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Date: Thu, 11 Aug 2011 15:13:41 +0200
From: Paolo Bonzini <pbonzini@redhat.com>
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Subject: Re: [Qemu-devel] [PATCH] tcg: Reload local variables after return
	from longjmp
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To: Peter Maydell <peter.maydell@linaro.org>
Cc: Blue Swirl <blauwirbel@gmail.com>, TeLeMan <geleman@gmail.com>, Jan Kiszka <jan.kiszka@web.de>, qemu-devel <qemu-devel@nongnu.org>, David Gilbert <david.gilbert@linaro.org>

On 08/11/2011 02:40 PM, Peter Maydell wrote:
> "All accessible objects have values [...] as of the time the longjmp
> function was called, except that the values of objects of automatic
> storage duration that are local to the function containing the
> invocation of the corresponding setjmp macro that do not have
> volatile-qualified type and have been changed between the setjmp
> invocation and longjmp call are indeterminate."
>   -- C99 section 7.13.2.1 para 3.
>
> So variables may only be destroyed if they are all of:
>   * local to the function calling setjmp
>   * not volatile
>   * changed between setjmp and longjmp

I didn't remember this third part.  Thanks for bringing up facts. :)

> We don't change env between the setjmp and longjmp so the compiler
> should not trash it. (Indeed according to Jan in
> http://lists.gnu.org/archive/html/qemu-devel/2011-07/msg00144.html
> -Wclobbered doesn't complain about this code.)

Then it's a compiler bug, not smartness.  Making env volatile (or making 
a volatile copy if there is a performance impact) should still be enough 
to work around it.

Paolo