Re: Performance analysis of Linux Kernel Markers 0.20 for 2.6.17

[dean gaudet <dean@arctic.org>]

On Sat, 30 Sep 2006, Mathieu Desnoyers wrote:

> - Optimized
> 
> static int my_open(struct inode *inode, struct file *file)
> {
>    0:   55                      push   %ebp
>    1:   89 e5                   mov    %esp,%ebp
>    3:   83 ec 0c                sub    $0xc,%esp
>         MARK(subsys_mark1, "%d %p", 1, NULL);
>    6:   b0 00                   mov    $0x0,%al <-- immediate load 0 in al
>    8:   84 c0                   test   %al,%al
>    a:   75 07                   jne    13 <my_open+0x13>

why not replace the mov+test with "xor %eax,%eax" and then change the 0x75 
to a 0x74 to change from jne to je when you want to enable the marker?

i.e. disabled:

	31 c0	xor %eax,%eax
	75 07	jne 13

enabled:

	31 c0	xor %eax,%eax
	74 07	je 13

it would save 2 bytes, 1 instruction and avoid partial register writes... 
and still has the nice property that a single byte store into the code is 
required for enable/disable (which sounds like a great property -- i 
assume you were deliberately going for that).

i assume there's probably no reason to tie the sequence to eax either -- 
you could let gcc choose it (or maybe you already do).

-dean
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