[PATCH] teach compute_objsize about placement new (PR 100876)

Jeff Law jeffreyalaw@gmail.com
Sun Jun 13 23:45:36 GMT 2021



On 6/2/2021 3:40 PM, Martin Sebor via Gcc-patches wrote:
> The two forms of placement operator new defined in <new> return their
> pointer argument and may not be displaced by user-defined functions.
> But because they are ordinary (not built-in) functions this property
> isn't reflected in their declarations alone, and there's no user-
> level attribute to annotate them with.  When they are inlined
> the property is transparent in the IL but when they are not (without
> inlining such as -O0), calls to the operators appear in the IL and
> cause -Wmismatched-new-delete to try to match them with the functions
> called to deallocate memory.  When the pointer to the memory was
> obtained from a function that matches the deallocator but not
> the placement new, the warning falsely triggers.
>
> The attached patch solves this by detecting calls to placement new
> and treating them the same as those to other pass-through calls (such
> as memset).  In addition, it also teaches -Wfree-nonheap-object about
> placement delete, for a similar reason as above.  Finally, it also
> adds a test for attribute fn spec indicating a function returns its
> argument.  It's not necessary for the fix (I had initially though
> placement new might have the attribute) but it seems appropriate
> to check.
>
> Tested on x86_64-linux.
>
> Martin
>
> gcc-100876.diff
>
> PR c++/100876 - -Wmismatched-new-delete should understand placement new when it's not inlined
>
> gcc/ChangeLog:
>
> 	PR c++/100876
> 	* builtins.c (gimple_call_return_array): Check for attribute fn spec.
> 	Handle calls to placement new.
> 	(ndecl_dealloc_argno): Avoid placement delete.
>
> gcc/testsuite/ChangeLog:
>
> 	PR c++/100876
> 	* g++.dg/warn/Wmismatched-new-delete-4.C: New test.
> 	* g++.dg/warn/Wmismatched-new-delete-5.C: New test.
> 	* g++.dg/warn/Wstringop-overflow-7.C: New test.
> 	* g++.dg/warn/Wfree-nonheap-object-6.C: New test.
> 	* g++.dg/analyzer/placement-new.C: Prune out expected warning.
>
> diff --git a/gcc/builtins.c b/gcc/builtins.c
> index af1fe49bb48..fb0717a0248 100644
> --- a/gcc/builtins.c
> +++ b/gcc/builtins.c
> @@ -5159,11 +5159,43 @@ static tree
>   gimple_call_return_array (gimple *stmt, offset_int offrng[2],
>   			  range_query *rvals)
>   {
> -  if (!gimple_call_builtin_p (stmt, BUILT_IN_NORMAL)
> -      || gimple_call_num_args (stmt) < 1)
> +  {
> +    /* Check for attribute fn spec to see if the function returns one
> +       of its arguments.  */
> +    attr_fnspec fnspec = gimple_call_fnspec (as_a <gcall *>(stmt));
> +    unsigned int argno;
> +    if (fnspec.returns_arg (&argno))
> +      {
> +	offrng[0] = offrng[1] = 0;
> +	return gimple_call_arg (stmt, argno);
> +      }
> +  }
> +
> +  if (gimple_call_num_args (stmt) < 1)
>       return NULL_TREE;
Nit.  You've got an unnecessary {} at the outer level of this hunk.

OK with the nit fixed.

THanks,
Jeff



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