linux-kernel - Re: [PATCH] firmware: declare __{start,end}_builtin

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Message-ID: <20160628122350.6688.qmail@ns.sciencehorizons.net>
Date:	28 Jun 2016 08:23:50 -0400
From:	"George Spelvin" <linux@...encehorizons.net>
To:	vegard.nossum@...il.com
Cc:	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] firmware: declare __{start,end}_builtin_fw as pointers

+#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".