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unix: improve uv__cpu_relax() on arm (#4863)

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The AArch64 YIELD instruction affects processors that support
symmetric multithreading, while on other implementations (which
are the majority on the market) it is equivalent to NOP, thus
failing to achieve the desired delay effect inside uv__async_spin().
Instead, use the ISB instruction, following one of Arm's
recommendations [1].

[1] https://community.arm.com/arm-community-blogs/b/architectures-and-processors-blog/posts/multi-threaded-applications-arm

Signed-off-by: Anton Kirilov <anton.kirilov@arm.com>
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Anton Kirilov 2025-08-20 20:15:37 +01:00 committed by GitHub
parent 07c8a3c4ca
commit 3d9d7c7ef6
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src/unix/async.c
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@ -409,7 +409,7 @@ static void uv__cpu_relax(void) {
#if defined(__i386__) || defined(__x86_64__)
__asm__ __volatile__ ("rep; nop" ::: "memory"); /* a.k.a. PAUSE */
#elif (defined(__arm__) && __ARM_ARCH >= 7) || defined(__aarch64__)
__asm__ __volatile__ ("yield" ::: "memory");
__asm__ __volatile__ ("isb" ::: "memory");
#elif (defined(__ppc__) || defined(__ppc64__)) && defined(__APPLE__)
__asm volatile ("" : : : "memory");
#elif !defined(__APPLE__) && (defined(__powerpc64__) || defined(__ppc64__) || defined(__PPC64__))