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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - Incorrect optimization of unique_ptr construction leads to segfault"
href="https://bugs.llvm.org/show_bug.cgi?id=42561">42561</a>
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<th>Summary</th>
<td>Incorrect optimization of unique_ptr construction leads to segfault
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<th>Product</th>
<td>clang
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<th>Version</th>
<td>8.0
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<th>Hardware</th>
<td>PC
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<th>OS</th>
<td>Linux
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<th>Status</th>
<td>NEW
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<th>Severity</th>
<td>enhancement
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<th>Priority</th>
<td>P
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<th>Component</th>
<td>C++
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<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
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<th>Reporter</th>
<td>jasonr@3db-labs.com
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<th>CC</th>
<td>blitzrakete@gmail.com, dgregor@apple.com, erik.pilkington@gmail.com, llvm-bugs@lists.llvm.org, richard-llvm@metafoo.co.uk
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<pre>This comes from a Stack Overflow question I asked a long time ago:
<a href="https://stackoverflow.com/questions/43624400/is-this-unsafe-usage-of-a-braced-initializer-list-in-a-range-based-for-loop">https://stackoverflow.com/questions/43624400/is-this-unsafe-usage-of-a-braced-initializer-list-in-a-range-based-for-loop</a>
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:
<a href="https://gcc.godbolt.org/z/_7MK9z">https://gcc.godbolt.org/z/_7MK9z</a>
It appears to work properly with versions before v3.9 (in which case it
optimizes out the entire program, which is reasonable).</pre>
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