https://gcc.gnu.org/bugzilla/show_bug.cgi?id=92323
Bug ID: 92323
Summary: bogus -Warray-bounds after unrolling despite
__builtin_unreachable
Product: gcc
Version: 10.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: msebor at gcc dot gnu.org
Target Milestone: ---
With the -Warray-bounds enhancement for pr91647, GCC issues a warning for the
unrolled loop in function f() below. No warning is issued for g(). The loop
in f() is unrolled but it results in pointless tests for the conditions that
the subsequent __builtin_unreachable call should obviate. The loop in g() is
transformed into a call to memmove which also seems suboptimal.
The code in f() was reduced from try_conditional_simplification() in
gimple-match-head.c.
$ cat z.c && gcc -O2 -S -Wall z.c
struct S { int a[5]; } s;
void sink (void*);
void f (unsigned n, struct S *p)
{
for (unsigned i = 1; i < n - 1; ++i)
s.a[i - 1] = p->a[i]; // bogus warning
if (n < 4 || n > 5)
__builtin_unreachable ();
}
void g (unsigned n, struct S *p)
{
if (n < 4 || n > 5)
__builtin_unreachable ();
for (unsigned i = 1; i < n - 1; ++i)
s.a[i - 1] = p->a[i];
}
z.c: In function ‘f’:
z.c:8:22: warning: array subscript 5 is above array bounds of ‘int[5]’
[-Warray-bounds]
8 | s.a[i - 1] = p->a[i]; // bogus warning
| ~~~~^~~
z.c:1:16: note: while referencing ‘a’
1 | struct S { int a[5]; } s;
| ^
