Re: Native compilation - specific optimisation surely possible?

[Andrea Corallo <akrl@sdf.org>]

Alan Mackenzie <acm@muc.de> writes:

> Hello, Emacs.
>
> The following very short function:
>
>
> ;; -*- lexical-binding: t -*-
> (defun comp-test-55 (x)
>   (unless (integerp x)
>     x))
>
>
> byte compiles to:
>
>
> byte code for comp-test-55:
>   doc:   ...
>     args: (arg1)
>     0       dup
>     1       integerp
>     2       not
>     3       goto-if-nil-else-pop 1
>     6       dup
>     7:1     return
>
>
> , then on an amd-64 machine, native compiles to (annotation added by
> me):
>
>
>
> 00000000000012c0 <F636f6d702d746573742d3535_comp_test_55_0>:
> Setup of the function:
>     12c0:	55                   	push   %rbp
>     12c1:	53                   	push   %rbx
>     12c2:	48 89 fb             	mov    %rdi,%rbx
>     12c5:	48 83 ec 08          	sub    $0x8,%rsp
>     12c9:	48 8b 05 18 2d 00 00 	mov    0x2d18(%rip),%rax        # 3fe8 <freloc_link_table@@Base-0x240>
>     12d0:	48 8b 28             	mov    (%rax),%rbp
> fixnump:
>     12d3:	8d 47 fe             	lea    -0x2(%rdi),%eax
>     12d6:	a8 03                	test   $0x3,%al
>     12d8:	75 26                	jne    1300 <F636f6d702d746573742d3535_comp_test_55_0+0x40>
>
>     12da:	48 8b 05 ff 2c 00 00 	mov    0x2cff(%rip),%rax        # 3fe0 <d_reloc@@Base-0x220>
>     12e1:	48 8b 78 10          	mov    0x10(%rax),%rdi
> Nil in %rdi?:
>     12e5:	31 f6                	xor    %esi,%esi
>     12e7:	ff 95 c0 27 00 00    	call   *0x27c0(%rbp)      `eq' <========================
>     12ed:	48 85 c0             	test   %rax,%rax
>     12f0:	48 0f 45 c3          	cmovne %rbx,%rax
> Tear down of the function:
>     12f4:	48 83 c4 08          	add    $0x8,%rsp
>     12f8:	5b                   	pop    %rbx
>     12f9:	5d                   	pop    %rbp
>     12fa:	c3                   	ret    
>     12fb:	0f 1f 44 00 00       	nopl   0x0(%rax,%rax,1)
> bignump:
>     1300:	8d 47 fb             	lea    -0x5(%rdi),%eax
>     1303:	a8 07                	test   $0x7,%al
>     1305:	74 09                	je     1310 <F636f6d702d746573742d3535_comp_test_55_0+0x50>
>
>     1307:	31 ff                	xor    %edi,%edi
>     1309:	eb da                	jmp    12e5 <F636f6d702d746573742d3535_comp_test_55_0+0x25>
>     130b:	0f 1f 44 00 00       	nopl   0x0(%rax,%rax,1)
> pseudovectorp:
>     1310:	be 02 00 00 00       	mov    $0x2,%esi
>     1315:	ff 55 08             	call   *0x8(%rbp)
>     1318:	84 c0                	test   %al,%al
>     131a:	75 be                	jne    12da <F636f6d702d746573742d3535_comp_test_55_0+0x1a>
>     131c:	31 ff                	xor    %edi,%edi
>     131e:	eb c5                	jmp    12e5 <F636f6d702d746573742d3535_comp_test_55_0+0x25>
>
> ..  The input parameter x (or arg1) is passed into the function in the
> register %rdi.  integerp is coded successively as fixnump followed (if
> necessary) by bignump.  The fixnump is coded beautifully in three
> instructions.
>
> I don't understand what's happening at 12da.  It seems that the address
> of a stack pointer is being loaded into %rax, from which the result of
> `fixnump' (which was already in %rax) is loaded into %rdi.  
>
> But my main point is the compilation of the `not' instruction at 12e5.
> The operand to `not' is in %rdi.  It is coded up as (eq %rdi nil) by
> loading 0 (nil) into %rsi at 12e5, then making a function call to `eq'
> at 12e7.
>
> Surely the overhead of the function call for `eq' makes this a candidate
> for optimisation?  `not' could be coded up in two instructions (test
> %rdi,%rdi followed by a conditional jump or (faster) the cmovne which is
> %already there).
>
> `not' is presumably a common opcode in byte compiled functions.  `eq'
> surely more so.  So why are we coding these up as function calls?
>
> Andrea?

Hi Alan,

could you attach the .c file produced with `native-comp-debug' >= 2?

Thanks

  Andrea

PS I might be a little slow answering mails for the coming week as I'm
on holiday :)