bytecodealliance/wasmtime

Eagerly cast pointers in aarch64 relocation code to `usize` to avoid `unsafe` blocks in cranelift-jit

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#7,588 opened on Nov 27, 2023

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good first issue

Description

When performing relocations in cranelift-jit, there are many introductions of a variable what that's defined as an unsafe pointer offset. These variables are almost always cast to usize or isize before they're used, so perhaps we should instead turn the base pointer into its target numeric type eagerly, to avoid needing the unsafe blocks for calling the offset method on the pointers.

https://github.com/bytecodealliance/wasmtime/blob/cd97c9f14713d81de5453fb9861319e78a761432/cranelift/jit/src/compiled_blob.rs#L55-L57

Contributor guide

Tech stack
rustwasm
Domain
backenddeveloper experience
Issue type
refactor
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
3
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
1-3 hours
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
stale
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
clear
Prerequisites
Rustunsafe Rustpointer arithmetic
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
40
Research direction
Examine the file `cranelift/jit/src/compiled blob.rs`, specifically lines 55-57 and surrounding code, to understand how the `what` pointer is used in relocation functions. Identify all occurrences where pointers are cast to `usize` or `isize` after offset calculations. Implement a change that casts the base pointer to `usize` early to avoid `unsafe` blocks for pointer offset calls.
Eagerly cast pointers in aarch64 relocation code to `usize` to avoid `unsafe` blocks in cranelift-jit · bytecodealliance/wasmtime#7588 | Good First Issue