Messages in this thread | | | Date | Sat, 27 Dec 2008 22:03:35 +1100 | From | Jeremy Fitzhardinge <> | Subject | Re: [PATCH 2/3] x86-64: Unify x86_*_percpu() functions. |
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Brian Gerst wrote: > Merge the 32-bit and 64-bit versions of these functions. Unlike 32-bit, > the segment base is the current cpu's PDA instead of the offset from the > original per-cpu area. This is because GCC hardcodes the stackprotector > canary at %gs:40. Since the assembler is incapable of relocating against > multiple symbols, the code ends up looking like: > > movq $per_cpu__var, reg > subq $per_cpu__pda, reg > movq %gs:(reg), reg > > This is still atomic since the offset is a constant (just calculated at > runtime) and not dependant on the cpu number. >
Yeah, it's a real pity we can't convince the linker to do this simple computation as a single %gs:ADDR addressing mode. On the other hand, if the compiler can reuse the computation of %reg 2-3 times, then the generated code could well end up being denser.
J
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