Re: [RFC][PATCH] Extend DCE to remove unnecessary new/delete-pairs

[Richard Biener <richard.guenther@gmail.com>]

On November 29, 2017 4:56:44 PM GMT+01:00, Martin Sebor <msebor@gmail.com> wrote:
>On 11/29/2017 01:30 AM, Jakub Jelinek wrote:
>> On Tue, Nov 28, 2017 at 09:11:00PM -0700, Martin Sebor wrote:
>>> On 11/27/2017 02:22 AM, Dominik Inführ wrote:
>>>> Thanks for all the reviews! I’ve revised the patch, the
>operator_delete_flag is now stored in tree_decl_with_vis (there already
>seem to be some FUNCTION_DECL-flags in there). I’ve also added the
>option -fallocation-dce to disable this optimization. It bootstraps and
>no regressions on aarch64 and x86_64.
>>>>
>>> It's great to be able to eliminate pairs of these calls.  For
>>> unpaired calls, though, I think it would be even more useful to
>>> also issue a warning.  Otherwise the elimination will mask bugs
>>
>> ??  I hope you're only talking about allocation where the returned
>> pointer can't leak elsewhere, doing allocation in one function
>> (e.g. constructor, or whatever other function) and deallocation in
>some
>> other one is so common such a warning would be not just useless, but
>> harmful with almost all occurrences being false positives.
>>
>> Warning on malloc/standard operator new or malloc/realloc-like
>function
>> when the return pointer can't escape the current function is
>reasonable.
>
>Yes, warn for leaks, or for calls to delete/free with no matching
>new/malloc (when they can be detected).
>
> From the test case included in the patch, warn on the first two
>of the following three functions:
>
>+++ b/gcc/testsuite/g++.dg/cpp1y/new1.C
>@@ -0,0 +1,65 @@
>+/* { dg-do compile } */
>+/* { dg-options "-O2 -fdump-tree-cddce-details" } */
>+
>+#include <stdlib.h>
>+
>+void
>+new_without_use() {
>+  int *x = new int;
>+}
>+
>+void
>+new_array_without_use() {
>+  int *x = new int[5];
>+}
>+
>+void
>+new_primitive() {
>+  int *x = new int;
>+  delete x;
>+}
>
>An obvious extension to such a checker would then be to also detect
>possible invalid deallocations, as in:
>
>   void f (unsigned n)
>   {
>     void *p = n < 256 ? alloca (n) : malloc (n);
>     // ...
>     free (p);
>   }
>
>David Malcolm was working on something like that earlier this year
>so he might have some thoughts on this as well.

Or

P = new x;
Free (P) ;

Richard. 

>Martin