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


On Wed, Nov 29, 2017 at 08:56:44AM -0700, Martin Sebor 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.

I also sent https://gcc.gnu.org/ml/gcc-patches/2017-05/msg00491.html a
while back, sorry I haven't gotten to  improving it yet though its gcc 9
stuff at this point I guess.

thanks

Trev

> 
> Martin


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