llvm dev - Aug 2020 - EmitTargetCodeForMemSet & LTO issue

Hi everyone,

I have 2 implementations of "memset". A standard one, and another one
optimized when the pointer and the size respect some specific constraints.
I am able to choose the proper one in the "EmitTargetCodeForMemSet"
method
that I implemented for my backend.

My issue is when I am compiling with the LTO optimisation, the linker tells
me that the optimized memset symbol is undefined ("*ld.lld: error:
undefined symbol: __memset_opt*"). I've looked into theLTO archive used
and
I found both memset functions.

It feels like when the compiler gets in "EmitTargetCodeForMemSet", it
has
already forgotten about the optimized memset function that was in the
archive because it was not used initially.

Any idea why?

Thanks in advance,
JODIN Romaric
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ThinLTO's whole program analysis is driven off of the linker symbol
resolution (at least in the case of lld, gold, bfd). Presumably when it
gets into the ThinLTO link, the linker does not see any uses of your
alternate memset and it is therefore removed as dead code. Have you tried
marking it with __attribute__((used))? Another thing that might work is to
mark it weak.

Teresa

On Fri, Aug 28, 2020 at 8:26 AM Romaric Jodin via llvm-dev <
llvm-dev at lists.llvm.org> wrote:
> Hi everyone,
>
> I have 2 implementations of "memset". A standard one, and another
one
> optimized when the pointer and the size respect some specific constraints.
> I am able to choose the proper one in the
"EmitTargetCodeForMemSet" method
> that I implemented for my backend.
>
> My issue is when I am compiling with the LTO optimisation, the linker
> tells me that the optimized memset symbol is undefined ("*ld.lld:
error:
> undefined symbol: __memset_opt*"). I've looked into theLTO archive
used
> and I found both memset functions.
>
> It feels like when the compiler gets in
"EmitTargetCodeForMemSet", it has
> already forgotten about the optimized memset function that was in the
> archive because it was not used initially.
>
> Any idea why?
>
> Thanks in advance,
> JODIN Romaric
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>

-- 
Teresa Johnson |  Software Engineer |  tejohnson at google.com |
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Hi Teresa,

Thank you for the help, adding the "used" attribute worked just fine.
It made me realize that the memset function is never inlined by the LTO
optimization even without my implementation of the
"EmitTargetCodeForMemSet" method.
I supposed that the passes dealing with the memset function happen too
late, is that correct?

Thank you again,
Romaric

On Fri, Aug 28, 2020 at 5:51 PM Teresa Johnson <tejohnson at google.com>
wrote:
> ThinLTO's whole program analysis is driven off of the linker symbol
> resolution (at least in the case of lld, gold, bfd). Presumably when it
> gets into the ThinLTO link, the linker does not see any uses of your
> alternate memset and it is therefore removed as dead code. Have you tried
> marking it with __attribute__((used))? Another thing that might work is to
> mark it weak.
>
> Teresa
>
> On Fri, Aug 28, 2020 at 8:26 AM Romaric Jodin via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
>> Hi everyone,
>>
>> I have 2 implementations of "memset". A standard one, and
another one
>> optimized when the pointer and the size respect some specific
constraints.
>> I am able to choose the proper one in the
"EmitTargetCodeForMemSet"
>> method that I implemented for my backend.
>>
>> My issue is when I am compiling with the LTO optimisation, the linker
>> tells me that the optimized memset symbol is undefined ("*ld.lld:
error:
>> undefined symbol: __memset_opt*"). I've looked into theLTO
archive used
>> and I found both memset functions.
>>
>> It feels like when the compiler gets in
"EmitTargetCodeForMemSet", it has
>> already forgotten about the optimized memset function that was in the
>> archive because it was not used initially.
>>
>> Any idea why?
>>
>> Thanks in advance,
>> JODIN Romaric
>> _______________________________________________
>> LLVM Developers mailing list
>> llvm-dev at lists.llvm.org
>> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>
>
>
> --
> Teresa Johnson |  Software Engineer |  tejohnson at google.com |
>

-- 
*Romaric JODIN*
UPMEM
*Software Engineer*
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