https://bugs.llvm.org/show_bug.cgi?id=42561
Bug ID: 42561
Summary: Incorrect optimization of unique_ptr construction
leads to segfault
Product: clang
Version: 8.0
Hardware: PC
OS: Linux
Status: NEW
Severity: enhancement
Priority: P
Component: C++
Assignee: unassignedclangb...@nondot.org
Reporter: jas...@3db-labs.com
CC: blitzrak...@gmail.com, dgre...@apple.com,
erik.pilking...@gmail.com, llvm-bugs@lists.llvm.org,
richard-l...@metafoo.co.uk
This comes from a Stack Overflow question I asked a long time ago:
https://stackoverflow.com/questions/43624400/is-this-unsafe-usage-of-a-braced-initializer-list-in-a-range-based-for-loop
A minimal test case is as follows:
----------
#include <initializer_list>
#include <memory>
struct Test
{
int x;
};
int main()
{
std::unique_ptr<Test> a(new Test);
std::unique_ptr<Test> b(new Test);
std::unique_ptr<Test> c(new Test);
int id = 0;
for(auto t : std::initializer_list<Test*>({a.get(), b.get(), c.get()}))
t->x = id++;
return 0;
}
----------
I compiled the above code with "-O3 -std=c++11". What I'm seeing is that,
since, clang v3.9, the above program crashes with a segfault pn my x86_64
system. From looking at the assembly that is generated, it appears that the
construction of the std::unique_ptr objects is optimized out, but the for loop
that dereferences the unique_ptr objects is not elided. Thus, an uninitialized
pointer is dereferenced and a segfault ensues.
I have the example code up on Compiler Explorer to show how the compiler output
evolves between clang versions:
https://gcc.godbolt.org/z/_7MK9z
It appears to work properly with versions before v3.9 (in which case it
optimizes out the entire program, which is reasonable).
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