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Re: [PATCH][RFC] Handle realloc in PTA and alias analysis
- From: Richard Biener <rguenther at suse dot de>
- To: gcc-patches at gcc dot gnu dot org
- Date: Wed, 21 May 2014 15:39:30 +0200 (CEST)
- Subject: Re: [PATCH][RFC] Handle realloc in PTA and alias analysis
- Authentication-results: sourceware.org; auth=none
- References: <alpine dot LSU dot 2 dot 11 dot 1405211448510 dot 13304 at zhemvz dot fhfr dot qr>
On Wed, 21 May 2014, Richard Biener wrote:
>
> PR56955 prompted me to handle BUILT_IN_REALLOC just the same
> way we already handle BUILT_IN_STR[N]DUP.
>
> Bootstrap and regtest running on x86_64-unknown-linux-gnu.
>
> Now this will disambiguate *p and *q for p = realloc (q, n)
> for any value of n (including those that don't actually
> trigger re-allocation and thus where p == q after the call).
> I don't think that any such use would be valid - but I can
> certainly play safer here and implement the points-to part
> as a pass-through (that is, make p point to what q points).
> That's of course less optimization.
Like with incremental
Index: gcc/tree-ssa-structalias.c
===================================================================
--- gcc/tree-ssa-structalias.c.orig 2014-05-21 15:37:58.762890034
+0200
+++ gcc/tree-ssa-structalias.c 2014-05-21 15:35:52.044898758 +0200
@@ -4313,6 +4313,17 @@ find_func_aliases_for_builtin_call (stru
process_all_all_constraints (lhsc, rhsc);
lhsc.release ();
rhsc.release ();
+ /* For realloc the resulting pointer can be equal to the
+ argument as well. But only doing this wouldn't be
+ correct because with ptr == 0 realloc behaves like malloc.
*/
+ if (DECL_FUNCTION_CODE (fndecl) == BUILT_IN_REALLOC)
+ {
+ get_constraint_for (gimple_call_lhs (t), &lhsc);
+ get_constraint_for (gimple_call_arg (t, 0), &rhsc);
+ process_all_all_constraints (lhsc, rhsc);
+ lhsc.release ();
+ rhsc.release ();
+ }
return true;
}
break;
that of course makes it a somewhat pointless excercise if
points-to doesn't figure out sth fancy for the argument to
realloc (like its NULL or the result of an earlier malloc call,
still optimizes the testcase as expected).
> I can't quite find language that specifies that the
> object can no longer accessed via the pointer argument q
> (at least if p didn't return NULL and size was not NULL).
> The C99 standard explicitely mentions that p may have
> the same pointer value as q though.
>
> Thoughts?
>
> Thanks,
> Richard.
>
> 2014-05-21 Richard Biener <rguenther@suse.de>
>
> * tree-ssa-alias.c (ref_maybe_used_by_call_p_1): Handle
> BUILT_IN_REALLOC like BUILT_IN_STRDUP.
> (call_may_clobber_ref_p_1): Likewise.
> * tree-ssa-structalias.c (find_func_aliases_for_builtin_call):
> Likewise.
> (handle_lhs_call): Use flags argument instead of recomputing it.
> (find_func_aliases_for_call): Call handle_lhs_call with proper
> call return flags.
>
> * gcc.dg/tree-ssa/alias-33.c: New testcase.
