> On Oct 22, 2022, at 12:54 PM, Martin Sebor <mse...@gmail.com> wrote:
>
> On 10/21/22 09:29, Qing Zhao wrote:
>> Hi,
>> (FAM below refers to Flexible Array Members):
>> I need inputs on how to handle the combination of -fstrict-flex-arrays +
>> -Warray-bounds.
>> Our initial goal is to update -Warray-bounds with multiple levels of
>> -fstrict-flex-arrays=N
>> to issue warnings according to the different levels of “N”.
>> However, after detailed study, I found that this goal was very hard to be
>> achieved.
>> 1. -fstrict-flex-arrays and its levels
>> The new option -fstrict-flex-arrays has 4 levels:
>> level trailing arrays
>> treated as FAM
>> 0 [],[0],[1],[n] the default without option
>> 1 [],[0],[1]
>> 2 [],[0]
>> 3 [] the default when option specified
>> without value
>> 2. -Warray-bounds and its levels
>> The option -Warray-bounds currently has 2 levels:
>> level trailing arrays
>> treated as FAM
>> 1 [],[0],[1] the default when option specified
>> without value
>> 2 []
>> i.e,
>> When -Warray-bounds=1, it treats [],[0],[1] as FAM, the same level as
>> -fstrict-flex-arrays=1;
>> When -Warray-bounds=2, it only treat [] as FAM, the same level as
>> -fstrict-flex-arrays=3;
>> 3. How to handle the combination of -fstrict-flex-arrays and -Warray-bounds?
>> Question 1: when -fstrict-flex-arrays does not present, the default is
>> -strict-flex-arrays=0,
>> which treats [],[0],[1],[n] as FAM, so should we update
>> the default behavior
>> of -Warray-bounds to treat any trailing array [n] as
>> FAMs?
>> My immediate answer to Q1 is NO, we shouldn’t, that will be a big regression
>> on -Warray-bounds, right?
>
> Yes, it would disable -Warray-bounds in the cases where it warns
> for past-the-end accesses to trailing arrays with two or more
> elements. Diagnosing those has historically (i.e., before recent
> changes) been a design goal.
>
>> Question 2: when -fstrict-flex-arrays=N1 and -Warray-bounds=N2 present at
>> the same time,
>> Which one has higher priority? N1 or N2?
>> -fstrict-flex-arrays=N1 controls how the compiler code generation treats the
>> trailing arrays as FAMs, it seems
>> reasonable to give higher priority to N1,
>
> I tend to agree. In other words, set N2' = min(N1, N2).
>
>> However, then should we completely disable the level of -Warray-bounds
>> N2 under such situation?
>> I really don’t know what’s the best way to handle the conflict between N1
>> and N2.
>> Can we completely cancel the 2 levels of -Warray-bounds, and always honor
>> the level of -fstrict-flex-arrays?
>> Any comments or suggestion will be helpful.
>
> The recent -fstrict-flex-array changes aside, IIRC, there's only
> a subtle distinction between the two -Warray-bounds levels (since
> level 1 started warning on a number of instances that only level
> 2 used to diagnose a few releases ago).
From the doc: (and I also checked the source code)
-Warray-bounds=2
This warning level also warns about out of bounds accesses to trailing
struct members of one-element array types (@pxref{Zero Length}) and about
the intermediate results of pointer arithmetic that may yield out of bounds
values. This warning level may give a larger number of false positives and
is deactivated by default.
As I understand, -Warray-bounds=1 (i.e., -Warray-bounds) will report
out-of-bounds access to trailing arrays with two or more elements, and treat
trailing arrays with 0 or 1 as FAMs;
-Warray-bounds=2 will report out-of-bounds access to trailing arrays with 0 or
1elements in addition to -Warray-bounds =1.
Is the above understanding correct?
> I think that subset of
> level 2 could be merged into level 1 without increasing the rate
> of false positives. Then level 2 could be assigned a new set of
> potential problems to detect (such as past-the-end accesses to
> trailing one-element arrays).
If I understand correctly, Current Level 2 already include warning about
past-the-end accesses to trailing one-element arrays (and also 0-length
arrays).
Qing
>
> Martin
