Alignment issue on x86_64?

John Fine johnsfine@verizon.net
Tue Jun 24 17:56:00 GMT 2008


Andrew Haley wrote:
>
>
> If so, it's a gcc bug either way.  Both caller and callee were compiled
> by gcc.  But we don't have enough information to determine what the cause
> of the problem really is.
>   
Sorry that I'm not taking the time to get a copy of the called code and 
I'm instead speculating.

But, it is perfectly possible for the reported symptom to be a bug in 
the called code, rather than in gcc.  Here is the simplest bug the 
called code might have fitting all described symptoms (not saying it has 
this bug, just saying we haven't checked that it doesn't).

It could internally use a reference to the passed CARD32 and then cast 
that (CARD32&) to a (long&) before using the value.

In a 32 bit build, that would always work, because a long is 32 bits.

In a 64 bit build, that would usually work, because most ways that the 
compilation of the calling code might place a 32 bit value in a 64 bit 
location would clear the high bits as an (unnecessary I think) side 
effect.  But sometimes it would fail.  And always it would be a bug, 
because the C++ standard doesn't let you expect defined behavior from 
such an action.



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