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RV64: Implement TLS relaxation #461

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rui314 opened this issue Apr 25, 2022 · 10 comments
Closed

RV64: Implement TLS relaxation #461

rui314 opened this issue Apr 25, 2022 · 10 comments
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@ksco
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enhancement New feature or request

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@rui314
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rui314 commented Apr 25, 2022

mold currently doesn't relax any TLS relocations at all. We should implement GD → LE and LD → LE relaxations just like we did to x86-64.
@rui314 rui314 added the enhancement New feature or request label Apr 25, 2022
@ksco
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ksco commented Sep 2, 2022

Hi Rui,

I would like to implement this but it may take some time as I am not familiar with the ELF TLS model.

@rui314
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rui314 commented Sep 2, 2022

Sure. I'll assign this bug to you to avoid duplicate effort. Feel free to ask any questions.

@ksco
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ksco commented Sep 2, 2022 •
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Firstly, I'm trying to open all the tls-* tests for RV32 and RV64 and everything works fine except for one issue.

In tls-gd.sh, in the end, we are trying to statically link the three PIC object files together. In this very case, the output becomes 0 2 0 0 0 0 instead of the expected 1 2 3 4 5 6, I haven't found the reason yet.

ksco@ac878cb#diff-8d8a35fafde296bbdfcf817940e5559dc0fb2a1bc4aaea5822870253cdbaac74R68

@rui314
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rui314 commented Sep 2, 2022

TLS is super tricky. I don't know what's wrong, let me take a look.

rui314 added a commit that referenced this issue Sep 2, 2022
@rui314
[ELF] Fix RISC-V TLSGD relocation
96524e4 
For RISC-V, DTPOFF is computed 0x800 off from the start of each TLS block
to maximize the accessible range for memory access instructions with
signed 12-bit immediate. DTPMOD is usually computed by the dynamic loader,
but when we do for static binaries, we need to subtract 0x800 to offset
the offset.

#461
@rui314
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rui314 commented Sep 2, 2022

I fixed the issue in the above commit.

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ksco commented Sep 2, 2022

That's amazing!

This was referenced Sep 2, 2022
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ksco commented Sep 4, 2022

I think this is done since we already handled the Thread-pointer Relaxation, right?

@rui314
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rui314 commented Sep 4, 2022

By TLS relaxation, I meant conversion of General Dynamic → Local Exec and Local Dynamic → Local Exec, and not the RISC-V-specific %tp optimization. But I found that the RISC-V psABI lacks a mechanism to allow GD → LD (riscv-non-isa/riscv-elf-psabi-doc#308), so maybe this is infeasible. It's a spec bug, though.

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ksco commented Sep 4, 2022

Oh, seems they're gonna add this mechanism after spec 1.0 is released, so I think I'll just wait till that happens.

@rui314
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rui314 commented Sep 30, 2023

TLSDESC for RISC-V has now been ratified, and we've implement it to mold. I think we can close this issue now.

@rui314 rui314 closed this as completed Sep 30, 2023
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