[AArch64] Fix codegen of AArch64 thread pointer lookup in Cilk functions #204
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…e effects to prevent misoptimization in programs where the executing thread may change across a function call. In principle, this change could be made more precise to enable some optimizations of this operation, including removal of redundant reads when there is no intervening function call. But it is not clear what attributes currently exist that could be attached to this operation to model its behavior more precisely.
…memory, rather than having unmodeled side effects.
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This change fixes a miscompilation of TLS accesses on non-macOS AArch64 systems, in which lookups of the thread pointer would be optimized across function calls where the executing thread might change across the call.
This change implements a conservative fix for this problem by marking that the thread-pointer lookup may have unmodeled side effects. As a result, this change may prevent some otherwise legal optimizations of the thread-pointer lookup. We can readdress this issue in the future if an attribute becomes available to model the effects of thread-pointer lookups more precisely.