quoted text > Hi,
>
> On Wed, 2009-10-21 at 13:23 -0400, Steven Rostedt wrote:
>> On Wed, 2009-10-21 at 10:07 -0700, David Daney wrote:
>>
>>> I have not used -pg, so I don't know for sure, I think all it does is
>>> add the calls to _mcount. Someone could investigate
>>> -fno-omit-frame-pointer, with that you may be able to use:
>> Note, -pg assumes -fno-omit-frame-pointer, since -fomit-frame-pointer
>> and -pg are incompatible.
>
> Ralf have told me -pg really works with -fomit-frame-pointer, although
> the gcc tool tell us they are not incompatible when we use both of them
> together, but when I remove -fno-omit-frame-pointer in
> KBUILD_FLAGS(enabled by CONFIG_FRAME_POINTER), it definitely remove the
> s8(fp) relative source code(Seems -fomit-frame-pionter is used by
> default by gcc), the leaf function becomes this:
>
> function:
>
> 80101144 <au1k_wait>:
> 80101144: 03e00821 move at,ra
> 80101148: 0c04271c jal 80109c70 <_mcount>
>
> No more instruction,
>
> and the non-leaf function becomes,
>
> 80126590 <copy_process>:
> 80126590: 27bdffa0 addiu sp,sp,-96
> 80126594: afbf005c sw ra,92(sp)
> 80126598: afbe0058 sw s8,88(sp)
> 8012659c: afb70054 sw s7,84(sp)
> 801265a0: afb60050 sw s6,80(sp)
> 801265a4: afb5004c sw s5,76(sp)
> 801265a8: afb40048 sw s4,72(sp)
> 801265ac: afb30044 sw s3,68(sp)
> 801265b0: afb20040 sw s2,64(sp)
> 801265b4: afb1003c sw s1,60(sp)
> 801265b8: afb00038 sw s0,56(sp)
> 801265bc: 03e00821 move at,ra
> 801265c0: 0c04271c jal 80109c70 <_mcount>
>
> It may save about two instructions for us.
>
> sw s8, offset(sp)
> move s8, fp
>
> and also, I have tried to just search "Save" instruction, if I find one,
> that should be a non-leaf function, otherwise, it's leaf function, but I
> can not prove no "Save" instruction before the leaf function's "move at,
> ra", for example:
>
> 8010113c: 03e00008 jr ra
> 80101140: 00020021 nop
>
> 80101144 <au1k_wait>:
> 80101144: 03e00821 move at,ra
> 80101148: 0c04271c jal 80109c70 <_mcount>
>
> if there is "save" instruction at address 80101140, it will fail.
> Although, I met not failure with several tries, but no prove on it! any
> ABI protection for this? if YES, this should be a better solution, for
> it may works without -fno-omit-frame-pointer and save several
> instructions for us.
This is what I was talking about up-thread. Leaf functions may have no
function prolog. If you do code scanning you will fail. While scanning
backwards, there is no way to know when you have entered a new function.
Looking for function return sequences 'jr ra' doesn't work as there
may be functions with multiple return sites, functions that never
return, or arbitrary data before the function. I think you have to
force a frame pointer to be established if you want this to work.
David Daney
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