Re: Shrinking the C core

[Ihor Radchenko <yantar92@posteo.net>]

"Alfred M. Szmidt" <ams@gnu.org> writes:

>    (progn
>      (setq x (list 'a 'b 'c))
>      (listp x))
>
>    `listp' call can definitely be optimized once the compiler knows that
>    `list' returns a list.
>
> A sufficiently smart compiler would optimize that to T sure, the Emacs
> Lisp compiler doesn't.  And that is one of the issues, native
> compilation or not, since right now native compilation gets this to
> work with, and it cannot do magic no matter how much magic dust you
> give it.  
>
> byte code for foo:
>   args: nil
> 0	constant  a
> 1	constant  b
> 2	constant  c
> 3	list3	  
> 4	dup	  
> 5	varset	  x
> 6	listp	  
> 7	return	  

AFAIK, native compilation should be able to optimize the above byte
code. At least, that's what I thought. But looking at disassembly, it
might actually be not the case.

Oh well. I am clearly missing something about how things work.

(defun test0 ()
  "Return value")

(defun test1 ()
  (let ((x (list 'a 'b 'c)))
    (when (listp x) "Return value")))

(disassemble (byte-compile #'test0))
byte code:
  doc:  Return value
  args: nil
0	constant  "Return value"
1	return	  

(native-compile #'test0 "/tmp/test0.eln")
(disassemble #'test0)
0000000000001100 <F7465737430_test0_0>:
    1100:	48 8b 05 c1 2e 00 00 	mov    0x2ec1(%rip),%rax        # 3fc8 <d_reloc@@Base-0x218>
    1107:	48 8b 00             	mov    (%rax),%rax
    110a:	c3                   	ret
    110b:	0f 1f 44 00 00       	nopl   0x0(%rax,%rax,1)

Now, test1

(disassemble (byte-compile #'test1))
byte code:
  args: nil
0	constant  a
1	constant  b
2	constant  c
3	list3	  
4	dup	  
5	varbind	  x
6	listp	  
7	goto-if-nil-else-pop 1
10	constant  "Return value"
11:1	unbind	  1
12	return	  

(native-compile #'test1 "/tmp/test1.eln")
(disassemble #'test1)
0000000000001100 <F7465737431_test1_0>:
    1100:	48 8b 05 c9 2e 00 00 	mov    0x2ec9(%rip),%rax        # 3fd0 <freloc_link_table@@Base-0x268>
    1107:	41 54                	push   %r12
    1109:	31 f6                	xor    %esi,%esi
    110b:	55                   	push   %rbp
    110c:	4c 8b 25 b5 2e 00 00 	mov    0x2eb5(%rip),%r12        # 3fc8 <d_reloc@@Base-0x218>
    1113:	53                   	push   %rbx
    1114:	48 8b 18             	mov    (%rax),%rbx
    1117:	49 8b 7c 24 10       	mov    0x10(%r12),%rdi
    111c:	ff 93 d0 20 00 00    	call   *0x20d0(%rbx)
    1122:	49 8b 7c 24 08       	mov    0x8(%r12),%rdi
    1127:	48 89 c6             	mov    %rax,%rsi
    112a:	ff 93 d0 20 00 00    	call   *0x20d0(%rbx)
    1130:	49 8b 3c 24          	mov    (%r12),%rdi
    1134:	48 89 c6             	mov    %rax,%rsi
    1137:	ff 93 d0 20 00 00    	call   *0x20d0(%rbx)
    113d:	49 8b 7c 24 20       	mov    0x20(%r12),%rdi
    1142:	48 89 c5             	mov    %rax,%rbp
    1145:	48 89 c6             	mov    %rax,%rsi
    1148:	ff 53 58             	call   *0x58(%rbx)
    114b:	48 89 ef             	mov    %rbp,%rdi
    114e:	ff 93 30 29 00 00    	call   *0x2930(%rbx)
    1154:	48 85 c0             	test   %rax,%rax
    1157:	74 17                	je     1170 <F7465737431_test1_0+0x70>
    1159:	49 8b 6c 24 30       	mov    0x30(%r12),%rbp
    115e:	bf 06 00 00 00       	mov    $0x6,%edi
    1163:	ff 53 28             	call   *0x28(%rbx)
    1166:	5b                   	pop    %rbx
    1167:	48 89 e8             	mov    %rbp,%rax
    116a:	5d                   	pop    %rbp
    116b:	41 5c                	pop    %r12
    116d:	c3                   	ret
    116e:	66 90                	xchg   %ax,%ax
    1170:	48 89 c5             	mov    %rax,%rbp
    1173:	bf 06 00 00 00       	mov    $0x6,%edi
    1178:	ff 53 28             	call   *0x28(%rbx)
    117b:	48 89 e8             	mov    %rbp,%rax
    117e:	5b                   	pop    %rbx
    117f:	5d                   	pop    %rbp
    1180:	41 5c                	pop    %r12
    1182:	c3                   	ret
    1183:	66 66 2e 0f 1f 84 00 	data16 cs nopw 0x0(%rax,%rax,1)
    118a:	00 00 00 00 
    118e:	66 90                	xchg   %ax,%ax


-- 
Ihor Radchenko // yantar92,
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