> On Aug 17, 2023, at 4:57 PM, Siddhesh Poyarekar <siddh...@gotplt.org> wrote:
>
> On 2023-08-17 16:23, Qing Zhao wrote:
>>>> Then, I think whatever MIN or MAX, the early phase has more precise
>>>> information than the later phase, we should use its result if it’s NOT
>>>> UNKNOWN?
>>>
>>> We can't be sure about that though, can we? For example for something like
>>> this:
>>>
>>> struct S
>>> {
>>> int a;
>>> char b[10];
>>> int c;
>>> };
>>>
>>> size_t foo (struct S *s)
>>> {
>>> return __builtin_object_size (s->b, 1);
>>> }
>>>
>>> size_t bar ()
>>> {
>>> struct S *in = malloc (8);
>>>
>>> return foo (in);
>>> }
>>>
>>> returns 10 for __builtin_object_size in early_objsz but when it sees the
>>> malloc in the later objsz pass, it returns 4:
>>>
>>> $ gcc/cc1 -fdump-tree-objsz-details -quiet -o - -O bug.c
>>> ...
>>> foo:
>>> .LFB0:
>>> .cfi_startproc
>>> movl $10, %eax
>>> ret
>>> .cfi_endproc
>>> ...
>>> bar:
>>> .LFB1:
>>> .cfi_startproc
>>> movl $4, %eax
>>> ret
>>> .cfi_endproc
>>> ...
>>>
>>> In fact, this ends up returning the wrong result for OST_MINIMUM:
>>>
>>> $ gcc/cc1 -fdump-tree-objsz-details -quiet -o - -O bug.c
>>> ...
>>> foo:
>>> .LFB0:
>>> .cfi_startproc
>>> movl $10, %eax
>>> ret
>>> .cfi_endproc
>>> ...
>>> bar:
>>> .LFB1:
>>> .cfi_startproc
>>> movl $10, %eax
>>> ret
>>> .cfi_endproc
>>> ...
>>>
>>> bar ought to have returned 4 too (and I'm betting the later objsz must have
>>> seen that) but it got overridden by the earlier estimate of 10.
>> Okay, I see.
>> Then is this the similar issue we discussed previously? (As following:)
>> "
>>> Hi, Sid and Jakub,
>>> I have a question in the following source portion of the routine
>>> “addr_object_size” of gcc/tree-object-size.cc:
>>> 743 bytes = compute_object_offset (TREE_OPERAND (ptr, 0), var);
>>> 744 if (bytes != error_mark_node)
>>> 745 {
>>> 746 bytes = size_for_offset (var_size, bytes);
>>> 747 if (var != pt_var && pt_var_size && TREE_CODE (pt_var) ==
>>> MEM_REF)
>>> 748 {
>>> 749 tree bytes2 = compute_object_offset (TREE_OPERAND (ptr,
>>> 0),
>>> 750 pt_var);
>>> 751 if (bytes2 != error_mark_node)
>>> 752 {
>>> 753 bytes2 = size_for_offset (pt_var_size, bytes2);
>>> 754 bytes = size_binop (MIN_EXPR, bytes, bytes2);
>>> 755 }
>>> 756 }
>>> 757 }
>>> At line 754, why we always use “MIN_EXPR” whenever it’s for OST_MINIMUM or
>>> not?
>>> Shall we use
>>> (object_size_type & OST_MINIMUM
>>> ? MIN_EXPR : MAX_EXPR)
>> That MIN_EXPR is not for OST_MINIMUM. It is to cater for allocations like
>> this:
>> typedef struct
>> {
>> int a;
>> } A;
>> size_t f()
>> {
>> A *p = malloc (1);
>> return __builtin_object_size (p, 0);
>> }
>> where the returned size should be 1 and not sizeof (int). The mode doesn't
>> really matter in this case.
>> “
>> If this is the same issue, I think we can use the same solution: always use
>> MIN_EXPR,
>> What do you think?
>
> It's not exactly the same issue, the earlier discussion was about choosing
> sizes in the same pass while the current one is about choosing between
> passes, but I agree it "rhymes". This is what I was alluding to originally
> (for OST_MINIMUM use MIN_EXPR if both passes returned a pass) but I haven't
> thought about it hard enough to be 100% confident that it's the better
> solution, especially for OST_MAXIMUM.
We have two different sources to get SIZE information for the subobject:
1. From the TYPESIZE information embedded in the IR;
2. From the initialization information propagated from data flow, this includes
both malloc call and the DECL_INIT.
We need to choose between these two when both available, (these two information
could be
in the same pass as we discussed before, or in different passes which is shown
in this discussion).
I think that the MIN_EXPR might be the right choice (especially for
OST_MAXIMUM) -:)
Qing
>
> Thanks,
> Sid
