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From: Alex =?utf-8?Q?Benn=C3=A9e?= <alex.bennee@linaro.org>
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Date: Tue, 01 Apr 2014 13:09:17 +0100
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Subject: Re: [Qemu-devel] [PATCH 1/8] exec-all.h: Use stl_p to avoid
	undefinedbehaviour patching x86 jumpss
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To: Richard Henderson <rth@twiddle.net>
Cc: peter.maydell@linaro.org, qemu-devel@nongnu.org, aurelien@aurel32.net


Richard Henderson <rth@twiddle.net> writes:

> From: Peter Maydell <peter.maydell@linaro.org>
>
> The code which patches x86 jump instructions assumes it can do an
> unaligned write of a uint32_t. This is actually safe on x86, but it's
> still undefined behaviour. We have infrastructure for doing efficient
> unaligned accesses which doesn't engage in undefined behaviour, so
> use it.
>
> This is technically fractionally less efficient, at least with gcc 4.6;
> instead of one instruction:
>  7b2:   89 3e                   mov    %edi,(%rsi)
> we get an extra spurious store to the stack slot:
>  7b2:   89 7c 24 64             mov    %edi,0x64(%rsp)
>  7b6:   89 3e                   mov    %edi,(%rsi)

Ehh? Is that gcc just being silly and putting parameters for an inline
on the stack frame?

>
> Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
> Signed-off-by: Richard Henderson <rth@twiddle.net>
<snip>

Reviewed-by: Alex Bennée <alex.bennee@linaro.org>

-- 
Alex Bennée