Re: [musl] Draft riscv64 TLSDESC implementation

[Tatsuyuki Ishi <ishitatsuyuki@gmail.com>]

> On Jan 22, 2024, at 9:03, Rich Felker <dalias@libc.org> wrote:
> 
> On Sun, Jan 21, 2024 at 03:48:55PM -0800, Fangrui Song wrote:
>> On Sun, Jan 21, 2024 at 2:28 PM Rich Felker <dalias@libc.org> wrote:
>>> 
>>> On Tue, Aug 22, 2023 at 01:38:21PM -0400, Rich Felker wrote:
>>>> The psABI work is not finalized, but based on the current status of
>>>> https://github.com/riscv-non-isa/riscv-elf-psabi-doc/pull/373, I think
>>>> the attached is a valid (but untested) implementation of TLSDESC for
>>>> riscv64. Actually activating it requires also adding the relocation
>>>> type macro to riscv64/reloc.h.
>>>> 
>>>> If any rv folks could look it over and make sure I haven't made any
>>>> stupid asm errors or missed any obvious optimizations, that would help
>>>> to quickly get this merged when the psABI is finalized.
>>>> 
>>>> Rich
>>> 
>>>> .text
>>>> .global __tlsdesc_static
>>>> .hidden __tlsdesc_static
>>>> .type __tlsdesc_static,%function
>>>> __tlsdesc_static:
>>>>      ld a0,8(a0)
>>>>      jr t0
>>>> 
>>>> .global __tlsdesc_dynamic
>>>> .hidden __tlsdesc_dynamic
>>>> .type __tlsdesc_dynamic,%function
>>>> __tlsdesc_dynamic:
>>>>      add sp,sp,-8
>>>>      sd t1,(sp)
>>>>      sd t2,8(sp)
>>>> 
>>>>      ld t2,-8(tp) # t2=dtv
>>>> 
>>>>      ld a0,8(a0)  # a0=&{modidx,off}
>>>>      ld t1,8(a0)  # t1=off
>>>>      ld a0,(a0)   # a0=modidx
>>>>      sll a0,a0,3  # a0=8*modidx
>>>> 
>>>>      add a0,a0,t2 # a0=dtv+8*modidx
>>>>      ld a0,(a0)   # a0=dtv[modidx]
>>>>      add a0,a0,t1 # a0=dtv[modidx]+off
>>>>      sub a0,a0,tp # a0=dtv[modidx]+off-tp
>>>> 
>>>>      ld t1,(sp)
>>>>      ld t2,8(sp)
>>>>      add sp,sp,8
>>>>      jr t0
>>> 
>>> Any feedback on this? Offhand, it looks like adjusting sp by 8 is
>>> wrong and that should be 16. Anything else? Does anyone have recent
>>> enough tooling to test this?
>> 
>> Tatsuyuki, do you have links to the latest version of
>> gcc/binutils/glibc patches?
>> Downloading patches from these mailing lists is probably a large
>> hurdle for many users, so having the relevant repositories online may
>> help.
>> 
>> mold has implemented RISC-V TLSDESC.
>> 
>> On the LLVM side, I have reviewed
>> https://github.com/llvm/llvm-project/pull/66915 and am waiting for it
>> to land, before I can check the lld status.
> 
> To test this, drop it in src/ldso/riscv64/tlsdesc.s, and add to
> arch/riscv64/reloc.h:
> 
> #define REL_TLSDESC R_RISCV_TLSDESC
> 
> or whatever the reloc name is (I don't think it's in elf.h yet so you
> probably need to either add it there too or just hard-code the number
> for testing).
> 
> Updated version with the sp bugfix attached.

The assembly looks fine to me. (It’s nice that musl don’t need to bother with save/restore at all since DTVs are initialized eagerly.)
For the patches mentioned in the other thread, most tests were done with glibc’s portable testsuite (tst-elf-*).
If musl has a similar one, you should be able to run it with my GCC / binutils fork (with either --with_tls=desc at configure time or -mtls-dialect=desc at compile time).

Tatsuyuki.

> Rich
> <tlsdesc.s>