[PATCH] PR63300 'const volatile' sometimes stripped in debug info.

Mark Wielaard mjw@redhat.com
Wed Sep 24 18:42:00 GMT 2014

On Wed, 2014-09-24 at 14:11 -0400, Jason Merrill wrote:
> > Anyway. I don't think this should hold up this specific bug fix. The
> > issue with type units vs qualified type DIEs has been there in all
> > previous versions of GCC (and only when using non-default flags). It
> > really comes from DWARF not specifying anything about the ordering of
> > type modifier tags.
> But GCC has been consistent about the ordering, and I would prefer not 
> to change that;

Sorry, I think I am missing what part was consistent that is being
changed now.

As my example shows even with older GCCs depending on whether a const
type or volatile type was encountered first a const volatile type comes
out either as a DIE chain const_type DIE -> volatile_type DIE -> type
DIE, or the other way around, in different compile units.

I believe this has always been done this way (at least since 4.6) and
the order in different compile units was not "consistent". It was just
optimal, preferring the shortest path to an already existing qualified
type in that compile unit if there was one. The only thing that has
changed is that we handle more qualifier modifiers now (well actually
only one for now, restrict).

What the patch fixes is a bug (PR63300, that I introduced, sorry) where
we would completely drop all qualifiers. It doesn't change making the
ordering of the type qualifier tags more or less consistent between
compile units I believe.

>  I would expect it to hurt dwz compression as well as type units.

dwz is somewhat similar to type units in this respect. But unlike type
units it doesn't prescribe using an ordered flattened DIE chain. So (in
theory) it can just swap around qualifier type tags if that is
beneficial for merging more type DIE trees.

But maybe I am missing something that makes things worse/different from
how they were before?



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