[PATCH] D56358: [AST] Move back BasePathSize to the bit-fields of CastExpr

[Bruno Ricci via Phabricator via cfe-commits]

riccibruno marked an inline comment as done.
riccibruno added inline comments.


================
Comment at: include/clang/AST/Expr.h:3029
+    assert((CastExprBits.BasePathSize == BasePathSize) &&
+           "BasePathSize overflow!");
     assert(CastConsistency());
----------------
lebedev.ri wrote:
> riccibruno wrote:
> > lebedev.ri wrote:
> > > rjmccall wrote:
> > > > riccibruno wrote:
> > > > > lebedev.ri wrote:
> > > > > > riccibruno wrote:
> > > > > > > It cannot overflow now, but someone in the future is going
> > > > > > > to shrink `CastExprBitfields::BasePathSize`.
> > > > > > Can you move that static assert here instead of deleting it?
> > > > > I am not sure it is really useful since when someone will want to
> > > > > shrink it, it will be stored as `unsigned BasePathSite : x;`. What we
> > > > > don't want is having `x` too small. However `numeric_limits` will
> > > > > be based on the underlying type of the bit-field and so will never fail.
> > > > > 
> > > > > Storing it as something other than an unsigned, say a short, would work,
> > > > > but it will pack poorly with MSVC.
> > > > > 
> > > > > Hopefully the comment in `CastExprBitfields` will tell people to not shrink it
> > > > > mindlessly.
> > > > I agree that the type-based numeric limit is not useful if we're anticipating
> > > > it becoming a bit-field.  The comment seems fine since it's in the code that
> > > > would need to be edited.
> > > I'm just worried that now there will be less of a test coverage,
> > > The actual run-time test already does not test template instantiation with depth of `16384`, but much less.
> > Would it make sense to go systematically through the limits recommended by
> > [implimits], and for each limit devise a test which will exercise it ?
> > 
> > Or would this take too long to be a test ?
> > 
> > It would perhaps also be nice to document these limits somewhere,
> > even if it is just "You can rely on [implimits]".
> > Would it make sense to go systematically through the limits recommended by
> > [implimits], and for each limit devise a test which will exercise it ?
> 
> Better test coverage is always good.
> 
> > Or would this take too long to be a test ?
> 
> The problem is, even this you can't really test with a run-time test.
> Template instantiation is recursive, so in most environments you overflow
> your stack long before you reach the recommended depth of `16384`.
> 
But you can perhaps test this without doing any template instantiation.
The limit is "Direct and indirect base classes [16384]". Therefore you can
just construct 2^16 classes with macros which should not be recursive,
and then somehow construct a class with all of these classes as a base
(although this step is perhaps recursive).


Repository:
  rC Clang

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  https://reviews.llvm.org/D56358/new/

https://reviews.llvm.org/D56358