On Wed, Jun 5, 2013 at 8:06 PM, Eli Friedman <[email protected]> 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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