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.
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.
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?
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.
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.
