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Re: [PATCH 2/2] xen/arm: Skip loops in init_pdx() when no PDX compression is used
- To: Julien Grall <julien@xxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxxx
- From: "Orzel, Michal" <michal.orzel@xxxxxxx>
- Date: Mon, 14 Jul 2025 14:37:14 +0200
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- Cc: Stefano Stabellini <sstabellini@xxxxxxxxxx>, Bertrand Marquis <bertrand.marquis@xxxxxxx>, Volodymyr Babchuk <Volodymyr_Babchuk@xxxxxxxx>
- Delivery-date: Mon, 14 Jul 2025 12:37:38 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 14/07/2025 12:09, Julien Grall wrote:
> Hi Michal,
>
> On 14/07/2025 10:50, Orzel, Michal wrote:
>>
>>
>> On 14/07/2025 11:45, Julien Grall wrote:
>>> Hi Michal,
>>>
>>> On 14/07/2025 08:37, Orzel, Michal wrote:
>>>>
>>>>
>>>> On 12/07/2025 12:29, Julien Grall wrote:
>>>>> Hi Michal,
>>>>>
>>>>> On 04/07/2025 08:54, Michal Orzel wrote:
>>>>>> When CONFIG_PDX_COMPRESSION=n, pdx_init_mask(), pdx_region_mask() and
>>>>>> pfn_pdx_hole_setup() are just stubs doing nothing. It does not make
>>>>>> sense to keep the two loops iterating over all the memory banks.
>>>>>
>>>>> I saw this was already committed. But I have a question. Wouldn't the
>>>>> compiler be able to optimize and remove the loops? Asking because we are
>>>>> trying to limit the number of #ifdef in the code hence why we have stubs.
>>>> Before submitting a patch I did disassembled init_pdx() with and without my
>>>> patch and in the latter case the compiler did not optimize out the loops.
>>>
>>> One more question. Was this in debug or non-debug build?
>> It was in debug build.
>
> Ok. I would be interested to know if this change in non-debug build
> because we have quite a few places in Xen relying on code elimination.
I did a test and with -O2 (non-debug), the loops are removed as oppose to -O1
(debug). That said, -fdce is part of -O1, so it's difficult to say what option
made impact here.
~Michal
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