Re: [patch 06/10] Immediate Value - i386 Optimization

[Mathieu Desnoyers]

* H. Peter Anvin (hpa@xxxxxxxxx) wrote:
> What is not clear to me is the exact code that is generated by these
> macros.  Nor can I find it anywhere in the documentation.
> 
> Could you please describe this in some detail?  In particular, it seems
> that the uses of these are largely as branch targets, where the extra
> indirection over modifying the jump target directly seems wasted.
> 

Hi Peter,

I understand your concern. If you find a way to let the code be compiled
by gcc, put at the end of the functions (never being a branch target)
and then, dynamically, get the address of the branch instruction and
patch it, all that in cooperation with gcc, I would be glad to hear from
it. What I found is that gcc lets us do anything that touches
variables/registers in an inline assembly, but does not permit to place
branch instructions ourselves; it does not expect the execution flow to
be changed in inline asms.

Here is an objdump of the interesting bits on an immediate value placed
in scheddule (inline schedule_debug).


00000000 <schedule>:
   0:   55                      push   %ebp
   1:   89 e5                   mov    %esp,%ebp
   3:   57                      push   %edi
   4:   56                      push   %esi
   5:   53                      push   %ebx
   6:   83 ec 40                sub    $0x40,%esp
   9:   b8 01 00 00 00          mov    $0x1,%eax
   e:   e8 fc ff ff ff          call   f <schedule+0xf>
  13:   e8 fc ff ff ff          call   14 <schedule+0x14>
  18:   89 45 dc                mov    %eax,0xffffffdc(%ebp)
  1b:   b8 00 00 00 00          mov    $0x0,%eax
  20:   8b 4d dc                mov    0xffffffdc(%ebp),%ecx
  23:   8b 14 8d 00 00 00 00    mov    0x0(,%ecx,4),%edx
  2a:   01 d0                   add    %edx,%eax
  2c:   89 45 d0                mov    %eax,0xffffffd0(%ebp)
  2f:   b8 00 00 00 00          mov    $0x0,%eax
  34:   c7 44 02 04 01 00 00    movl   $0x1,0x4(%edx,%eax,1)
  3b:   00
  3c:   8b 5d d0                mov    0xffffffd0(%ebp),%ebx
  3f:   8b 9b f0 03 00 00       mov    0x3f0(%ebx),%ebx
  45:   89 5d c8                mov    %ebx,0xffffffc8(%ebp)
  48:   81 c3 94 01 00 00       add    $0x194,%ebx
  4e:   89 5d cc                mov    %ebx,0xffffffcc(%ebp)
  51:   8b 45 c8                mov    0xffffffc8(%ebp),%eax
  54:   8b 40 14                mov    0x14(%eax),%eax
  57:   85 c0                   test   %eax,%eax
  59:   0f 89 30 03 00 00       jns    38f <schedule+0x38f>
  5f:   89 e0                   mov    %esp,%eax
  61:   25 00 e0 ff ff          and    $0xffffe000,%eax
  66:   8b 40 14                mov    0x14(%eax),%eax
  69:   25 ff ff ff ef          and    $0xefffffff,%eax
  6e:   83 e8 01                sub    $0x1,%eax
  71:   0f 85 fb 02 00 00       jne    372 <schedule+0x372>
<branch site>
  77:   b8 00 00 00 00          mov    $0x0,%eax
  7c:   85 c0                   test   %eax,%eax
  7e:   0f 85 16 03 00 00       jne    39a <schedule+0x39a>
    here, we just loaded 0 in eax (movl used to make sure we populate the
    whole register so we do not stall the pipeline)a
    When we activate the site,
    line 77 becomes: b8 01 00 00 00    mov    $0x1,%eax
</branch site>

  84:   8b 45 d0                mov    0xffffffd0(%ebp),%eax
  87:   e8 fc ff ff ff          call   88 <schedule+0x88>
  8c:   8b 4d c8                mov    0xffffffc8(%ebp),%ecx
  8f:   8b 41 04                mov    0x4(%ecx),%eax
  92:   f0 0f ba 70 08 02       lock btrl $0x2,0x8(%eax)
...

<profile_hit inline function>
 39a:   8b 55 04                mov    0x4(%ebp),%edx
 39d:   b9 01 00 00 00          mov    $0x1,%ecx
 3a2:   b8 02 00 00 00          mov    $0x2,%eax
 3a7:   e8 fc ff ff ff          call   3a8 <schedule+0x3a8>
 3ac:   e9 d3 fc ff ff          jmp    84 <schedule+0x84>
</profile_hit inline function>
-- 
Mathieu Desnoyers
Computer Engineering Ph.D. Student, Ecole Polytechnique de Montreal
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F  BA06 3F25 A8FE 3BAE 9A68
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