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fix(port_riscv)!: make
riscv-rt
optional
Fixes linker errors in an application that is linked to `r3_port_riscv` and does not use the `riscv-rt`-compatible startup routine provided by `r3_port_riscv::use_rt!`. Rust implements `rlib`s as static libraries (archives). Linkers treat archives differently from object files: all object files participate in linking, while archives will only participate in linking if they can satisfy at least one undefined reference (version scripts don't count). This means that in an application that does not use `use_rt!`, `libriscv_rt*.rlib` does not participate in linking at all. This behavior also causes `#[no_mangle]` and `#[used]` items to be ignored by the linker (`KEEP` in linker scripts can't keep them either), leading to a long-standing bug in Rust ([rust-lang/rust#47384][2]). The situation changed with the merge of [rust-lang/rust#95604][1]. To fix [rust-lang/rust#47384][2], this PR introduced a synthetic object file containing references to all symbols pertaining to `#[no_mangle]` and `#[used]` items. `libriscv_rt*.rlib` now participates in linking, but the expectation is that `--gc-sections` will remove unused items in the end, unless they are explicitly retained by `KEEP` in linker scripts or other means. This change unexpectedly caused breakage in the tests for `qemu_sifive_u _s_rv(64,32)` targets, which use a custom startup routine instead of `use_rt!`. For some reason, the linker didn't respect `--gc-sections` for some items from `libriscv_rt*.rlib` and decided to include them in the output binary. These items reference symbols that are defined by the `riscv-rt` linker script, which isn't used by `qemu_sifive_u_s_ rv(64,32)` targets, hence the linker error. The thing is, `riscv-rt` outputs these items in the `.init` section[3], which is one of the section names hard-coded in LLD[4] to be excluded from `--gc-sections`. As a result, these items were considered for inclusion despite being referenced from nowhere. This commit works around this problem by making `r3_port_riscv`'s `riscv-rt` dependency optional, ensuring that `riscv-rt` participates in linking only if needed by the application. P.S. I'll try to use an exclamation mark (`!`) in the commit headline to indicate breaking changes from now on, as per [Conventional Commits 1.0.0][5]. [1]: rust-lang/rust#95604 [2]: rust-lang/rust#47384 [3]: https://github.com/rust-embedded/riscv-rt/blob/7de3d2744a465ad723519c04f13c56664e138cb9/asm.S#L20 [4]: https://github.com/llvm/llvm-project/blob/b86440ecde5c1dec5b898a3f1bc08ab9853d5ed9/lld/ELF/MarkLive.cpp#L183 [5]: https://www.conventionalcommits.org/en/v1.0.0/
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