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Switch from llvm_asm!() to new asm!() macro #304

merged 2 commits into from Nov 16, 2021


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To be honest I'm not the most familiar with inline assembly, but afaict the new version is correct. The assembly output is identical, at least, checking RawRwLock::bump_shared_slow (which calls both elision impls) using cargo asm.

This should be ready for use when asm!() is stabilized, cf (well, your own stabilization report I guess) rust-lang/rust#72016 (comment)

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Amanieu commented Nov 11, 2021

This code is actually incorrect on the x32 ABI: you need to force the use of the :e modifier for both registers so that a 32-bit cmpxchg is used instead of a 64-bit one.

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coolreader18 commented Nov 11, 2021

Not that I'd know more about this than you, but are you sure? The unstable book looks like it implies that eax will be chosen on x86 32-bit (

Architecture Register class Modifier Example output LLVM modifier
x86-32 reg None eax k
x86-64 reg None rax q

Is it just by chance that it's picking the correct e registers?

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Amanieu commented Nov 12, 2021

x32 is not the same as x86. It's the 32-bit ABI for x86_64 where it uses the x86_64 ISA but with 32-bit pointers. You can try it yourself with the x86_64-unknown-linux-gnux32 target.

"xadd [{}], {}",
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You also need :e here for 32-bit pointer size.

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2 participants