RISC-V psABI doesn't seem to allow relaxation of TLS GD

[rui314]

https://github.com/riscv-non-isa/riscv-elf-psabi-doc/blob/master/riscv-elf.adoc#global-dynamic

In TLS GD access model, the address of a thread-local variable is obtained using three instructions

    auipc a0,0                    # R_RISCV_TLS_GD_HI20 (symbol)
    addi  a0,a0,0                 # R_RISCV_PCREL_LO12_I (label)

    call  __tls_get_addr@plt

However, if a linker knows the the thread-local variable's offset from tp, it can relax the instructions by setting the offset directly to a0 and replaces call ___tls_get_addr@plt with a nop.

The problem here is this: in this RISC-V psABI, there seems to be no way to find a call instruction for an auipc with a R_RISCV_TLS_GD_HI20 relocation. It doesn't mandate that the call instruction immediately follow the addi instruction, so the call can be placed at any place after addi (as long as a0 is preserved).

As a result, it's impossible to relax TLS GD to TLS LE. All TLS accesses has to be resolved using a function call at runtime which is pretty expensive.

[kito-cheng]

My first impression is adding a R_RISCV_PCREL_LO12_I-like relocation maybe R_RISCV_TLS_CALL_LINK which point to label, then linker will know the auipc linked with addi and call?

Need inputs from all other linker hackers @MaskRay @jrtc27 @Nelson1225

label:
    auipc a0,0                    # R_RISCV_TLS_GD_HI20 (symbol)
    addi  a0,a0,0                 # R_RISCV_PCREL_LO12_I (label)

    call  __tls_get_addr@plt # R_RISCV_TLS_CALL_LINK(label) R_RISCV_CALL_PLT(__tls_get_addr)

[rui314]

If we have to make a change to the toolchain to fix this problem, I believe the best approach is to define TLSDESC (#94) and implement it instead, and keep the existing TLSGD as-is for backward compatibility.

[MaskRay]

I think we just need TLSDESC (#94) and initial-exec to local-exec optimization (#122). We don't need to add optimizations to TLS GD which will be superseded by TLSDESC.

[aswaterman]

Agreed with @rui314 and @MaskRay.

[Nelson1225]

I think we just need TLSDESC (#94) and initial-exec to local-exec optimization (#122).

From my side, this is the right way to do. There are many PRs in the sourceware for this topic, https://sourceware.org/bugzilla/show_bug.cgi?id=27953. Just that I am not sure if we should do these while relaxing, or just do these while relocating. Maybe we also can do these while relaxing, but like the alignment relaxation that cannot be disabled.

label:
    auipc a0,0                    # R_RISCV_TLS_GD_HI20 (symbol)
    addi  a0,a0,0                 # R_RISCV_PCREL_LO12_I (label)

    call  __tls_get_addr@plt # R_RISCV_TLS_CALL_LINK(label) R_RISCV_CALL_PLT(__tls_get_addr)

Yeah, if there are any patterns that need to chain all the related instructions, then currently define a new relocation like pcrel_lo is the way, or refer to the corresponding symbol directly is another way (like R_RISCV_TLS_ADD). Personally I prefer the later. BTW, for this case, we will have two relocations for the call, R_RISCV_CALL_PLT and R_RISCV_TLS_CALL_LINK (label or symbol), so make sure they are both together should be necessary.