Question about RISCV gp-relaxtion

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Maybe the LLD doesn’t support linker relaxtion (It is difficult to implement).
(Correct me if I’m wrong)

I want to implement lwgp, 17-bit/128KB gp-offset. (riscv-code-size-reduction/Zce_spec.adoc at after_v0.50.1_dev · riscv/riscv-code-size-reduction · GitHub)

Eg: lui a0, %hi(sym); load/store a1, %lo(sym)(a0) → lwgp/swgp a1, gp_offset(gp)

question:

  1. Do I have to support linker relaxtion if I want to implement lwgp?

  2. Is there any other way to solve the problem？ assmembler?

2

CC

3

Hi,

To my understanding, lwgp/swgp might need linker relaxation because linker can resolve the symbol address and determine whether the gp-offset fit in to the instructions.
There are linker relaxation patches on lld but didn’t land yet.
https://reviews.llvm.org/D77694

https://reviews.llvm.org/D100835

Hope this helps,
Shiva

liao <liaochunyu126@126.com> 於 2021年10月18日 週一 上午10:08寫道：

4

Hi,

To my understanding, lwgp/swgp might need linker relaxation because linker
can resolve the symbol address and determine whether the gp-offset fit in
to the instructions.

I am always of the opinion that if global variable load/store
has been a performance/code size bottleneck, the application
author needs to think why.

Even 64kb range is pretty limited.
It may appear that adding such an instruction can decrease code size for
some use cases or perhaps benchmarks specifically written for them, but
I wonder whether it would be a good use of an opcode.