https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107277
Bug ID: 107277
Summary: Spurious -Wformat-overflow when combined with
__builtin_expect()
Product: gcc
Version: 12.2.1
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
Assignee: unassigned at gcc dot gnu.org
Reporter: tomerv at gmail dot com
Target Milestone: ---
Incorrect format-overflow warning/error:
error: '%d' directive writing between 1 and 11 bytes into a region of size 10
[-Werror=format-overflow=]
note: directive argument in the range [-2147483648, 5]
On this code:
__attribute((__noreturn__)) void do_panic();
int get_number();
#define unlikely(x) __builtin_expect((x),0)
int foo()
{
char buff[10]={0};
int index = get_number();
if (unlikely(index < 0)) { // <-- index cannot be negative
do_panic();
}
if (6 <= index) {
index = 0;
}
int n = sprintf(buff, "%d", index);
if (unlikely(n < 0)) {
do_panic();
}
return n;
}
Removing "unlikely" from the first condition fixes the issue.
Also - and this is a missing warning, which is the reverse problem - removing
the whole "if (6 <= index)" block removes the warning, but only with the
"unlikely" present.
https://godbolt.org/z/9esTTTbGq
