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<base href="https://bugs.llvm.org/">
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<body><table border="1" cellspacing="0" cellpadding="8">
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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - [Regression] Clang 6 generated code produces incorrect result with -02 and above"
href="https://bugs.llvm.org/show_bug.cgi?id=36929">36929</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>[Regression] Clang 6 generated code produces incorrect result with -02 and above
</td>
</tr>
<tr>
<th>Product</th>
<td>clang
</td>
</tr>
<tr>
<th>Version</th>
<td>6.0
</td>
</tr>
<tr>
<th>Hardware</th>
<td>PC
</td>
</tr>
<tr>
<th>OS</th>
<td>Linux
</td>
</tr>
<tr>
<th>Status</th>
<td>NEW
</td>
</tr>
<tr>
<th>Severity</th>
<td>release blocker
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
</tr>
<tr>
<th>Component</th>
<td>-New Bugs
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>budai@tresorit.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr></table>
<p>
<div>
<pre>Created <span class=""><a href="attachment.cgi?id=20137" name="attach_20137" title="Source code, runner script and results for the original example">attachment 20137</a> <a href="attachment.cgi?id=20137&action=edit" title="Source code, runner script and results for the original example">[details]</a></span>
Source code, runner script and results for the original example
We have a code that uses a custom allocator with `std::vector` to provide some
extra memory safety features. After switching to Clang 6, our tests started to
fail on 32-bit x86 Linux platforms.
We boiled down our code to this simple example with no dependencies but STL:
#include <cstddef>
#include <vector>
template<typename T>
struct my_allocator {
typedef T value_type;
T* allocate(size_t n) {
return new T[n];
}
void deallocate(T* p, size_t n) {
delete[] p;
}
template<typename U, typename... A>
void construct(U* p, A... a) {
new ((void*)p)U(std::forward<A>(a)...);
}
};
int main() {
std::vector<char, my_allocator<char>> vec(32, 'a');
vec.insert(vec.begin(), 1, 'b');
return vec.size() == 33 ? 0 : 1;
}
This code should extend `vec` by one by inserting a single element on the
beginning, but it produces incorrect results (exit code 1) with Clang 6 with
libcxx, cross-compiling from 64 to 32-bit Linux in optimization level 2 and
above:
clang 5, x86_32, libstdc++, -O0: ok
clang 5, x86_32, libstdc++, -O1: ok
clang 5, x86_32, libstdc++, -O2: ok
clang 5, x86_32, libstdc++, -O3: ok
clang 5, x86_32, libc++, -O0: ok
clang 5, x86_32, libc++, -O1: ok
clang 5, x86_32, libc++, -O2: ok
clang 5, x86_32, libc++, -O3: ok
clang 5, x86_64, libstdc++, -O0: ok
clang 5, x86_64, libstdc++, -O1: ok
clang 5, x86_64, libstdc++, -O2: ok
clang 5, x86_64, libstdc++, -O3: ok
clang 5, x86_64, libc++, -O0: ok
clang 5, x86_64, libc++, -O1: ok
clang 5, x86_64, libc++, -O2: ok
clang 5, x86_64, libc++, -O3: ok
clang 6, x86_32, libstdc++, -O0: ok
clang 6, x86_32, libstdc++, -O1: ok
clang 6, x86_32, libstdc++, -O2: ok
clang 6, x86_32, libstdc++, -O3: ok
clang 6, x86_32, libc++, -O0: ok
clang 6, x86_32, libc++, -O1: ok
clang 6, x86_32, libc++, -O2: fail 1
clang 6, x86_32, libc++, -O3: fail 1
clang 6, x86_64, libstdc++, -O0: ok
clang 6, x86_64, libstdc++, -O1: ok
clang 6, x86_64, libstdc++, -O2: ok
clang 6, x86_64, libstdc++, -O3: ok
clang 6, x86_64, libc++, -O0: ok
clang 6, x86_64, libc++, -O1: ok
clang 6, x86_64, libc++, -O2: ok
clang 6, x86_64, libc++, -O3: ok
With using either Clang 5, stdlibc++, or x86_64 target, or removing/changing
the `construct` method fixes the generated code, yielding in correct results.
You can find the source code, runner script and the above results in the
attachment `original.tar.gz`.</pre>
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