On Tue, Jan 17, 2023 at 7:36 AM Alexey Lapshin via Gcc-patches
<gcc-patches@gcc.gnu.org> wrote:
>
> After updating to GCC newer than 11.4.0 we found that some code started
> to fail if it was built with size optimization (-Os).
> You can find testsuite for reproduction in the attached patch.
>
> The simplified version affected code looks like this:
>
> void alloc_function (unsigned char **data_p) {
> *data_p = malloc (8);
> assert(*data_p != NULL);
> }
> int main () {
> int *data;
> alloc_function (&data);
> printf ("data pointer is %p", data); // prints NULL(compile with -Os)
> }
This code is violating C/C++ aliasing rules.
You store to data via a "unsigned char*" and then do a load via "int*".
You can just use -fno-strict-aliasing if you want your code to just work.
Thanks,
Andrew Pinski
>
> If the type of passed argument is equal to the type in alloc_function
> declaration it works perfectly. Also helps change one or both types to
> void.
>
> I found that issue started to appear from commit
> d119f34c952f8718fdbabc63e2f369a16e92fa07
> if-statement which leads to this issue was found and after being
> removed seems it works well.
>
> Could you please elaborate on what cases exactly this checking should
> optimize?
> I think it should also contain at least one more check for accessing
> variable's memory to write..
>
>
> ---
> gcc/testsuite/gcc.dg/tree-ssa/alloc-in-func.c | 17 +++++++++++++++++
> gcc/tree-ssa-alias.cc | 2 --
> 2 files changed, 17 insertions(+), 2 deletions(-)
> create mode 100644 gcc/testsuite/gcc.dg/tree-ssa/alloc-in-func.c
>
> diff --git a/gcc/testsuite/gcc.dg/tree-ssa/alloc-in-func.c
> b/gcc/testsuite/gcc.dg/tree-ssa/alloc-in-func.c
> new file mode 100644
> index 00000000000..b30c1cedcb9
> --- /dev/null
> +++ b/gcc/testsuite/gcc.dg/tree-ssa/alloc-in-func.c
> @@ -0,0 +1,17 @@
> +/* { dg-do run } */
> +/* { dg-options "-Os" } */
> +
> +#define assert(x) if (!(x)) __builtin_abort ()
> +
> +static inline void alloc_function (unsigned char **data_p)
> +{
> + *data_p = (unsigned char *) __builtin_malloc (10);
> + assert (*data_p != (void *)0);
> +}
> +
> +int main ()
> +{
> + int *data = (void *)0;
> + alloc_function ((unsigned char **) &data);
> + assert (data != (void *)0);
> +}
> diff --git a/gcc/tree-ssa-alias.cc b/gcc/tree-ssa-alias.cc
> index b8f107dfa52..9068db300e5 100644
> --- a/gcc/tree-ssa-alias.cc
> +++ b/gcc/tree-ssa-alias.cc
> @@ -2608,8 +2608,6 @@ modref_may_conflict (const gcall *stmt,
> if (num_tests >= max_tests)
> return true;
> alias_stats.modref_tests++;
> - if (!alias_sets_conflict_p (base_set, base_node->base))
> - continue;
> num_tests++;
> }
>
> --
> 2.34.1
>
