https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106074
Bug ID: 106074
Summary: Spurious Wstringop-overflow for int-to-string with
SSE4
Product: gcc
Version: 13.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: ed at catmur dot uk
Target Milestone: ---
Code adapted from
https://github.com/capnproto/capnproto/blob/be7a80b4add706ddaeb41689221146b86c3e0f5f/c%2B%2B/src/kj/string.c%2B%2B#L157-L203
:
auto f(short i) {
struct R { char data[6]; } result;
bool negative = i < 0;
unsigned u = i;
if (negative)
u = -u;
unsigned char reverse[5];
unsigned char* p = reverse;
if (u == 0)
*p++ = 0;
else
for (; u > 0; u /= 10)
*p++ = u % 10;
char* p2 = result.data;
if (negative)
*p2++ = '-';
while (p > reverse) {
#ifdef ASSUME
if (p2 >= (&result.data)[1]) __builtin_unreachable();
#endif
*p2++ = '0' + *--p;
}
if (p2 < (&result.data)[1])
*p2 = '\0';
return result;
}
When compiled with -O3 -msse4, from 12.1.0 through 13.0.0 20220619:
<source>: In function 'auto f(short int)':
<source>:21:11: warning: writing 1 byte into a region of size 0
[-Wstringop-overflow=]
21 | *p2++ = '0' + *--p;
| ~~~~~~^~~~~~~~~~~~
<source>:2:19: note: at offset 6 into destination object 'f(short
int)::R::data' of size 6
2 | struct R { char data[6]; } result;
| ^~~~
<source>:2:30: note: at offset 7 into destination object 'result' of size 6
2 | struct R { char data[6]; } result;
| ^~~~~~
<source>:2:19: note: at offset 6 into destination object 'f(short
int)::R::data' of size 6
2 | struct R { char data[6]; } result;
| ^~~~
<source>:2:19: note: at offset 6 into destination object 'f(short
int)::R::data' of size 6
<source>:2:30: note: at offset 7 into destination object 'result' of size 6
2 | struct R { char data[6]; } result;
| ^~~~~~
<source>:2:19: note: at offset 6 into destination object 'f(short
int)::R::data' of size 6
2 | struct R { char data[6]; } result;
| ^~~~
<source>:2:19: note: at offset 6 into destination object 'f(short
int)::R::data' of size 6
<source>:2:30: note: at offset 7 into destination object 'result' of size 6
2 | struct R { char data[6]; } result;
| ^~~~~~
<source>:2:19: note: at offset 6 into destination object 'f(short
int)::R::data' of size 6
2 | struct R { char data[6]; } result;
| ^~~~
<source>:2:19: note: at offset 6 into destination object 'f(short
int)::R::data' of size 6
<source>:2:30: note: at offset 7 into destination object 'result' of size 6
2 | struct R { char data[6]; } result;
| ^~~~~~
<source>:2:19: note: at offset 6 into destination object 'f(short
int)::R::data' of size 6
2 | struct R { char data[6]; } result;
| ^~~~
Adding the assumption at line 19 (-DASSUME) works around this.
