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). 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).
Martin
