[NativeAOT] Use 8.1 atomics, if available, in RhpCheckedXchg/RhpCheckedLockCmpXchg #85283
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Tagging subscribers to this area: @agocke, @MichalStrehovsky, @jkotas Issue DetailsWe could, in theory, test for presence of LSE instructions dynamically, - set a global flag early on, and check it when need to do an object exchange. I am not sure if potential improvement could be more than the cost of the check. When it is guaranteed that LSE is present, it is a trivial change.
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The improvement is more than the cost of the check based on what we have seen earlier. We use the dynamic check for standard CoreCLR: runtime/src/coreclr/vm/util.hpp Line 95 in 11a0671 |
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I have added a dynamic check when |
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| ;; Bit position for the ARM64IntrinsicConstants_Atomics flags, to be used with tbz / tbnz instructions | ||
| ;; ARM64IntrinsicConstants_Atomics = 0x0080 | ||
| ARM64_ATOMICS_FEATURE_FLAG_BIT equ 7 |
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Can we validate that the value of this constant matches the C++ version?
This looks very likely to get forgotten if we ever shuffle the enum for whatever reason.
We have a mechanism for compile-time validation of these things in AsmOffsets.h. It's not an offset per se, but this is used to ensure MAX_STRING_LENGTH agree, etc.
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I'll put this into offsets (with appropriate check).
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ARM64_ATOMICS_FEATURE_FLAG_BIT is in the AsmOffsets.h now and there is an assert that couples it with ARM64IntrinsicConstants_Atomics
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hmm arm64 asm on linux is not happy about LSE instructions. I wonder if LSE can be enabled locally |
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/azp run runtime-extra-platforms |
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clang defines __GNUC__ for compat, so this is #if 1 for all practical purposes.
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hmm, that is not very useful. Is there a way to detect GCC, or do we ever compile this file with GCC?
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GCC does not document mte as something it would accept.
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We can just put #if 0 around this whole block for now. We do not need support for mtag in our copy of libunwind.
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Although it might be that both clang and gcc support mte:
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Is there a way to detect GCC
We can instead use #ifdef __llvm__ (if it is really needed)
Please reference the commit modifying external source in src/native/external/llvm-libunwind-version.txt.
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The problem is with older versions of llvm that do not understand this extension at all.
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Maybe just replace it with assert(false) until we have to support ‘mte’?
We’d have to switch to compiler that supports it by then.
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This is a dead code as we are setting stage2 to false: https://github.com/dotnet/runtime/pull/85179/files#diff-342f897208801d38daad0d3b917fae03bdcc260357ebfc132231be828977dae0 and this code is under if (stage2 && ..) condition, so we can just skip this branch from compilation as @jkotas has done in #85431.
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We could, in theory, test for presence of LSE instructions dynamically, - set a global flag early on, and check it when need to do an object exchange. I am not sure if potential improvement could be more than the cost of the check.
When it is guaranteed that LSE is present, it is a trivial change.