are always a jalr to %x1.
+    // The return address is provided as a special-purpose link argument.
+    put_i(func.encodings[inst].bits(),
+          1, // rs1 = %x1
+          0, // no offset.
+          0, // rd = %x0: no address written.
+          sink);
 }
 
 /// U-type instructions.