Re: [cxx-mem-model] bitfield tests

[Aldy Hernandez <aldyh at redhat dot com>]

> The memory model is not implementable on strict-alignment targets
> that do not have a byte store operation.  But we previously said that ;)

Yes.  I think we should issue an error when we have such a target and 
the user tries -fmemory-model=c++0x.  However, how many strict-alignment 
targets are not byte addressable nowadays?

> Also consider global vars
>
> char a;
> char b;
>
> accessing them on strict-align targets may access adjacent globals
> (that's a problem anyway, also with alias analysis).

Good point.  I am adding a test to that effect (see attached patch).

BTW, I assume you mean strict-align targets WITHOUT byte-addressability 
as above.  I have spot-checked your scenario on a handful of important 
targets that have strict alignment, and all of them work without 
touching adjacent global vars:

	arm-elf		OK
	sparc-linux	OK
	ia64-linux	OK
	alpha-linux	OK, but only with -mbwx (byte addressability)

rth tells me that we shouldn't worry about ancient non-byte addressable 
Alphas, so the last isn't an issue.

So... do you have any important targets in mind, because I don't see 
this being a problem for most targets?  As can be expected, I am only 
interested in x86*, powerpc*, and s390, especially since a cursory 
glance on other important targets didn't exhibit any problems.  However, 
given my target bias, I am willing to look into any important targets 
that are problematic (I'm hoping none :)).

Let me know if you see anything else, and please take a quick peek at 
the attached patch below, which I will be committing shortly.

As usual, thanks.
Aldy

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