Generate SSA code from returns (#5172)

Modify return pseudo-instructions to have pairs of registers: virtual and real. This allows us to constrain the virtual registers to the real ones specified by the abi, instead of directly emitting moves to those real registers.

cranelift/codegen/src/prelude_lower.isle

@@ -158,6 +158,7 @@
 (type BoxExternalName (primitive BoxExternalName))
 (type RelocDistance (primitive RelocDistance))
 (type VecArgPair extern (enum))
+(type VecRetPair extern (enum))
 
 ;;;; Helper Clif Extractors ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
 
@@ -614,10 +615,6 @@
 
 ;;;; Helpers for generating returns ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
 
-;; The (writable) register(s) that will contain the n'th return value.
-(decl retval (usize) WritableValueRegs)
-(extern constructor retval retval)
-
 ;; Extractor to check for the special case that a `WritableValueRegs`
 ;; contains only a single register.
 (decl only_writable_reg (WritableReg) WritableValueRegs)
@@ -725,31 +722,14 @@
 (decl gen_move (Type WritableReg Reg) MInst)
 (extern constructor gen_move gen_move)
 
-;; Copy a return value to a set of registers.
-(decl copy_to_regs (WritableValueRegs Value) Unit)
-(rule (copy_to_regs dsts val @ (value_type ty))
-      (let ((srcs ValueRegs (put_in_regs val)))
-        (copy_to_regs_range ty (value_regs_range srcs) dsts srcs)))
-
-;; Helper for `copy_to_regs` that uses a range to index into the reg/value
-;; vectors. Fails for the empty range.
-(decl copy_to_regs_range (Type Range WritableValueRegs ValueRegs) Unit)
-
-(rule (copy_to_regs_range ty (range_empty) dsts srcs)
-      (unit))
-
-(rule (copy_to_regs_range ty (range_unwrap head tail) dsts srcs)
-      (let ((dst WritableReg (writable_regs_get dsts head))
-            (src Reg (value_regs_get srcs head))
-            (_ Unit (emit (gen_move ty dst src))))
-        (copy_to_regs_range ty tail dsts srcs)))
-
-
+;; Generate a return instruction
 (decl lower_return (Range ValueSlice) InstOutput)
-(rule (lower_return (range_empty) _) (output_none))
-(rule (lower_return (range_unwrap head tail) args)
-      (let ((_ Unit (copy_to_regs (retval head) (value_slice_get args head))))
-        (lower_return tail args)))
+(rule (lower_return _ vals)
+      (let ((_ Unit (gen_return vals)))
+        (output_none)))
+
+(decl gen_return (ValueSlice) Unit)
+(extern constructor gen_return gen_return)
 
 ;;;; Automatic conversions ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;