[Bug c++/106921] New: [11/12.1] -O1 and -fipa-icf -fpartial-inlining causes wrong code
lutztonineubert at gmail dot com
gcc-bugzilla@gcc.gnu.org
Tue Sep 13 06:55:29 GMT 2022
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106921
Bug ID: 106921
Summary: [11/12.1] -O1 and -fipa-icf -fpartial-inlining causes
wrong code
Product: gcc
Version: 11.3.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: lutztonineubert at gmail dot com
Target Milestone: ---
Short summary:
The following code returns 1 if compiled with -O2 (which is wrong) and does
return 0 if compiled without optimization.
```
#include <array>
#include <cstddef>
#include <exception>
#define GCC_VERSION (__GNUC__ * 10000 \
+ __GNUC_MINOR__ * 100 \
+ __GNUC_PATCHLEVEL__)
static_assert(GCC_VERSION == 110300);
template <size_t Bits>
class bitset {
private:
using word_t = size_t;
static constexpr size_t bits_per_word = sizeof(word_t) * 8;
static constexpr size_t number_of_words = (Bits / bits_per_word) + (((Bits %
bits_per_word) == 0) ? 0 : 1);
public:
bool all_first(size_t n) const {
{
if (n > Bits) {
#ifdef RETURN_INSTEAD_TERMINATE
return false;
#else
std::terminate();
#endif
}
size_t i = 0;
for (; n > bits_per_word; n -= bits_per_word, i++) {
if (words_[i] != ~word_t{0}) {
return false;
}
}
word_t last_word = words_[i];
for (; n != 0; n--) {
if ((last_word & 1) != 1) {
return false;
}
last_word >>= 1;
}
return true;
}
}
void fill() noexcept {
for (auto& word : words_) {
word = ~word_t{0};
}
}
private:
std::array<word_t, number_of_words> words_{};
};
volatile int X = 0;
int main() {
if (X == 1) {
bitset<123> bitset;
static_cast<void>(bitset.all_first(123));
} else {
bitset<256> bitset;
bitset.fill();
if (!bitset.all_first(255)) {
return 1;
}
}
return 0;
}
```
See: https://gcc.godbolt.org/z/bEexjrKP4
This issue does not exist in GCC 10 or GCC > 12.1. I couldn't test if it does
work in GCC 11.3.1 (or the trunk of it).
Additional:
* I could also trigger the issue with -O1 -fipa-icf -fpartial-inlining
* If we do a return false instead of a std::terminate, no wrong code is
generated.
I am sorry, but I couldn't reduced the code any further - this already took so
much time to figure out it is a compiler bug.
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