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SubjectRe: ARM unaligned MMIO access with attribute((packed))
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From: Russell King - ARM Linux <linux@arm.linux.org.uk>
Date: Wed, 2 Feb 2011 16:37:02 +0000

> 1. there's no way to tell GCC that the inline assembly is a load
> instruction and therefore it needs to schedule the following
> instructions appropriately.

Just add a dummy '"m" (pointer)' asm input argument to the inline asm
statement. Just make sure "typeof(pointer)" has a size matching the
size of the load your are performing.

> 2. GCC will needlessly reload pointers from structures and other such
> behaviour because it can't be told clearly what the inline assembly
> is doing, so the inline asm needs to have a "memory" clobber.

This behavior is correct, and in fact needed. Writing to chip registers
can trigger changes to arbitrary main memory locations.

> 3. It seems to misses out using the pre-index addressing, prefering to
> create add/sub instructions prior to each inline assembly load/store.

Yes, this is indeed a problem.

But you really need that memory clobber there whether you like it or
not, see above.


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