Patch to force SuitableAlign's alignment when loading on object with larger alignment

[jahanian]

Hi John,

Here is the revised patch.

	
- Fariborz

On Jul 28, 2014, at 6:58 PM, John McCall <rjmccall at apple.com> wrote:

> On Jul 28, 2014, at 4:22 PM, jahanian <fjahanian at apple.com> wrote:
>> This patch enforces SuitableAlign’s alignment when loading objects with more relaxed alignment. Currently, SuitableAlign is used in a
>> warning when type is over aligned. This patch optionally enforces this in IRGen. I defined a new field in TargetInfo which is optionally set to
>> SuitableAlign. Currently, this is only defined for Apple’s targets as I don’t know its implication for all other supported targets. Long term,
>> we want to use one value for both (Sema and code gen). This is rdar://16254558
> 
> We can’t change the ABI alignment; that would change struct layout.
> 
> The right way of doing this is a code generation option that caps the effective alignment that we’ll honor on an access to a pointer lacking an explicit alignment attribute.  It should be exposed to users; I suggest spelling it something like -fmax-type-align=N, with an option to disable it completely called -fno-max-type-align.  You can have the driver pass down -fmax-type-align=16 by default when targeting Darwin.
> 
> +  static bool RestrictedCompleteObjectAlign(QualType T) {
> 
> What this is really asking is whether the type has an explicit alignment attribute.  The most reasonable thing to do is to have ASTContext::getTypeInfoImpl compute (and cache) this information; that will correctly look through recursive sugar, which you are not doing.  You can add a new accessor (ASTContext::getTypeAlignInCharsAndIsExplicit?) which returns a pair of the alignment and this bool.
> 
> You should add a test case that checks that a typedef of an aligned typedef is still considered explicitly aligned.
> 
> There is nothing here that should be specific to ElaboratedType.  Whatever that logic is, it should be testing for RecordType.  But I actually think you should be applying the same logic to structs and unions that you do to any other type: we should be capping the (non-ABI) alignment of structs and unions unless they have an explicit AlignedAttr.
> 
> John.

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