bionic/: implement TLSDESC (Documented in psabi; implemented in llvm)

[enh-google]

risc-v is the first Android ABI to ship after we added ELF TLS support. emutls doesn't even seem to work (with LLVM at least) for risc-v, but we can't go straight to using TLSDESC relocations because they haven't been standardized for risc-v yet. blocked on riscv-non-isa/riscv-elf-psabi-doc#94 getting some traction.

[MaoHan001]

What is the current status of TLS support? Recently, llvm received a fix for emulate_tls on RISC-V. But in AOSP emulate_tls is currently disabled on RISC-V, it seems the traditional GD/LD TLS mode is being used; and it appears that someone in the community has started to support TLS DESC in llvm so all that's needed going forward is to update the tools and enable relocation support in Bionic once llvm support is available?

[enh-google]

What is the current status of TLS support? Recently, llvm received a fix for emulate_tls on RISC-V. But in AOSP emulate_tls is currently disabled on RISC-V, it seems the traditional GD/LD TLS mode is being used;

correct. we don't want to use emutls for rv64 --- we'll use the best option available, which is currently the one you mention.

and it appears that someone in the community has started to support TLS DESC in llvm so all that's needed going forward is to update the tools and enable relocation support in Bionic once llvm support is available?

yeah, i'm excited to see some progress on that. the bionic work will be easy+quick once the details are fixed. (i'm assuming the same will be true for llvm too, given that other architectures already support it.)

but for now we're blocked on the TLS DESC psabi proposal.

[appujee]

Reference to sourceware implementation: https://inbox.sourceware.org/binutils/DS7PR12MB576522B309E27EBA26708A85CB1BA@DS7PR12MB5765.namprd12.prod.outlook.com/T/

[appujee]

Support for RISC-V TLSDESC Relocations has landed in llvm: llvm/llvm-project#66916 Thanks to @ilovepi and @MaskRay

[enh-google]

@appujee --- is there anything left to do here? do we have a prebuilt clang where this is on by default?

[appujee]

Yes the patch is in the current clang-r530567

[appujee]

And enabled by default for Android targets in llvm/llvm-project#81198

[ilovepi]

Yeah, TLSDESC should be on by default in Clang for all Android RISC-V targets. I don't think there's anything left to do, since we've resolved all the psABI issues, updated the compiler and linker, and bionic.