llvm dev - Nov 2020 - Fwd: Select output section for a function based on a subtarget feature

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

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