RISC-V LLVM feature +relax not recognized by rustc #109426
Comments
zyedidia
added
A-diagnostics
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Does |
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I tried it out and it does not seem to enable linker relaxation unfortunately. |
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@nikic I wonder where could I add |
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Why is a target feature being used for linker relaxation...? |
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This optimization/feature is specific to RISC-V. For the RISC-V target by default LLVM does not emit relocations (in the generated object file) for global variable lookups that tell the linker to make use of linker relaxation (generating addresses for globals using the In any case it should be possible to enable with Rust without getting a warning, and perhaps should even be enabled by default for RISC-V targets. |
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As mentioned in the other issue, the warning is in large part because LLVM and Rust don't always agree on the definition of a target feature, in part because LLVM has proven surprisingly willing to change the definitions. Proooobably we should just enable this by default, then, if gcc does? Hmm. |
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.
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.
Code
Current output
Desired output
Rationale and extra context
I would like to enable linker relaxation for the RISC-V target. To do so I passed
-C target-feature=+relaxto tell LLVM to use linker relaxation. It seems likerustcis not aware of this option, so it emits a warning (but the feature is still properly enabled). I can't find any way to disable this warning, so I am submitting this issue in the hopes that either the feature will become recognized, or a way to disable the warning will be added.Related: #96472.
The program is compiled by running
rustc --target riscv64imac-unknown-none-elf -C target-feature=+relax test.rs --emit=objHopefully I've posted this to the correct place. Thanks!
Other cases
No response
Anything else?
No response
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