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* [Bug c/48267] New: incorrect signed overflow warning when a pointer cannot possibly overflow
@ 2011-03-24 8:53 eggert at gnu dot org
2011-03-24 12:18 ` [Bug middle-end/48267] " rguenth at gcc dot gnu.org
2021-09-28 9:09 ` pinskia at gcc dot gnu.org
0 siblings, 2 replies; 3+ messages in thread
From: eggert at gnu dot org @ 2011-03-24 8:53 UTC (permalink / raw)
To: gcc-bugs
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=48267
Summary: incorrect signed overflow warning when a pointer
cannot possibly overflow
Product: gcc
Version: 4.5.2
Status: UNCONFIRMED
Severity: minor
Priority: P3
Component: c
AssignedTo: unassigned@gcc.gnu.org
ReportedBy: eggert@gnu.org
I ran into this problem when compiling the GNU Emacs trunk with a GCC
4.5.2 that I built on RHEL 5.5 (x86-64). I narrowed it down to the
following stripped-down test case. This smells different from the previous
bug report I filed in this area (PR48228) on the same platform.
When I compile the following program with "gcc -S -Wstrict-overflow -O2"
GCC reports "warning: assuming pointer wraparound does not occur when comparing
P +- C1 with P +- C2". This warning is incorrect, since
signed overflow is obviously impossible in this function: the only
pointers computed are head_table and head_table + 1, which are both
in range. Changing the "+ 1" to "+ 7" generates even more warnings,
though the program is still correct.
int head_table[7];
int
foo (void)
{
const int *p;
int x = 0;
for (p = head_table; p < head_table + 1; p++)
x ^= *p;
return x;
}
^ permalink raw reply [flat|nested] 3+ messages in thread
* [Bug middle-end/48267] incorrect signed overflow warning when a pointer cannot possibly overflow
2011-03-24 8:53 [Bug c/48267] New: incorrect signed overflow warning when a pointer cannot possibly overflow eggert at gnu dot org
@ 2011-03-24 12:18 ` rguenth at gcc dot gnu.org
2021-09-28 9:09 ` pinskia at gcc dot gnu.org
1 sibling, 0 replies; 3+ messages in thread
From: rguenth at gcc dot gnu.org @ 2011-03-24 12:18 UTC (permalink / raw)
To: gcc-bugs
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=48267
Richard Guenther <rguenth at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Keywords| |diagnostic
Status|UNCONFIRMED |NEW
Last reconfirmed| |2011.03.24 12:14:46
Component|c |middle-end
Ever Confirmed|0 |1
Known to fail| |4.5.2, 4.7.0
--- Comment #1 from Richard Guenther <rguenth at gcc dot gnu.org> 2011-03-24 12:14:46 UTC ---
Confirmed. Happens from
#1 0x00000000007307e7 in fold_undefer_overflow_warnings (issue=1 '\001',
stmt=0x7ffff5b373c0, code=2)
at /space/rguenther/src/svn/trunk/gcc/fold-const.c:281
281 warning_at (locus, OPT_Wstrict_overflow, "%s", warnmsg);
(gdb)
#2 0x0000000000afdfdf in evaluate_stmt (stmt=0x7ffff5b373c0)
at /space/rguenther/src/svn/trunk/gcc/tree-ssa-ccp.c:2117
2117 fold_undefer_overflow_warnings (is_constant, stmt, 0);
(gdb)
#3 0x0000000000aff7c5 in visit_cond_stmt (stmt=0x7ffff5b373c0,
taken_edge_p=0x7fffffffda48)
at /space/rguenther/src/svn/trunk/gcc/tree-ssa-ccp.c:2443
2443 val = evaluate_stmt (stmt);
(gdb)
#4 0x0000000000aff8f8 in ccp_visit_stmt (stmt=0x7ffff5b373c0,
taken_edge_p=0x7fffffffda48, output_p=0x7fffffffda40)
at /space/rguenther/src/svn/trunk/gcc/tree-ssa-ccp.c:2501
2501 return visit_cond_stmt (stmt, taken_edge_p);
(gdb) call debug_gimple_stmt (stmt)
if (p_1 < &head_table[1])
which calls fold_binary with &head_table < &head_table[1] and in the end warns
here:
8688 if (code != EQ_EXPR
8689 && code != NE_EXPR
8690 && bitpos0 != bitpos1
8691 && (pointer_may_wrap_p (base0, offset0, bitpos0)
8692 || pointer_may_wrap_p (base1, offset1, bitpos1)))
8693 fold_overflow_warning (("assuming pointer wraparound
does not "
8694 "occur when comparing P +- C1
with "
8695 "P +- C2"),
8696
WARN_STRICT_OVERFLOW_CONDITIONAL)
because we call pointer_may_wrap_p with base0 which isn't an address
but an object which is bogus. It's argument has to depend on
indirect_base in which case we stripped an ADDR_EXPR.
^ permalink raw reply [flat|nested] 3+ messages in thread
* [Bug middle-end/48267] incorrect signed overflow warning when a pointer cannot possibly overflow
2011-03-24 8:53 [Bug c/48267] New: incorrect signed overflow warning when a pointer cannot possibly overflow eggert at gnu dot org
2011-03-24 12:18 ` [Bug middle-end/48267] " rguenth at gcc dot gnu.org
@ 2021-09-28 9:09 ` pinskia at gcc dot gnu.org
1 sibling, 0 replies; 3+ messages in thread
From: pinskia at gcc dot gnu.org @ 2021-09-28 9:09 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=48267
Andrew Pinski <pinskia at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Status|NEW |RESOLVED
Resolution|--- |FIXED
Known to work| |6.1.0
Known to fail| |5.5.0
Target Milestone|--- |6.0
--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Fixed in GCC 6.
^ permalink raw reply [flat|nested] 3+ messages in thread
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2011-03-24 8:53 [Bug c/48267] New: incorrect signed overflow warning when a pointer cannot possibly overflow eggert at gnu dot org
2011-03-24 12:18 ` [Bug middle-end/48267] " rguenth at gcc dot gnu.org
2021-09-28 9:09 ` pinskia at gcc dot gnu.org
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