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We currently expose R_LARCH_DELETE in our specs but it could be hard to properly use: for producers of object code, why emit the code at all, if you plan to stick this reloc to it right away to have the code removed by another program later?

For reference, the similar R_RISCV_DELETE is exclusively an internal concept only for BFD's relaxation pass:

// bfd/elfnn-riscv.c

/* Internal relocations used exclusively by the relaxation pass.  */
#define R_RISCV_DELETE (R_RISCV_max + 1)

And LLVM doesn't define it at all.

So I suggest removing this reloc in a future revision of the ELF psABI document, making it reserved in the meantime. No program would consciously write this reloc kind to object files so compatibility would be preserved.

How to know a register is all zero and generate a mask to compare?

8.1. The registers
...
The registers that can be used for passing arguments and returning values are the argument registers,
which include:
• GARs: 8 general-purpose registers $a0 - $a7, where $a0 and $a1 are also used for integral values.
• FARs: 8 floating-point registers $fa0 - $fa7, where $fa0 and $fa1 are also used for returning values.
...

Should it be the word returning like FARs rather than integral?

https://github.com/loongson/la-abi-specs/blob/release/lapcs.adoc

https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2019/p0709r4.pdf
https://www.open-std.org/JTC1/SC22/WG14/www/docs/n2429.pdf

According to C and C++ standard, there is a proposal on defining a new exception handling system. It runs by returning 2 registers + a boolean discriminant.

I suggest to define the ABI now by using r12 register as the register for storing that boolean discriminant flag to indicate the function throws an exception or not.