https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105823
Bug ID: 105823
Summary: -Wrestrict / -Wstringop-overflow / -Warray-bounds
warnings for uninitialized values
Product: gcc
Version: 13.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: fiesh at zefix dot tv
Target Milestone: ---
Some of our code when compiled with enough optimization resulted in weird
-Wrestrict warnings that I think were somehow drawn in through std::string and
then from libstdc++'s char_traits.h.
I tried reducing it and arrived at code which behaves as follows:
% g++ -Wall -Wextra -Wno-nonnull -Wno-stringop-overflow -Wno-array-bounds
-Werror -std=c++20 -O3 -c a.ii
fails because of -Wrestrict, and removing -Wno-stringop-overflow or
-Wno-array-bounds makes these trigger the same warning. (stringop-overflow is
disabled in char_traits.h which I think is why we hit -Wrestrict instead.)
% g++ -Wall -Wextra -Wno-restrict -Wno-nonnull -Wno-stringop-overflow
-Wno-array-bounds -Werror -std=c++20 -O3 -c a.ii
succeeds.
The warning is:
error: 'void* __builtin_memcpy(void*, const void*, long unsigned int)'
accessing 9223372036854775808 or more bytes at offsets 0 and 0 may overlap up
to 9223372036854775809 bytes at offset -1
The code is:
char aq_ai, bi_bc;
struct ah {
auto aq(long aj) {
return __builtin_memcpy(0, &aq_ai, aj);
}
long ba_bg;
void ba() { bi((ba_bg)); }
ah &bi(long);
};
char *bi_ar;
ah &ah::bi(long bp) {
if (bp) {
if (bi_ar >= &bi_bc + bp)
;
else {
long bt = &bi_bc + bp - bi_ar;
aq(-bt);
}
}
return *this;
}
void cn() {
ah container;
container.ba();
}
Note that changing "bi((ba_bq));" to "bi(ba_bq);", i.e. removing the double
parentheses, makes gcc correctly determine that ba_bq is used uninitialized.
(Maybe this is the actual bug and assigning -1 to unused values is just what
results in this warning here and is legitimate?)
