[Sbcl-devel] Problem in handling of multiple values (1.0.9.32, but also on 1.0.9.5)

[Paul K. <pk...@gm...>]

If I compile this reduced test case:
(defun test-case (x)
  (labels ((inner ()
             (values 1 0)))
    (if x
        (inner)
        (multiple-value-bind (a b)
            (inner)
          (values b a)))))

(test-case t)    => 1, 0
(test-case nil) => 0, 0

A disassembly of the function (at (speed 3) and (safety 0) to make it
more readable for me) reveals this:

; 031412BC:       4881FA17001020   CMP RDX, 537919511         ;
no-arg-parsing entry point
;      2C3:       752F             JNE L0
;      2C5:       488BC5           MOV RAX, RBP
;      2C8:       488BCC           MOV RCX, RSP
;      2CB:       4883EC40         SUB RSP, 64
;      2CF:       488941F8         MOV [RCX-8], RAX
;      2D3:       488BE9           MOV RBP, RCX
;      2D6:       488D0506000000   LEA RAX, [RIP+6]
;      2DD:       488945F0         MOV [RBP-16], RAX
;      2E1:       EB11             JMP L0
;      2E3:       488BCB           MOV RCX, RBX
;      2E6:       488BD9           MOV RBX, RCX
;      2E9:       488D65F0         LEA RSP, [RBP-16]
;      2ED:       488B6DF8         MOV RBP, [RBP-8]
;      2F1:       C20800           RET 8
;      2F4: L0:   B908000000       MOV ECX, 8                 ;
no-arg-parsing entry point
;      2F9:       31DB             XOR EBX, EBX
;      2FB:       488D65F0         LEA RSP, [RBP-16]
;      2FF:       488B6DF8         MOV RBP, [RBP-8]
;      303:       C20800           RET 8

Note the code at 2E3 and 2E6, which seems like a really bad way to
exchange the contents of 2 registers :)

Paul Khuong