On 6/28/22 01:16, Richard Biener wrote:
On Mon, Jun 27, 2022 at 4:20 PM Qing Zhao via Gcc-patches
<gcc-patches@gcc.gnu.org> wrote:
Hi,
Per our discussion in the bug report, I came up with the following patch:
=======
PR101836: Add a new option -fstrict-flex-array[=n]
Add the new option and use it in __builtin_object_size.
Treat the trailing array of a structure as a flexible array member in a
stricter way. The value of 'n' controls the level of strictness.
'n'=0 is the least strict, all trailing arrays of structures are treated
as flexible array members; This is the default behavior of GCC without specify
this option.
'n'=3 is the strictest, only when the trailing array is declared as a
flexible array member per C99 standard onwards ([]), it is treated as a
flexible array member;
There are two more levels in between 0 and 3, which are provided to support
older codes that use GCC zero-length array extension ([0]), or one-size array as
flexible array member ([1]):
When 'n' is 1, the trailing array is treated as a flexible array member
when it is declared as either [], [0], or [1];
When 'n' is 2, the trailing array is treated as a flexible array member
when it is declared as either [], or [0].
There are other places in GCC that conservatively treat flexible array members.
A follow-up patch will make -ftrict-flex-array option to control all these
places consistently.
Bootstrapped and regression tested on both X86 and aarch64, no issues.
Any comment and suggestion?
Since this aims at the C or C++ frontends but the middle-end eventually consumes
this it would be much nicer to encode this in the types themselves.
Since the least
strict reading is the default right now it would be a flag (on the
FIELD_DECL I suppose)
like DECL_NOT_FLEXARRAY or DECL_FIXED_SIZE? Alternatively the flag could
also be on the record type enclosing the trailing array member (but
type sharing might
make this more difficult in the end).
There's also array_at_struct_end_p which is supposed to be the main
query interface
for this (but it seems people sneaked in more variants with eventually
different semantics ... :/)
The conservative array_at_struct_end_p has historically been used
for codegen. component_ref_size was added as a separate function
with more flexible (including stricter) semantics to implement
warnings without running the risk of interfering with codegen.
Martin
Richard.
Okay for commit to Gcc13?
thanks.
Qing
=======================
gcc/
PR tree-optimization/101836
* common.opt (fstrict-flex-array, fstrict-flex-array=): New options.
* doc/invoke.texi (-fstrict-flex-array, -fstrict-flex-array=): Document.
* tree-object-size.cc (addr_object_size): Call is_flexible_array_p to
check whether an array is a flexible array.
* tree.cc (special_array_member_type): New routine.
(is_flexible_array_p): New routine.
(component_ref_size): Call special_array_member_type to decide the
type of special array member.
* tree.h (enum struct special_array_member): Add is_vla, trail_flex.
(special_array_member_type): New prototype.
(is_flexible_array_p): New prototype.
gcc/testsuite/
PR tree-optimization/101836
* gcc.dg/pr101836.c: New test.
* gcc.dg/pr101836_1.c: New test.
* gcc.dg/pr101836_2.c: New test.
* gcc.dg/pr101836_3.c: New test.
* gcc.dg/pr101836_4.c: New test.
* gcc.dg/pr101836_5.c: New test.
The complete patch is:
