arm64: uaccess: permit __smp_store_release() to use zero register
Currently the asm constraints for __smp_store_release() require that the value is placed in a "real" GPR (i.e. one other than [XW]ZR or SP). This means that for cases such as: __smp_store_release(ptr, 0) ... the compiler has to move '0' into "real" GPR, e.g. mov xN, #0 stlr xN, [<addr>] This is unfortunate, as using the zero register would require fewer instructions and save a "real" GPR for other usage, allowing the compiler to generate: stlr xzr, [<addr>] Modify the asm constaints for __smp_store_release() to permit the use of the zero register for the value. There should be no functional change as a result of this patch. Signed-off-by: Mark Rutland <mark.rutland@arm.com> Cc: Catalin Marinas <catalin.marinas@arm.com> Cc: Robin Murphy <robin.murphy@arm.com> Cc: Will Deacon <will@kernel.org> Link: https://lore.kernel.org/r/20230314153700.787701-3-mark.rutland@arm.comSigned-off-by: Will Deacon <will@kernel.org>
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