Stream: git-wasmtime

Topic: wasmtime / issue #7191 riscv64: Add Support For Linker Re...

Wasmtime GitHub notifications bot (Oct 08 2023 at 14:19):

afonso360 opened issue #7191:

:wave: Hey,

RISC-V has a number of linker relaxation optimizations that it can perform. These require the R_RISCV_RELAX relocation to be used in order for the linker to be able to apply them.

This is also a continuation of the discussion in #7047. There we decided that it would not be a good idea to implement compressed conditional branches due to their low offset range. We can instead implement them as normal branches and use RELAX to let the linker compress them if it finds it beneficial.

All of the possible relaxation sequences are available in the Linker Relaxation section of the RISC-V ELF document.

Implementation

We can't just apply RELAX to the existing relocations and expect everything to work. Since cranelift has a similar Label mechanism that computes offsets, adding RELAX at any point in the middle of those would cause these offsets to become wrong.

The easiest way that I can think of to implement this would be to add a flag to cranelift that would preserve all labels that were emitted during lowering and transform all LabelUse's into their equivalent relocation.

This would allow the linker to have knowledge of where all the branches are, and adjust them based on what relaxations it has performed.

Wasmtime GitHub notifications bot (Oct 08 2023 at 14:19):

afonso360 added the cranelift:area:riscv64 label to Issue #7191.

Last updated: Nov 22 2024 at 16:03 UTC