[llvm-dev] XMMs unused

[Dangeti Tharun kumar via llvm-dev]

Hi

On Tue, Oct 30, 2018 at 10:47 PM Matthias Braun <mbraun at apple.com> wrote:

> At a first glance I see nothing obviously wrong with the assembly, but it
> is a big file. So if you have a specific part in mind, please copy into the
> E-Mail discussion.
>
> I assume you are compiling for a mac or linux system? In that case none of
> the xmm registers are callee saved (as you already explained) so the
> register allocator has to spill them if they are alive across a call. So I
> don't see proof yet that using xmm10-xmm15 would have helped in this
> function...
>

Yes, I am compiling for linux system.
So the RA will not consider assigning a scratch register to a live range
crossing function call, though it may reduce spills?


> - Matthias
>
> On Oct 30, 2018, at 12:24 AM, Dangeti Tharun kumar via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
> Hi Craig,
>
> The test case is from spec-17.
>
> The attached is the assembly file for the function MeanShiftImage from
> spec17/538.imagick_r/src/magick/feature.c.
> As I was saying, registers XMM10-15 are not used.
>
>
> On Thu, Oct 25, 2018 at 2:16 PM Dangeti Tharun kumar <
> cs15mtech11002 at iith.ac.in> wrote:
>
>> Hi,
>>
>> It can be reproduced with the following command:
>>
>> clang -S -I spec17/benchspec/CPU/538.imagick_r/src/ -O3 -mavx spec17/benchspec/CPU/538.imagick_r/src/magick/feature.c -DMAGICKCORE_HDRI_ENABLE
>>
>> I don't see any register above *xmm9* been used in any function.
>>
>>
>> On Thu, Oct 25, 2018 at 10:44 AM Craig Topper <craig.topper at gmail.com>
>> wrote:
>>
>>> Can you provide an example?
>>>
>>> ~Craig
>>>
>>>
>>> On Wed, Oct 24, 2018 at 9:52 PM Dangeti Tharun kumar via llvm-dev <
>>> llvm-dev at lists.llvm.org> wrote:
>>>
>>>> Hi,
>>>>
>>>> I see that, in Greedy register allocation not all the XMM registers are
>>>> used(even there is a need) if there is a function call crossing the live
>>>> range.
>>>> There are spills which can be avoided just by using them.
>>>>
>>>> The reason I see is the CCC declares XMMs are not callee saved. This
>>>> means they are caller saved, correct me if I am wrong.
>>>> Is the greedy RA intentionally not using XMMs in order to avoid saving
>>>> and restoring regs at call site?
>>>>
>>>>
>>>> --
>>>> Regards,
>>>> DTharun
>>>> _______________________________________________
>>>> LLVM Developers mailing list
>>>> llvm-dev at lists.llvm.org
>>>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>>>
>>>
>>
>> --
>> Regards,
>> DTharun
>>
>
>
> --
> Regards,
> DTharun
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>
>

-- 
Regards,
DTharun
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