>
> Index: gcc/tree-ssa-alias.c
> ===================================================================
> *** gcc/tree-ssa-alias.c.orig 2014-05-21 14:38:57.841133822 +0200
> --- gcc/tree-ssa-alias.c 2014-05-21 14:39:05.954133263 +0200
> *************** ref_maybe_used_by_call_p_1 (gimple call,
> *** 1594,1599 ****
> --- 1594,1600 ----
> /* These read memory pointed to by the first argument. */
> case BUILT_IN_STRDUP:
> case BUILT_IN_STRNDUP:
> + case BUILT_IN_REALLOC:
> {
> ao_ref dref;
> tree size = NULL_TREE;
> *************** call_may_clobber_ref_p_1 (gimple call, a
> *** 1958,1963 ****
> --- 1959,1965 ----
> case BUILT_IN_MALLOC:
> case BUILT_IN_ALIGNED_ALLOC:
> case BUILT_IN_CALLOC:
> + case BUILT_IN_REALLOC:
> case BUILT_IN_STRDUP:
> case BUILT_IN_STRNDUP:
> /* Unix98 specifies that errno is set on allocation failure. */
> Index: gcc/tree-ssa-structalias.c
> ===================================================================
> *** gcc/tree-ssa-structalias.c.orig 2014-05-21 14:38:57.844133822 +0200
> --- gcc/tree-ssa-structalias.c 2014-05-21 14:45:50.896105384 +0200
> *************** handle_lhs_call (gimple stmt, tree lhs,
> *** 3974,3980 ****
>
> /* If the call returns an argument unmodified override the rhs
> constraints. */
> - flags = gimple_call_return_flags (stmt);
> if (flags & ERF_RETURNS_ARG
> && (flags & ERF_RETURN_ARG_MASK) < gimple_call_num_args (stmt))
> {
> --- 3974,3979 ----
> *************** find_func_aliases_for_builtin_call (stru
> *** 4299,4307 ****
> return true;
> case BUILT_IN_STRDUP:
> case BUILT_IN_STRNDUP:
> if (gimple_call_lhs (t))
> {
> ! handle_lhs_call (t, gimple_call_lhs (t), gimple_call_flags (t),
> vNULL, fndecl);
> get_constraint_for_ptr_offset (gimple_call_lhs (t),
> NULL_TREE, &lhsc);
> --- 4298,4308 ----
> return true;
> case BUILT_IN_STRDUP:
> case BUILT_IN_STRNDUP:
> + case BUILT_IN_REALLOC:
> if (gimple_call_lhs (t))
> {
> ! handle_lhs_call (t, gimple_call_lhs (t),
> ! gimple_call_return_flags (t) | ERF_NOALIAS,
> vNULL, fndecl);
> get_constraint_for_ptr_offset (gimple_call_lhs (t),
> NULL_TREE, &lhsc);
> *************** find_func_aliases_for_call (struct funct
> *** 4535,4541 ****
> else
> handle_rhs_call (t, &rhsc);
> if (gimple_call_lhs (t))
> ! handle_lhs_call (t, gimple_call_lhs (t), flags, rhsc, fndecl);
> rhsc.release ();
> }
> else
> --- 4536,4543 ----
> else
> handle_rhs_call (t, &rhsc);
> if (gimple_call_lhs (t))
> ! handle_lhs_call (t, gimple_call_lhs (t),
> ! gimple_call_return_flags (t), rhsc, fndecl);
> rhsc.release ();
> }
> else
> Index: gcc/testsuite/gcc.dg/tree-ssa/alias-33.c
> ===================================================================
> *** /dev/null 1970-01-01 00:00:00.000000000 +0000
> --- gcc/testsuite/gcc.dg/tree-ssa/alias-33.c 2014-05-21 14:39:06.003133260 +0200
> ***************
> *** 0 ****
> --- 1,20 ----
> + /* { dg-do run } */
> + /* { dg-options "-O -fdump-tree-fre1-details" } */
> +
> + int j;
> + int main ()
> + {
> + int i = 1;
> + int **p;
> + j = 0;
> + p = __builtin_malloc (sizeof (int *));
> + *p = &i;
> + p = __builtin_realloc (p, 2 * sizeof (int *));
> + **p = 0;
> + if (i != 0)
> + __builtin_abort ();
> + return j;
> + }
> +
> + /* { dg-final { scan-tree-dump "Replaced j with 0" "fre1" } } */
> + /* { dg-final { cleanup-tree-dump "fre1" } } */
>