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Re: [PATCH] Fix missed DSE opportunity with operator delete.
- From: Richard Biener <richard dot guenther at gmail dot com>
- To: Mikhail Maltsev <maltsevm at gmail dot com>
- Cc: gcc-patches mailing list <gcc-patches at gcc dot gnu dot org>, Marc Glisse <marc dot glisse at inria dot fr>
- Date: Wed, 20 Apr 2016 16:12:28 +0200
- Subject: Re: [PATCH] Fix missed DSE opportunity with operator delete.
- Authentication-results: sourceware.org; auth=none
- References: <5712AF84 dot 5080002 at gmail dot com> <CAFiYyc2wz_a-92BbvMGa5mtUtWTgk2wafxWmmzum1r_t-_-vnw at mail dot gmail dot com> <5716998B dot 8010907 at gmail dot com>
On Tue, Apr 19, 2016 at 10:48 PM, Mikhail Maltsev <maltsevm@gmail.com> wrote:
> On 04/18/2016 12:14 PM, Richard Biener wrote:
>>
>> Enlarging tree_function_decl is bad.
> Probably using 3 bits for malloc_flag, operator_new_flag and free_flag is
> redundant. I packed the state into 2 bits.
>>
>> Passes should get at the info via flags_from_decl_or_type () and a new
>> ECF_FREE.
> Fixed.
Thanks - much better.
@@ -2117,6 +2127,13 @@ call_may_clobber_ref_p_1 (gcall *call, ao_ref *ref)
/* Fallthru to general call handling. */;
}
+ if (callee != NULL_TREE
+ && (flags_from_decl_or_type (callee) & ECF_FREE) != 0)
+ {
as you have a stmt here please use gimple_call_flags (call) & ECF_FREE.
@@ -2402,6 +2409,16 @@ stmt_kills_ref_p (gimple *stmt, ao_ref *ref)
default:;
}
+
+ if (callee != NULL_TREE
+ && (flags_from_decl_or_type (callee) & ECF_FREE) != 0)
+ {
Likewise.
@@ -1728,6 +1729,15 @@ ref_maybe_used_by_call_p_1 (gcall *call, ao_ref *ref)
/* Fallthru to general call handling. */;
}
+ /* free-like functions may not reference their first argument. */
+ if (callee != NULL_TREE && (flags & ECF_FREE) != 0)
+ {
+ tree ptr = gimple_call_arg (call, 0);
+ tree base = ao_ref_base (ref);
+ if (base && TREE_CODE (base) == MEM_REF && TREE_OPERAND (base, 0) == ptr)
+ return false;
+ }
+
So this is less aggressive than what we do for BUILT_IN_FREE which simply
returns false as "not reading from (any) memory". I suspect we might want
to amend the documentation of the "free" attribute to that effect, or find
a better wording ...
Otherwise using DECL_SET_MALLOC sometimes and sometimes
DECL_ALLOC_FN_KIND () = ALLOC_FN_MALLOC looks somewhat
inconsistent. I'd prefer removing DECL_SET_MALLOC.
You have
+static tree
+handle_free_attribute (tree *node, tree name, tree /*args*/, int /*flags*/,
+ bool *no_add_attrs)
+{
+ tree decl = *node;
+ if (TREE_CODE (decl) == FUNCTION_DECL
+ && type_num_arguments (TREE_TYPE (decl)) != 0
+ && POINTER_TYPE_P (TREE_VALUE (TYPE_ARG_TYPES (TREE_TYPE (decl)))))
+ DECL_ALLOC_FN_KIND (decl) = ALLOC_FN_FREE;
+ else
+ {
+ warning_at (DECL_SOURCE_LOCATION (decl), OPT_Wattributes,
+ "%qE attribute ignored", name);
+ *no_add_attrs = true;
+ }
so one can happily apply the attribute to
void foo (void *, void *);
but then
@@ -2117,6 +2127,13 @@ call_may_clobber_ref_p_1 (gcall *call, ao_ref *ref)
/* Fallthru to general call handling. */;
}
+ if (callee != NULL_TREE
+ && (flags_from_decl_or_type (callee) & ECF_FREE) != 0)
+ {
+ tree ptr = gimple_call_arg (call, 0);
+ return ptr_deref_may_alias_ref_p_1 (ptr, ref);
+ }
will ignore the 2nd argument. I think it's better to ignore the attribute
if type_num_arguments () != 1.
Richard.
> --
> Regards,
> Mikhail Maltsev