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Re: [Xen-devel] [PATCH] x86/current: Provide additional information to optimise get_cpu_info()

>>> On 01.09.14 at 12:58, <andrew.cooper3@xxxxxxxxxx> wrote:
> Exactly as with c/s d55c5eefe "x86: use compiler visible "add" instead of
> inline assembly "or" in get_cpu_info()", this is achieved by providing more
> information to the compiler.
> With this modification, gcc replaces the older:
>     mov imm, %reg
>     and %rsp, %reg
> with:
>     mov %rsp, %reg
>     and imm, %reg
> which is one byte shorter.

I'm in no way opposed to the change, but is that really true? Afaict
it can be 1 byte shorter only when %rax gets selected as the register

>  It also considers all general purpose registers
> for %reg rather than just the legacy ones (i.e. will now use %r12 etc), 
> which
> allows for better register scheduling in larger functions.

Same here - why would with the old code not all registers be
available for selection by the compiler?


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