Re: [RFC] propagate malloc attribute in ipa-pure-const pass

[Jan Hubicka <hubicka@ucw.cz>]

> 
> * LTO and memory management
> This is a general question about LTO and memory management.
> IIUC the following sequence takes place during normal LTO:
> LGEN: generate_summary, write_summary
> WPA: read_summary, execute ipa passes, write_opt_summary
> 
> So I assumed it was OK in LGEN to allocate return_callees_map in
> generate_summary and free it in write_summary and during WPA, allocate
> return_callees_map in read_summary and free it after execute (since
> write_opt_summary does not require return_callees_map).
> 
> However with fat LTO, it seems the sequence changes for LGEN with
> execute phase takes place after write_summary. However since
> return_callees_map is freed in pure_const_write_summary and
> propagate_malloc() accesses it in execute stage, it results in
> segmentation fault.
> 
> To work around this, I am using the following hack in pure_const_write_summary:
> // FIXME: Do not free if -ffat-lto-objects is enabled.
> if (!global_options.x_flag_fat_lto_objects)
>   free_return_callees_map ();
> Is there a better approach for handling this ?

I think most passes just do not free summaries with -flto.  We probably want
to fix it to make it possible to compile multiple units i.e. from plugin by
adding release_summaries method...
So I would say it is OK to do the same as others do and leak it with -flto.
> diff --git a/gcc/ipa-pure-const.c b/gcc/ipa-pure-const.c
> index e457166ea39..724c26e03f6 100644
> --- a/gcc/ipa-pure-const.c
> +++ b/gcc/ipa-pure-const.c
> @@ -56,6 +56,7 @@ along with GCC; see the file COPYING3.  If not see
>  #include "tree-scalar-evolution.h"
>  #include "intl.h"
>  #include "opts.h"
> +#include "ssa.h"
>  
>  /* Lattice values for const and pure functions.  Everything starts out
>     being const, then may drop to pure and then neither depending on
> @@ -69,6 +70,15 @@ enum pure_const_state_e
>  
>  const char *pure_const_names[3] = {"const", "pure", "neither"};
>  
> +enum malloc_state_e
> +{
> +  PURE_CONST_MALLOC_TOP,
> +  PURE_CONST_MALLOC,
> +  PURE_CONST_MALLOC_BOTTOM
> +};

It took me a while to work out what PURE_CONST means here :)
I would just call it something like STATE_MALLOC_TOP... or so.
ipa_pure_const is outdated name from the time pass was doing only
those two.
> @@ -109,6 +121,10 @@ typedef struct funct_state_d * funct_state;
>  
>  static vec<funct_state> funct_state_vec;
>  
> +/* A map from node to subset of callees. The subset contains those callees
> + * whose return-value is returned by the node. */
> +static hash_map< cgraph_node *, vec<cgraph_node *>* > *return_callees_map;
> +

Hehe, a special case of return jump function.  We ought to support those more generally.
How do you keep it up to date over callgraph changes?
> @@ -921,6 +1055,23 @@ end:
>    if (TREE_NOTHROW (decl))
>      l->can_throw = false;
>  
> +  if (ipa)
> +    {
> +      vec<cgraph_node *> v = vNULL;
> +      l->malloc_state = PURE_CONST_MALLOC_BOTTOM;
> +      if (DECL_IS_MALLOC (decl))
> +	l->malloc_state = PURE_CONST_MALLOC;
> +      else if (malloc_candidate_p (DECL_STRUCT_FUNCTION (decl), v))
> +	{
> +	  l->malloc_state = PURE_CONST_MALLOC_TOP;
> +	  vec<cgraph_node *> *callees_p = new vec<cgraph_node *> (vNULL);
> +	  for (unsigned i = 0; i < v.length (); ++i)
> +	    callees_p->safe_push (v[i]);
> +	  return_callees_map->put (fn, callees_p);
> +	}
> +      v.release ();
> +    }
> +

I would do non-ipa variant, too.  I think most attributes can be detected that way
as well.

The patch generally makes sense to me.  It would be nice to make it easier to write such
a basic propagators across callgraph (perhaps adding a template doing the basic
propagation logic). Also I think you need to solve the problem with keeping your
summaries up to date across callgraph node removal and duplications.

Honza