https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113540
Richard Biener <rguenth at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Blocks| |56456
Keywords| |diagnostic
Status|UNCONFIRMED |NEW
Ever confirmed|0 |1
Last reconfirmed| |2024-01-23
--- Comment #1 from Richard Biener <rguenth at gcc dot gnu.org> ---
If you remove the volatile, like
#include <stdlib.h>
char *foo (void)
{
char *t;
t = malloc (4);
for (int i = 0; i <= 4; i++)
t[i] = 0;
return t;
}
you get
t.c: In function 'foo':
t.c:8:10: warning: '__builtin_memset' writing 5 bytes into a region of size 4
[-Wstringop-overflow=]
8 | t[i] = 0;
| ~~~~~^~~
t.c:6:7: note: destination object of size 4 allocated by 'malloc'
6 | t = malloc (4);
| ^~~~~~~~~~
note this is because we then unroll the loop. If you change it like
#include <stdlib.h>
short *foo (void)
{
short *t;
t = malloc (8);
for (int i = 0; i <= 4; i++)
t[i] = 13;
return t;
}
you get
t.c: In function 'foo':
t.c:8:6: warning: array subscript 4 is outside array bounds of 'short int[4]'
[-Warray-bounds=]
8 | t[i] = 13;
| ~^~~
t.c:6:7: note: at offset 8 into object of size 8 allocated by 'malloc'
6 | t = malloc (8);
| ^~~~~~~~~~
because we unroll the loop. Upping the bounds like
#include <stdlib.h>
short *foo (void)
{
short *t;
t = malloc (64);
for (int i = 0; i <= 32; i++)
t[i] = 13;
return t;
}
no longer warns because we hit unroll limits. This is also the reason
we do not diagnose the original testcase - there's currently no analysis
done to compute the set of values 'i' must reach for the purpose of
array-bound diagnostics. Instead we use value-ranges which are
conservative, aka [-INF, INF] is "correct". But that means we only
diagnose cases where _all_ values of the range fall outside of the
array.
Using niter analysis and SCEV we could do a better job in cases like the
one in this bug.
I'm quite sure we have related/duplicate bugreports for this already.
Referenced Bugs:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=56456
[Bug 56456] [meta-bug] bogus/missing -Warray-bounds
