On Windows, r18_tls is used to store the pointer to the current thread's TEB. Therefore, this register should never be modified (see details in register_aarch64.hpp). However, C1's restore_live_registers can modify r18_tls when it pops saved registers off the stack if it is executed on a different thread than the one that saved the registers. This causes hangs and occasional crashes in the virtual threads jtreg tests on Windows AArch64 after https://github.com/openjdk/jdk/commit/78b80150e009745b8f28d36c3836f18ad0ca921f.