On Wed, Apr 6, 2022 at 4:26 PM Jason Merrill via Gcc-patches
<gcc-patches@gcc.gnu.org> wrote:
>
> This bug was an object/value confusion; we are interested in the size
> of *b.ip, but instead the code was calculating the size of b.ip itself.
>
> This seems to be because compute_objsize will compute the size of whatever
> object it can find in the argument: if you pass it a VAR_DECL, it gives you
> the size of that variable. If you pass it an ADDR_EXPR of a VAR_DECL, it
> again gives you the size of the variable. The way you can tell the
> difference is by looking at the deref member of access_ref: if it's -1, the
> argument is a pointer to the object. Since that's what we're interested in,
> we should check for that, like check_dangling_stores does.
>
> This regressed some tests because compute_objsize_r was wrongly zeroing
> deref in the POINTER_PLUS_EXPR handling; adding an offset to a pointer
> doesn't change whether the pointer is itself a variable or a pointer to
> one. In fact, handling POINTER_PLUS_EXPR only really makes sense for deref
> == -1, where we're adjusting a pointer to the variable.
>
> Tested x86_64-pc-linux-gnu, OK for trunk?
OK.
Thanks,
Richard.
> PR c++/100370
>
> gcc/cp/ChangeLog:
>
> * init.cc (warn_placement_new_too_small): Check deref.
>
> gcc/ChangeLog:
>
> * pointer-query.cc (compute_objsize_r) [POINTER_PLUS_EXPR]: Require
> deref == -1.
>
> gcc/testsuite/ChangeLog:
>
> * g++.dg/warn/Wplacement-new-size-11.C: New test.
> ---
> gcc/cp/init.cc | 5 +++++
> gcc/pointer-query.cc | 7 ++++---
> .../g++.dg/warn/Wplacement-new-size-11.C | 15 +++++++++++++++
> 3 files changed, 24 insertions(+), 3 deletions(-)
> create mode 100644 gcc/testsuite/g++.dg/warn/Wplacement-new-size-11.C
>
> diff --git a/gcc/cp/init.cc b/gcc/cp/init.cc
> index 01e762320f3..43097121244 100644
> --- a/gcc/cp/init.cc
> +++ b/gcc/cp/init.cc
> @@ -2811,6 +2811,11 @@ warn_placement_new_too_small (tree type, tree nelts,
> tree size, tree oper)
> if (!objsize)
> return;
>
> + /* We can only draw conclusions if ref.deref == -1,
> + i.e. oper is the address of the object. */
> + if (ref.deref != -1)
> + return;
> +
> offset_int bytes_avail = wi::to_offset (objsize);
> offset_int bytes_need;
>
> diff --git a/gcc/pointer-query.cc b/gcc/pointer-query.cc
> index 4390535ef56..d93657f3206 100644
> --- a/gcc/pointer-query.cc
> +++ b/gcc/pointer-query.cc
> @@ -2299,9 +2299,10 @@ compute_objsize_r (tree ptr, gimple *stmt, bool addr,
> int ostype,
> if (!compute_objsize_r (ref, stmt, addr, ostype, pref, snlim, qry))
> return false;
>
> - /* Clear DEREF since the offset is being applied to the target
> - of the dereference. */
> - pref->deref = 0;
> + /* The below only makes sense if the offset is being applied to the
> + address of the object. */
> + if (pref->deref != -1)
> + return false;
>
> offset_int orng[2];
> tree off = pref->eval (TREE_OPERAND (ptr, 1));
> diff --git a/gcc/testsuite/g++.dg/warn/Wplacement-new-size-11.C
> b/gcc/testsuite/g++.dg/warn/Wplacement-new-size-11.C
> new file mode 100644
> index 00000000000..a6fe82e90ae
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/warn/Wplacement-new-size-11.C
> @@ -0,0 +1,15 @@
> +// PR c++/100370
> +// { dg-do compile { target c++11 } }
> +
> +using size_t = decltype(sizeof(1));
> +inline void *operator new (size_t s, void *p) { return p; }
> +
> +int main()
> +{
> + struct s1 { int iv[4]; };
> + struct s2 { union { char* cp; int* ip; }; };
> +
> + s2 b;
> + b.ip=new int[8];
> + new (b.ip+4) s1; // { dg-bogus "-Wplacement-new" }
> +}
>
> base-commit: 44fe49401725055a740ce47e80561b6932b8cd01
> --
> 2.27.0
>
