Fix issue #3806 - Implement core.atomic.pause() for some archs

[kinke]

No description provided.

[JohanEngelen]

From some internet searching, yield seems to be the ARM instruction corresponding to pause on x86. pause also works for MIPS. See code here: https://chromium.googlesource.com/chromium/src/third_party/WebKit/Source/wtf/+/823d62cdecdbd5f161634177e130e5ac01eb7b48/SpinLock.cpp

#if CPU(X86_64) || CPU(X86)
#define YIELD_PROCESSOR __asm__ __volatile__("pause")
#elif CPU(ARM) || CPU(ARM64)
#define YIELD_PROCESSOR __asm__ __volatile__("yield")
#elif CPU(MIPS)
// The MIPS32 docs state that the PAUSE instruction is a no-op on older
// architectures (first added in MIPS32r2). To avoid assembler errors when
// targeting pre-r2, we must encode the instruction manually.
#define YIELD_PROCESSOR __asm__ __volatile__(".word 0x00000140")
#elif CPU(MIPS64) && __mips_isa_rev >= 2
// Don't bother doing using .word here since r2 is the lowest supported mips64
// that Chromium supports.
#define YIELD_PROCESSOR __asm__ __volatile__("pause")
#endif

[kinke]

Thx for looking it up. I came up with

    void pause() pure nothrow @nogc @trusted
    {
        version (X86)
            enum inst = "pause";
        else version (X86_64)
            enum inst = "pause";
        else version (ARM)
            enum inst = "yield"; // apparently requires v6k+ (e.g., -mtriple=armv6k-linux-gnueabihf)
        else version (AArch64)
            enum inst = "yield";
        else version (MIPS32)
            enum inst = "pause"; // apparently requires ISA r2+ (e.g., -mcpu=mips32r2)
        else version (MIPS64)
            enum inst = "pause"; // apparently requires ISA r2+ (e.g., -mcpu=mips64r2)
        else
            enum inst = null; // TODO?

        static if (inst !is null)
            asm pure nothrow @nogc @trusted { (inst); }
    }

but see the according comments. Neither that internal function nor the core.atomic.pause() wrapper are templates, so this might easily lead to druntime compilation trouble for such archs, for probably extremely little benefit. The AArch64 addition seems trouble-free and trivial to add though.

[kinke]

Nevermind, __traits(targetHasFeature) works nicely, tested manually.