[llvm-dev] EmitTargetCodeForMemSet & LTO issue

[Fangrui Song via llvm-dev]

On 2020-09-01, Teresa Johnson via llvm-dev wrote:
>Are you compiling with -fno-builtin-memset? If not the memset calls are
>replaced with an llvm builtin memset intrinsic call, which is expanded
>after inlining. With -fno-builtin-memset the memset calls are left as
>regular calls, since it tells clang that you are not using the builtin
>version. However, if the definition is in another translation unit, and the
>linker doesn't think that version is prevailing, it won't get imported or
>inlined by LTO/ThinLTO in any case.
>
>Teresa

Another possibility, if you arrange for EmitTargetCodeForMemSet to call
a customized library function which is not a registered libcall, it may
not be fetched if it is in an archive. See lld/ELF/Driver.cpp handleLibCall.

>On Sun, Aug 30, 2020 at 11:11 PM Romaric Jodin <rjodin at upmem.com> wrote:
>
>> 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*
>>
>>
>
>-- 
>Teresa Johnson |  Software Engineer |  tejohnson at google.com |

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