Re: [PATCH] c++: do not hash TYPENAME_TYPEs on pointers [PR124811]

On Mon, 10 Aug 2026, Jason Merrill wrote:

> On 8/7/26 9:55 AM, Richard Biener wrote:
> > From: Bernhard M. Wiedemann <[email protected]>
> > 
> > typename_htab is written to a precompiled header.  A hash table is
> > streamed out slot array and all: gt_pch_nx() relocates the pointers
> > inside the entries but leaves every entry in the slot it happened to
> > occupy.  typename_hasher hashed on the addresses of the scope and the
> > fullname, so the slots were chosen from addresses that ASLR randomises
> > in the process writing the header, and they no longer correspond to the
> > hash of anything once the header has been read back at a different
> > address.
> > 
> > Lookups then find a restored TYPENAME_TYPE only when it happens to lie
> > on the probe sequence of the slot the new hash points at, so most miss
> > and build a duplicate, and which ones miss depends on the layout the
> > writing process had.  That makes a compile using a PCH differ from the
> > same compile without one, and differ from itself between runs: the
> > duplicates consume DECL_UIDs, every later DECL_UID shifts, and
> > var-tracking hashes on DECL_UID, so .debug_loclists comes out different.
> > 
> > Hash on TYPE_UID/DECL_UID and IDENTIFIER_HASH_VALUE instead, which the
> > header preserves.  The name replaces the fullname in the hash because a
> > TEMPLATE_ID_EXPR fullname has no address-independent hash of its own;
> > both are compared by equal() either way.
> 
> We could handle TEMPLATE_ID_EXPR by using iterative_hash_template_arg instead
> of iterative_hash_object?
> 
> But I suppose it's unlikely that we'd need a bunch of typenames that differ
> only in template args, so OK either way.

IIRC the PR65328 compile-time-hog testcase had a lot of TEMPLATE_ID_EXPR
typenames, and indeed this patch as-is significantly regresses compile
time of the PR65328#c6 testcase since it effectively undoes part of
r13-1047-g343d83c7a89d0c.  So I think we need to go with using
iterative_hash_template_arg instead (with comparing_specializations
set so that we produce distinct hashes for nested TYPENAME_TYPE).

> 
> > Compiling qgstopologicalmesh.cpp of qgis 4.2.1, 4757 lookups in the
> > table:
> > 
> >    no PCH                 3046 misses, 3046 entries
> >    PCH, before            483-484 misses, 3135-3136 entries, varies per run
> >    PCH, after             394 misses, 3046 entries
> > 
> > and the object file now matches the one built without a PCH, byte for
> > byte, from any of eight independently generated headers.
> > 
> > Bootstrapped and tested on x86_64-unknown-linux-gnu.
> > 
> > OK?
> > 
> > Thanks,
> > Richard.
> > 
> > gcc/cp/ChangeLog:
> > 
> > 	PR pch/124811
> > 	* decl.cc (typename_hasher::hash): Hash the UIDs of the context
> > 	and the name rather than their addresses.
> > 
> > Assisted-by: Claude
> > ---
> >   gcc/cp/decl.cc | 20 +++++++++++++-------
> >   1 file changed, 13 insertions(+), 7 deletions(-)
> > 
> > diff --git a/gcc/cp/decl.cc b/gcc/cp/decl.cc
> > index b5d9ed05874..c66f59fef79 100644
> > --- a/gcc/cp/decl.cc
> > +++ b/gcc/cp/decl.cc
> > @@ -4920,27 +4920,33 @@ struct typename_hasher : ggc_ptr_hash<tree_node>
> >   {
> >     typedef typename_info *compare_type;
> >   -  /* Hash a TYPENAME_TYPE.  */
> > +  /* Hash a TYPENAME_TYPE.  This table goes into a precompiled header,
> > which
> > +     moves everything it contains to a different address; entries keep the
> > +     slot they were put in, so hashing on the addresses of CONTEXT and NAME
> > +     would leave them unfindable once the header has been read back.  Hash
> > on
> > +     the UIDs instead, which the header preserves.  */

Since this is a common problem with GC'd hash tables and PCH and not
specific to this hash table, I don't think this comment is necessary
here FWIW.  It'd be better suited in a more central piece of
documentation such as in the gccint manual, if it's not already
documented there.

> >       static hashval_t
> > -  hash (tree context, tree fullname)
> > +  hash (tree context, tree name)
> >     {
> >       hashval_t hash = 0;
> > -    hash = iterative_hash_object (context, hash);
> > -    hash = iterative_hash_object (fullname, hash);
> > -    return hash;
> > +    if (context)
> > +      hash = iterative_hash_hashval_t (TYPE_P (context) ? TYPE_UID
> > (context)
> > +				       : DECL_P (context) ? DECL_UID (context)
> > +				       : 0, hash);
> > +    return iterative_hash_hashval_t (IDENTIFIER_HASH_VALUE (name), hash);
> >     }
> >       static hashval_t
> >     hash (const typename_info *ti)
> >     {
> > -    return typename_hasher::hash (ti->scope, ti->template_id);
> > +    return typename_hasher::hash (ti->scope, ti->name);
> >     }
> >       static hashval_t
> >     hash (tree t)
> >     {
> > -    return typename_hasher::hash (TYPE_CONTEXT (t), TYPENAME_TYPE_FULLNAME
> > (t));
> > +    return typename_hasher::hash (TYPE_CONTEXT (t), TYPE_IDENTIFIER (t));
> >     }
> >       /* Compare two TYPENAME_TYPEs.  */
> 
>