Add support for RISC-V relax target feature #109860
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Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @jackh726 (or someone else) soon. Please see the contribution instructions for more information. Namely, in order to ensure the minimum review times lag, PR authors and assigned reviewers should ensure that the review label (
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For embedded and bare-metal RISC-V applications, we can send patches to them before changes in this pull request ( |
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Not my area of expertise r? compiler |
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Me neither :( |
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The feature is unstable, so it should be fine in any case. |
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Dylan-DPC
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Add support for RISC-V relax target feature This adds `relax` as an allowed RISC-V target feature. The relax feature in LLVM enables [linker relaxation](https://www.sifive.com/blog/all-aboard-part-3-linker-relaxation-in-riscv-toolchain), an optimization specific to RISC-V that allows global variable accesses to be resolved by the linker by using the global pointer (`gp`) register (rather than constructing the addresses from scratch for each access). Enabling `relax` will cause LLVM to emit relocations in the object file that support this. The feature can be enabled in rustc with `-C target-feature=+relax`. Currently this feature is disabled by default, but maybe it should be enabled by default since it is an easy performance improvement (but requires the `gp` register to be set up properly). GCC/Clang enable this feature by default (for both hosted/bare-metal targets), and include the `-mno-relax` flag to disable it (see [here](https://github.com/llvm/llvm-project/blob/466d554dcab39c3d42fe0c5b588b795e0e4b9d0d/clang/lib/Driver/ToolChains/Arch/RISCV.cpp#L145) for the code that enables it in Clang). I think it would make sense to enable by default, at least for all hosted targets since the `gp` register should be automatically set up by the runtime. For bare-metal targets, `gp` must be set up manually, so it is probably best to leave off by default to avoid breaking existing applications that do not set up `gp`. Leaving it disabled by default for all targets is also reasonable though. Let me know your thoughts. Thanks! Fixes rust-lang#109426.
JohnTitor
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Add support for RISC-V relax target feature This adds `relax` as an allowed RISC-V target feature. The relax feature in LLVM enables [linker relaxation](https://www.sifive.com/blog/all-aboard-part-3-linker-relaxation-in-riscv-toolchain), an optimization specific to RISC-V that allows global variable accesses to be resolved by the linker by using the global pointer (`gp`) register (rather than constructing the addresses from scratch for each access). Enabling `relax` will cause LLVM to emit relocations in the object file that support this. The feature can be enabled in rustc with `-C target-feature=+relax`. Currently this feature is disabled by default, but maybe it should be enabled by default since it is an easy performance improvement (but requires the `gp` register to be set up properly). GCC/Clang enable this feature by default (for both hosted/bare-metal targets), and include the `-mno-relax` flag to disable it (see [here](https://github.com/llvm/llvm-project/blob/466d554dcab39c3d42fe0c5b588b795e0e4b9d0d/clang/lib/Driver/ToolChains/Arch/RISCV.cpp#L145) for the code that enables it in Clang). I think it would make sense to enable by default, at least for all hosted targets since the `gp` register should be automatically set up by the runtime. For bare-metal targets, `gp` must be set up manually, so it is probably best to leave off by default to avoid breaking existing applications that do not set up `gp`. Leaving it disabled by default for all targets is also reasonable though. Let me know your thoughts. Thanks! Fixes rust-lang#109426.
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Apr 11, 2023
Rollup of 8 pull requests Successful merges: - rust-lang#109527 (Set up standard library path substitution in rust-gdb and gdbgui) - rust-lang#109752 (Stall auto trait assembly in new solver for int/float vars) - rust-lang#109860 (Add support for RISC-V relax target feature) - rust-lang#109923 (Update `error [E0449]: unnecessary visibility qualifier` to be more clear) - rust-lang#110070 (The `wrapping_neg` example for unsigned types shouldn't use `i8`) - rust-lang#110146 (fix(doc): do not parse inline when output is json for external crate) - rust-lang#110147 (Add regression test for rust-lang#104916) - rust-lang#110149 (Update books) Failed merges: r? `@ghost` `@rustbot` modify labels: rollup
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This adds
relaxas an allowed RISC-V target feature. The relax feature in LLVM enables linker relaxation, an optimization specific to RISC-V that allows global variable accesses to be resolved by the linker by using the global pointer (gp) register (rather than constructing the addresses from scratch for each access). Enablingrelaxwill cause LLVM to emit relocations in the object file that support this. The feature can be enabled in rustc with-C target-feature=+relax.Currently this feature is disabled by default, but maybe it should be enabled by default since it is an easy performance improvement (but requires the
gpregister to be set up properly). GCC/Clang enable this feature by default (for both hosted/bare-metal targets), and include the-mno-relaxflag to disable it (see here for the code that enables it in Clang). I think it would make sense to enable by default, at least for all hosted targets since thegpregister should be automatically set up by the runtime. For bare-metal targets,gpmust be set up manually, so it is probably best to leave off by default to avoid breaking existing applications that do not set upgp. Leaving it disabled by default for all targets is also reasonable though.Let me know your thoughts. Thanks!
Fixes #109426.