From: "George Spelvin" <linux@sciencehorizons.net>
To: vegard.nossum@gmail.com
Cc: linux-kernel@vger.kernel.org
Subject: Re: [PATCH] firmware: declare __{start,end}_builtin_fw as pointers
Date: 28 Jun 2016 08:23:50 -0400 [thread overview]
Message-ID: <20160628122350.6688.qmail@ns.sciencehorizons.net> (raw)
+#define external_array(type, name) \
+ ({ \
+ extern type name[]; \
+ type *name_ptr = name; \
+ asm ("" : "+r" (name_ptr)); \
+ name_ptr; \
+ })
I've had to pull similar tricks to persuade GCC to generate
the code I wanted (in my case, it was optimization: "evaluate it
in this order, damn it!"), and I prefer to use the asm itself
with overlapping operands to do the assignment.
#define external_array(type, name) \
({ \
extern type name[]; \
type *name_ptr;
asm ("" : "=g" (name_ptr) : "0" (name)); \
name_ptr; \
})
You could define a wrapper if you like, something like
/*
* Assign dst = src, but prevent the compiler from inferring anything
* about the assigned value, so it can't do any unwanted optimization.
*/
#define blind_assign(dst,src) asm("" : "=X" (dst) : "0" (src))
In case it helps, here's a list of architecture-independent operand
constraints (that I think is exhaustive, but I'm not 100% sure):
r - general-purpose register
f - floating-point register
m - memory
o - offsettable memory (excluding pre/post inc/decrement modes)
i - immediate
g - "general", any of the above
X - "wildcard". This matches anything at all, including special-purpose
registers that are not included in "r".
next reply other threads:[~2016-06-28 12:23 UTC|newest]
Thread overview: 9+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-06-28 12:23 George Spelvin [this message]
2016-06-28 20:08 ` [PATCH] firmware: declare __{start,end}_builtin_fw as pointers Linus Torvalds
-- strict thread matches above, loose matches on Subject: below --
2016-06-25 15:04 Vegard Nossum
2016-06-25 21:06 ` Vegard Nossum
2016-06-26 9:24 ` Vegard Nossum
2016-06-26 17:17 ` Linus Torvalds
2016-10-14 5:52 ` Jiri Slaby
2016-10-14 6:25 ` Vegard Nossum
2016-10-14 6:30 ` Jiri Slaby
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