kees added a comment.
In D135727#3853896 <https://reviews.llvm.org/D135727#3853896>, @void wrote:
> @kees @serge-sans-paille: It appears to me that a terminating array of size >
> 2 *isn't* treated as a FAM in Clang, at least Clang warns about it. The first
> failure above (`clang/test/Sema/array-bounds-ptr-arith.c`) shows that. It
> turns out that the same failure occurs in that testcase when the array isn't
> the last in a structure, so I'll change it.
Okay, what the heck is even going on in this test? The diagnostic appears to
think the array changes in size based on the cast??
<source>:16:24: warning: the pointer incremented by 80 refers past the end of
the array (that contains 64 elements) [-Warray-bounds-pointer-arithmetic]
return (void *)es->s_uuid + 80;
^ ~~
s_uuid is 8, not 64. 64 would be 8 "void *"s. This seems like a very very
broken diagnostic?
These should all trip the diagnostic, but don't:
es->s_uuid + 8; /* this is past the end */
(void *)es->s_uuid + 9; /* this is past the end by 1, but doesn't trip
because it thinks it's suddenly 8 times larger */
For -Warray-bounds, GCC isn't fooled by the "void *" casts (but doesn't have
-Warray-bounds-arithmetic):
https://godbolt.org/z/5eqEEv4of
Note that while the diagnostics of both GCC and Clang complain only about >1
terminal arrays, they both return -1 for `__builtin_object_size` regardless of
length.
So, we're facing, again, a disconnect between diagnostics, bos, and sanitizer.
GCC's sanitizer follows bos rules, where-as Clang's sanitizer followed
diagnostics rules. Given that bos is used for run-time analysis, it follows
that sanitizer and bos should match.
> There's another failure that I'm not too sure about. The
> `Clang.SemaCXX::array-bounds.cpp` failure is due to a union that's acting
> like an FAM. I have a question for you. Should `a` and `c` in the union in
> this code be considered an FAM?
This test looks correct to me:
> struct {
> union {
> short a[2]; // expected-note 4 {{declared here}}
> char c[4];
> };
> int ignored;
> };
I would **not** expect this to warn or trap because it's not the trailing
member of the //structure//.
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