Detect CPU features with Linux methods on Android for non-Intel CPUs. #1351
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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 @Amanieu (or someone else) soon. Please see the contribution instructions for more information. |
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Regarding CI, there are currently some issues on the Rust side that need to be resolved (rust-lang/rust#103673). Unfortunately I currently don't have the time to look into these issues. |
Applied suggestion to require `feature = "libc"` on Android targets. Co-authored-by: Amanieu d'Antras <[email protected]>
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I don't think we use the |
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We do use it, otherwise feature detection wouldn't work on non-x86 platforms. |
I believe it doesn't?
I don't see how the libc feature would be enabled. I thought this was intentional (Last time I looked, some of that code seemed to be a bit sketchy, but looking now it's probably fine?) |
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It's detected via these features: Which are enabled by default here: |
76: detect: Workaround Exynos 9810 bug on aarch64 Android r=taiki-e a=taiki-e Samsung Exynos 9810 has a bug that big and little cores have different ISAs. And on older Android (pre-9), the kernel incorrectly reports that features available only on some cores are available on all cores. See https://reviews.llvm.org/D114523 for details. Our own run-time detection code has not been released yet and is not a problem, but portable-atomic < 1.1 may have been affected by this issue since rustc 1.69-nightly when is_aarch64_feature_detected supported run-time detection on Android. (rust-lang/stdarch#1351, rust-lang/rust#105784) A patch on stdarch side: rust-lang/stdarch#1378 Co-authored-by: Taiki Endo <[email protected]>
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After digging into why the
is_aarch64_feature_detected!macro didn't work as I expected on my Android device (onlyasimd,fp,neonwere detected by the macro), I came to this line that dispatches the detection depending on the OS.My understanding is that Android should behave like Linux w.r.t. CPU feature detection on ARM - at least on the devices I've tested with the various methods (
getauxval, checking/proc/cpuinfo) worked fine. However, Rust defines a distincttarget_osfor Android (https://github.com/rust-lang/rust/blob/1.65.0/compiler/rustc_target/src/spec/android_base.rs#L5), so the filtering bytarget_os = "linux"doesn't work (even though Android is arguably a flavor of Linux).I've tested this patch manually on a stage 1 compiler and it worked on physical devices (Samsung Galaxy S7 and Samsung Galaxy S20).
I wonder if the
feature = "libc"is relevant on Android?I didn't try the CI script but I guess it's not so relevant at the moment since Android CI was removed (#1346)? I wonder if this PR makes a CI for Android more relevant - although it may be that under the hood qemu doesn't implement as many CPU features as physical devices do?