[release/9.0] [mono][mini] Interlocked.CompareExchange and Interlocked.Exchange intrinsics for small types and enums #106897
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also add mono_atomic_cas_u8 utility function
maybe we can't actually do a 1 or 2 byte move from RAX ?
fixup u16 win32 atomic
for the 16-bit addressing prefix
…t fail to allocate a table index and generate a warning. This shouldn't happen in prod anyway Implement cmpxchg atomics natively in jiterpreter Remove unnecessary jiterp cas helpers Do cmpxchg result fixups as needed Add runtime option for jiterpreter atomics Implement atomic exchanges in the jiterpreter
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jeffschwMSFT
approved these changes
Aug 23, 2024
jkurdek
approved these changes
Aug 26, 2024
| @@ -1216,12 +1216,42 @@ typedef union { | |||
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| #define amd64_prefix_size(inst,p,size) do { x86_prefix((inst), p); } while (0) | |||
| #define amd64_rdtsc_size(inst,size) do { amd64_codegen_pre(inst); amd64_emit_rex ((inst),(size),0,0,0); x86_rdtsc(inst); amd64_codegen_post(inst); } while (0) | |||
| // FIXME: wrong for size == 1 or 2 | |||
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those limitations do not concern current usage scenarios?
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Correct. These macros are not currently used for size 1 or 2
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PR is green, reviewed and signed off. Ready to merge /cc @carlossanlop |
This was referenced Aug 26, 2024
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Backport of #106660 to release/9.0
/cc @lambdageek
Customer Impact
After the BCL started to use the new
Interlocked.Exchange<T>(...)andInterlocked.CompareExchange<T>(...)APIs withboolandenumtypes for certain internal lock-free data structures, Mono experienced a severe performance regression across multiple form factors because we did not implement intrinsics for types backed by 8-bit and 16-bit integers. As a result the slower IL fallbacks were used. See #105335Regression
This is a performance regression, not a correctness regression.
Testing
We have test coverage for these cases in the JIT and libraries testsuires.
Risk
Medium.
Additionally on WebAssembly we now always emit the atomic instructions in the jiterpreter. It is possible customers may deploy apps on older browser that do not implement those instructions. (Although all modern browsers support the opcodes even on non-shared memory). As a mitigation the jiterpreter includes a configuration flag to turn off JITing for traces that require the new opcodes.
It is possible that there are correctness criteria that are missed by our testsuites and we may JIT or incorrect code on some architectures. It is unlikely that we will AOT incorrect code as changes to LLVM codegen were minor. Possible customer workarounds in case of a JIT bug would be to produce AOT profiles that force AOT for the affected methods.
Fixes #105335
Fixes #93488