[PATCH] Fix alignment of data in TypeLocs (PR16144)

Richard Smith richard at metafoo.co.uk
Wed Jun 5 20:34:45 PDT 2013

On Wed, Jun 5, 2013 at 8:06 PM, Eli Friedman <eli.friedman at gmail.com> wrote:

> Patch attached; fixes PR16144.
> Currently, the data in TypeLocs consists of a bunch of tightly packed
> structures, and the structures can become misaligned because there isn't
> any padding.
> There are basically three possible approaches to fixing the alignment
> issues in TypeLocs:
> 1) Force every piece of the TypeLoc's data to have alignment 8.
> 2) Perform dynamic alignment adjustments.
> 3) Use #pragma pack to let the compiler know the data is intentionally
> misaligned.
> (1) has a substantial impact on memory usage (something like 1% on
> Cocoa.h), so I'd like to avoid it if possible.  (2) is the attached patch;
> it avoids both misaligned loads and unnecessary memory usage.  The primary
> downside is that TypeLocBuilder becomes a lot more complicated, because it
> doesn't know in advance where it needs to insert padding.  (3) keeps around
> to misaligned data: there's a potential performance penalty, it requires
> being careful not to introduce incorrect accesses to the data, and it's
> just plain ugly.
> Two questions to focus on for review: would (3) be a better approach?

I've tried this. We expose pointers into the type source info block in a
couple of places (for instance, the array of ParmVarDecl*s on a
FunctionTypeLoc) and the misalignment is then exposed to quite a large body
of code. Maybe a MisalignedArrayRef<...> would help, but the damage is
still not very contained.

And is there any way to make the TypeLocBuilder implementation a bit less
> ugly?

Perhaps we could remove the guarantee that the child locations of a TypeLoc
produced by push<T> are valid, or require some explicit action to fix them?
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