diary at Telent Netowrks

Something odd is happening somewhere in the assembler#

Sun, 08 Feb 2004 15:12:17 +0000

Something odd is happening somewhere in the assembler. Here's a bit of the SBCL source code

(defun allocation (alloc-tn size &optional ignored)
  (declare (ignore ignored))
  (flet ((load-size (dst-tn size)
           (unless (and (tn-p size) (location= alloc-tn size))
             (inst mov dst-tn size))))
    (let ((caller-saves (list rax-tn rcx-tn ;rdx-tn rsi-tn
                              ;rdi-tn r8-tn r9-tn r10-tn r11-tn
      (dolist (r caller-saves)
        (inst push r))
      (move rdi-tn size)
      (inst call (make-fixup (extern-alien-name "alloc") :foreign))
      (move alloc-tn rax-tn)
      (dolist (r (reverse caller-saves))
        (if (location= r alloc-tn)
            (inst add rsp-tn 8)
            (inst pop r)))))

As the name suggests, this function outputs some code for allocating memory that Lisp will use: it frobs some registers around, then calls the C alloc() function, arranging that its return value ends up in alloc-tn, whichever register that is. At least, that's what it's supposed to do - apart from anything else, you'll notice that it's not actually saving most of the registers it's supposed to save. But it's not actually relevant what the output code does right now; what's important is we don't seem to be getting the right stuff assembled. Witness:

0x000000000b22c57f:     push   %rax
0x000000000b22c580:     push   %rcx
0x000000000b22c581:     mov    $0x10,%rdi
0x000000000b22c58b:     callq  0x4162d0 <alloc>
0x000000000b22c590:     rexXYZ mov    %r8d,%r11d
0x000000000b22c593:     pop    %rcx
0x000000000b22c594:     pop    %rax
or if you'd like to see that mov instruction in more detail

(gdb) x/8bx 0x000000000b22c590
0xb22c590:      0x47    0x8b    0xd8    0x59    0x58    0xc7    0x03    0x12

That 'rexXYZ' prefix is presumably gdb's disassembler getting more than slightly confused by our 0x47 rex byte. I'm reasonably certain that %r8d should be %rax, likewise supposing that the operand size (rex bit 3, counting from lsb=0) should be 64 not 32, and I'll give odds of about 1 in 2 that that's r11 not rdx.

Strangre thing is, even after placing assertions where we output the rex prefix that "you're not outputting #x47, are you?" it still doesn't stop. And there's still #x47 bytes in the core file. Very odd.