Re: HELP: Questions on unshare_expr

[Qing Zhao <qing.zhao@oracle.com>]

> On Jan 17, 2024, at 1:43 AM, Richard Biener <richard.guenther@gmail.com> wrote:
> 
> On Wed, Jan 17, 2024 at 7:42 AM Richard Biener
> <richard.guenther@gmail.com> wrote:
>> 
>> On Tue, Jan 16, 2024 at 9:26 PM Qing Zhao <qing.zhao@oracle.com> wrote:
>>> 
>>> 
>>> 
>>>> On Jan 15, 2024, at 4:31 AM, Richard Biener <richard.guenther@gmail.com> wrote:
>>>> 
>>>>> All my questions for unshare_expr relate to a  LTO bug that I currently stuck with
>>>>> when using .ACCESS_WITH_SIZE in bound sanitizer (only with -flto, without -flto, no issue):
>>>>> 
>>>>> [opc@qinzhao-aarch64-ol8 gcc]$ sh t
>>>>> during IPA pass: modref
>>>>> t.c:20:1: internal compiler error: tree code ‘ssa_name’ is not supported in LTO streams
>>>>> 0x14c3993 lto_write_tree
>>>>>       ../../latest-gcc-write/gcc/lto-streamer-out.cc:561
>>>>> 0x14c3aeb lto_output_tree_1
>>>>> 
>>>>> And the value of the tree node that triggered the ICE is:
>>>>> (gdb) call debug_tree(expr)
>>>>> <ssa_name 0xfffff5761e60 type <error_mark 0xfffff56c0e58>
>>>>>   nothrow
>>>>>   def_stmt
>>>>>   version:13 in-free-list>
>>>>> 
>>>>> Is there any good way to debug LTO bug?
>>>> 
>>>> This happens usually when you have a VLA type and its type fields are not
>>>> properly gimplified which usually happens because the frontend fails to
>>>> insert a gimplification point for it (a DECL_EXPR).
>>> 
>>> I found an old gcc bug
>>> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97172
>>> ICE: tree code ‘ssa_name’ is not supported in LTO streams since r11-3303-g6450f07388f9fe57
>>> 
>>> Which is very similar to the bug I am having right now.
>>> 
>>> After further study, I suspect that the issue I am having right now with the LTO streaming also
>>> relate to “unshare_expr”, “save_expr”, and the combination of these two, I suspect that
>>> the current gcc cannot handle the combination of these two correctly for my case.
>>> 
>>> My testing case is:
>>> 
>>> #include <stdlib.h>
>>> void __attribute__((__noinline__)) setup_and_test_vla (int n1, int n2, int m)
>>> {
>>>   struct foo {
>>>       int n;
>>>       int p[][n2][n1] __attribute__((counted_by(n)));
>>>   } *f;
>>> 
>>>   f = (struct foo *) malloc (sizeof(struct foo) + m*sizeof(int[n2][n1]));
>>>   f->n = m;
>>>   f->p[m][n2][n1]=1;
>>>   return;
>>> }
>>> 
>>> int main(int argc, char *argv[])
>>> {
>>>  setup_and_test_vla (10, 11, 20);
>>>  return 0;
>>> }
>>> 
>>> Failed with
>>> my_gcc -Os -fsanitize=bounds -flto
>>> 
>>> If changing either n1 or n2 to a constant, the testing passed.
>>> If deleting -flto, the testing passed too.
>>> 
>>> I double checked my code per the suggestions provided by you and Jakub in this
>>> email thread, and I think the code should be fine.
>>> 
>>> The code is following:
>>> 
>>> =====
>>> 504 /* Instrument array bounds for INDIRECT_REFs whose pointers are
>>> 505    POINTER_PLUS_EXPRs of calls to .ACCESS_WITH_SIZE. We create special
>>> 506    builtins that gets expanded in the sanopt pass, and make an array
>>> 507    dimension of it.  ARRAY is the pointer to the base of the array,
>>> 508    which is a call to .ACCESS_WITH_SIZE, *OFFSET is the offset to the
>>> 509    beginning of array.
>>> 510    Return NULL_TREE if no instrumentation is emitted.  */
>>> 511
>>> 512 tree
>>> 513 ubsan_instrument_bounds_indirect_ref (location_t loc, tree array, tree *offset)
>>> 514 {
>>> 515   if (!is_access_with_size_p (array))
>>> 516     return NULL_TREE;
>>> 517   tree bound = get_bound_from_access_with_size (array);
>>> 518   /* The type of the call to .ACCESS_WITH_SIZE is a pointer type to
>>> 519      the element of the array.  */
>>> 520   tree element_size = TYPE_SIZE_UNIT (TREE_TYPE (TREE_TYPE (array)));
>>> 521   gcc_assert (bound);
>>> 522
>>> 523   /* Given the offset, and the size of each element, the index can be
>>> 524      computed as: offset/element_size.  */
>>> 525   *offset = save_expr (*offset);
>>> 526   tree index = fold_build2 (EXACT_DIV_EXPR,
>>> 527                            sizetype, *offset,
>>> 528                            unshare_expr (element_size));
>>> 529   /* Create a "(T *) 0" tree node to describe the original array type.
>>> 530      We get the original array type from the first argument of the call to
>>> 531      .ACCESS_WITH_SIZE (REF, COUNTED_BY_REF, 1, num_bytes, -1).
>>> 532
>>> 533      Originally, REF is a COMPONENT_REF with the original array type,
>>> 534      it was converted to a pointer to an ADDR_EXPR, and the ADDR_EXPR's
>>> 535      first operand is the original COMPONENT_REF.  */
>>> 536   tree ref = CALL_EXPR_ARG (array, 0);
>>> 537   tree array_type
>>> 538     = unshare_expr (TREE_TYPE (TREE_OPERAND (TREE_OPERAND(ref, 0), 0)));
>>> 539   tree zero_with_type = build_int_cst (build_pointer_type (array_type), 0);
>>> 540   return build_call_expr_internal_loc (loc, IFN_UBSAN_BOUNDS,
>>> 541                                        void_type_node, 3, zero_with_type,
>>> 542                                        index, bound);
>>> 543 }
>>> 
>>> =====
>>> 
>>> Inside gdb, the guilty IR failed in LTO streaming is from the above line 520:
>>> TYPE_SIZE_UNIT (TREE_TYPE (TREE_TYPE (array))),
>>> 
>>> When I use this tree node as an operand of the expression at line 526, I added
>>> unshare_expr.
>>> 
>>> However, I still see the guilty IR as in gdb:
>>> 
>>>            unit-size <mult_expr 0xfffff5aabf90 type <integer_type 0xfffff57c0000 sizetype>
>>>                side-effects
>>>                arg:0 <mult_expr 0xfffff5aabf68 type <integer_type 0xfffff57c0000 sizetype>
>>> 
>>>                    arg:0 <ssa_name 0xfffff5761e18 type <error_mark 0xfffff56c0e58>
>>>                        nothrow
>>>                        def_stmt
>>>                        version:12 in-free-list>
>>>                    arg:1 <ssa_name 0xfffff5761e60 type <error_mark 0xfffff56c0e58>
>>>                        nothrow
>>>                        def_stmt
>>>                        version:13 in-free-list>>
>>>                arg:1 <integer_cst 0xfffff56c10c8 constant 4>>
>>> 
>>> 
>>> I have been stuck with this bug for quite some time.
>>> Any help is helpful.
>> 
>> The above hasn't been gimplified correctly, you'd instead see
>> a D.1234 in there, not an expression with SSA names.  That happens
>> when the frontend fails to emit a DECL_EXPR for a decl with this
>> type.
> 
> .. which then also results in missing unsharing of this expression
> (so the SSA names leak in)

