Re: [PATCH] libgccjit: Fix infinite recursion in gt_ggc_mx_lang_tree_node

[David Malcolm <dmalcolm@redhat.com>]

On Thu, 2022-06-02 at 21:23 -0400, Antoni Boucher via Gcc-patches
wrote:
> Sorry, forgot to attach the patch.
> 
> Here it is.
> 
> On Thu, 2022-06-02 at 21:20 -0400, Antoni Boucher via Jit wrote:
> > Hi.
> > The attached patch fix bug 105827:
> > https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105827
> > 
> > I'm not sure how to test this, so please share ideas.

Do you have a reproducer for this?

With garbage-collections issues in libgccjit, you can set:

  gcc_jit_context_set_bool_option (ctxt,
                                   GCC_JIT_BOOL_OPTION_SELFCHECK_GC,
                                   1);

which will force a full garbage collection at every opportunity that
the collector considers doing one (rather than following heuristics)

This really slows things down, but makes reproducing crashes much more
deterministic, often turning "it crashes every now and then" to "it
crashes every time" (and the test suite runs with this enabled).

> > 
> > Thanks for the review.
> 

> diff --git a/gcc/jit/dummy-frontend.cc b/gcc/jit/dummy-frontend.cc
> index 84ff359bfe3..8bb5d03d630 100644
> --- a/gcc/jit/dummy-frontend.cc
> +++ b/gcc/jit/dummy-frontend.cc
> @@ -506,13 +506,14 @@ struct GTY(()) lang_identifier
>  
>  /* The resulting tree type.  */
>  
> +/* See lang_tree_node in gcc/c/c-decl.cc.  */
>  union GTY((desc ("TREE_CODE (&%h.generic) == IDENTIFIER_NODE"),
> -	   chain_next ("CODE_CONTAINS_STRUCT (TREE_CODE (&%h.generic), TS_COMMON) ? ((union lang_tree_node *) TREE_CHAIN (&%h.generic)) : NULL")))
> -lang_tree_node
> -{
> -  union tree_node GTY((tag ("0"),
> -		       desc ("tree_node_structure (&%h)"))) generic;
> -  struct lang_identifier GTY((tag ("1"))) identifier;
> +       chain_next ("(union lang_tree_node *) jit_tree_chain_next (&%h.generic)"))) lang_tree_node
> + {
> +  union tree_node GTY ((tag ("0"),
> +			desc ("tree_node_structure (&%h)")))
> +    generic;
> +  struct lang_identifier GTY ((tag ("1"))) identifier;
>  };

Those GTY markings on gcc/jit/dummy-frontend.cc's lang_tree_node union
have been like that since my initial proof-of-concept patch back in
2013:
  https://gcc.gnu.org/legacy-ml/gcc-patches/2013-10/msg00228.html
so presumably I simply copied and pasted that from another frontend
when I was initially prototyping libgccjit.  There was an element of
"cargo cult programming" as I was getting the project started.

Jakub had changed the C and C++ frontends in June 2011 with
563007852e8d19b66ec8c1e42e431efaaa967dc6, which introduced the
c_tree_chain_next that you're now copying in your patch, so presumably
I copied from a different frontend.  My notes say I created the first
prototype in July 2013, so that's when I would have copied&pasted the
code.

Dave