[llvm-dev] Fwd: Select output section for a function based on a subtarget feature

[Vlad Ivanov via llvm-dev]

For anyone stumbling upon this, here's what I ended up with:

1. in llvm/lib/CodeGen/TargetLoweringObjectFileImpl.cpp,
   selectELFSectionForGlobal returns a section for a given object.
   Since TargetMachine is available there as well, it's possible to
   call checkFeatures() on MCSubtargetInfo

2. In lld/elf/OutputSections.cpp, getPhdrFlags can be updated to
   propagate section header flags to program header flags

10.11.2020, 14:41, "Vlad Ivanov via llvm-dev" <llvm-dev at lists.llvm.org>:
> Hello,
>
> I'm implementing a port of LLVM for PowerPC VLE. It's a compressed
> instruction set similar to mips16 and ARM Thumb. Instruction
> encoding (VLE/non-vle) is selected for a given memory region by an
> attribute in a memory area descriptor. Targets supporting this that
> I know of are all bare-metal (so powerpc-none-elf).
>
> I'm trying to implement ELF support right now. VLE ELF files should
> have a section header flag set (SHF_PPC_VLE = 0x10000000). I have
> added a subtarget feature "vle", similar to Altivec, SPE, VSX etc
> and I'm enabling this feature for appropriate CPUs.
>
> Now, functions that are compiled with "vle" feature should go into
> a different text section (text_vle). I can see that TargetMachine
> has a method getSubtargetImpl which can be overridden for the
> implementation and it can return different subtargets for different
> function attributes. What I'm struggling with is connecting it
> to section selection; in MCObjectFileInfo::initELFMCObjectFileInfo
> a default text section is created but since here section should
> be selected based on function, I'm not sure where object file
> writer implementation should be "connected" to the subtarget info.
>
> What would be the appropriate place for this kind of logic?
>
> Best Regards,
>
> Vlad