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

[Reid Kleckner]

What is HasAlignAttribute, and how is it different from
AlignmentIsRequired? I have a feeling that they are different names for the
same thing. If you want to rename it, that's fine with me.


On Fri, Aug 1, 2014 at 10:45 AM, jahanian <fjahanian at apple.com> wrote:

> 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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