linux-kernel - Unnecessary Relocation Hiding?

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Message-ID: <a2ebde260608230500o3407b108hc03debb9da6e62c@mail.gmail.com>
Date:	Wed, 23 Aug 2006 20:00:59 +0800
From:	"Dong Feng" <middle.fengdong@...il.com>
To:	linux-kernel@...r.kernel.org
Subject: Unnecessary Relocation Hiding?

Hi, all,

I have a question. Why shall we need a RELOC_HIDE() macro in the
definition of per_cpu()? Maybe the question is actually why we need
macro RELOC_HIDE() at all. I changed the following line in
include/asm-generic/percpu.h, from

#define per_cpu(var, cpu) (*RELOC_HIDE(&per_cpu__##var, __per_cpu_offset[cpu]))

to

#define per_cpu(var, cpu) (*((unsigned long)(&per_cpu__##var) +
__per_cpu_offset[cpu]))

I recompiled the code and it works well on my Intel Dual-core laptop.
It essentially the same as to change the definition of RELOC_HIDE(),
from

#define RELOC_HIDE(ptr, off) \
  ({ unsigned long __ptr; \
    __asm__ ("" : "=r"(__ptr) : "0"(ptr)); \
    (typeof(ptr)) (__ptr + (off)); })

to

#define RELOC_HIDE(ptr, off) \
  ({ unsigned long __ptr; \
    __ptr = (unsigned long)ptr; \
    (typeof(ptr)) (__ptr + (off)); })

Why shouldn't we have a pure C solution in this part?

Best Regards.
Feng,Dong
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