Re: [musl] Draft riscv64 TLSDESC implementation

[Rich Felker <dalias@libc.org>]

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?