https://gcc.gnu.org/bugzilla/show_bug.cgi?id=117322
Bug ID: 117322
Summary: Another spurios "may be used uninitialized" warning
when using alloca.
Product: gcc
Version: 15.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: akryuk at gmail dot com
Target Milestone: ---
The following code produces a "may be used uninitialized warning", though there
is no visible path that would leave the `arr` pointer uninitialized
(https://godbolt.org/z/6dbEafEGb).
This happens when using GCC versions starting with 14.1. It does not seem to
happen with prior versions.
I ran into it when building on Ubuntu 24.04 with "-O2 -Wall" flags.
If I use alloca or heap allocation unconditionally, the warning goes away.
If I do an early return (`if(len == 0) return;`), the warning goes away.
If I actually skip heap memory allocation the `else` clause ( /*
heapBuff.reset(new Storage[len]);*/ ), the warning goes away.
If I drop optimization to "-O1", the warning goes away.
//*********************************************************
#include <memory>
#include <alloca.h>
#include <type_traits>
using T = long long;
//----------------------------------------------------------
void f(T const* data, size_t len);
//----------------------------------------------------------
void h(int const* data, size_t len)
{
using Storage = typename std::aligned_storage<sizeof(T), alignof(T)>::type;
T* arr;
std::unique_ptr<Storage[]> heapBuff;
auto nbytes = len * sizeof(T);
if (nbytes <= 256*1024) {
arr = reinterpret_cast<T*>(alloca(nbytes));
}
else {
heapBuff.reset(new Storage[len]);
arr = reinterpret_cast<T*>(heapBuff.get());
}
// for(size_t i = 0; i != len; ++i)
// arr[i] = data[i];
std::uninitialized_copy(data, data + len, arr);
f(arr, len);
}
//*********************************************************
<source>: In function 'void h(const int*, size_t)':
<source>:29:4: warning: 'arr' may be used uninitialized [-Wmaybe-uninitialized]
29 | f(arr, len);
| ~^~~~~~~~~~
<source>:7:6: note: by argument 1 of type 'const T*' {aka 'const long long
int*'} to 'void f(const T*, size_t)' declared here
7 | void f(T const* data, size_t len);
| ^