Thanks a lot for the hints.

One correction first, the LTO bug is not related to -fsanitize=bounds.  Deleting -fsanitize=bounds still can
repeat the failure.

After further debugging into the gimplification phase related with the SAVE_EXPR, I finally locate the place 
where the unshareing of the expression is missing.   This is in the routine “pointer_int_sum” of c-family/c-common.cc:

3330     {
3331       if (!complain && !COMPLETE_TYPE_P (TREE_TYPE (result_type)))
3332         return error_mark_node;
3333       size_exp = size_in_bytes_loc (loc, TREE_TYPE (result_type));
3334       /* Wrap the pointer expression in a SAVE_EXPR to make sure it
3335          is evaluated first when the size expression may depend
3336          on it for VM types.  */
3337       if (TREE_SIDE_EFFECTS (size_exp)
3338           && TREE_SIDE_EFFECTS (ptrop)
3339           && variably_modified_type_p (TREE_TYPE (ptrop), NULL))
3340         {
3341           ptrop = save_expr (ptrop);
3342           size_exp = build2 (COMPOUND_EXPR, TREE_TYPE (intop), ptrop, size_exp);
3343         }
3344     }

In the above, at line 3333, the tree node, TYPE_SIZE_UNIT (TREE_TYPE(result_type)), is returned directly as
the size_exp, 

 (gdb) call debug_tree(size_exp)
 <mult_expr 0xfffff5a6f910
    type <integer_type 0xfffff57c0000 sizetype public unsigned DI
        size <integer_cst 0xfffff56c0e70 constant 64>
        unit-size <integer_cst 0xfffff56c0e88 constant 8>
        align:64 warn_if_not_align:0 symtab:0 alias-set -1 canonical-type 0xfffff57c0000 precision:64 min <integer_cst 0xfffff56c0ea0 0> max <integer_cst 0xfffff56d05e0 18446744073709551615>>
    side-effects
    arg:0 <mult_expr 0xfffff5a6f8e8 type <integer_type 0xfffff57c0000 sizetype>
        side-effects
        arg:0 <nop_expr 0xfffff56dc540 type <integer_type 0xfffff57c0000 sizetype>
            side-effects
            arg:0 <save_expr 0xfffff56dc4c0 type <integer_type 0xfffff57c05e8 int>
                side-effects arg:0 <parm_decl 0xfffff76b6f80 n1>>>
        arg:1 <nop_expr 0xfffff56dc600 type <integer_type 0xfffff57c0000 sizetype>
            side-effects
            arg:0 <save_expr 0xfffff56dc580 type <integer_type 0xfffff57c05e8 int>
                side-effects arg:0 <parm_decl 0xfffff76b7000 n2>>>>
    arg:1 <integer_cst 0xfffff56c10c8 type <integer_type 0xfffff57c0000 sizetype> constant 4>>


Without unshare_expr to this size_exp, the above TYPE_SIZE_UNIT node containing SAVE_EXPRs 
is gimpflified to expressions with SSA_NAME during gimplification.  (This is unaccepted by LTO).

Adding an unshare_expr (size_exp) resolved this problem. 

Although I still think that there might be potential issue with the gimpflication of SAVE_EXPRs, I dare not
to modify that part of the code. 

At this moment, I will add unshare_expr to the routine “pointer_int_sum” to workaround this issue.

Let me know if you have any comment and suggestion.

Thanks a lot.

Qing

>> 
>>> 
>>> Qing
>>> 
>>>> 
>>>>> Thanks a lot for the help.
>>>>> 
>>>>> Qing