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<base href="https://bugs.llvm.org/">
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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - Missed optimization: useless write of zero to memory"
href="https://bugs.llvm.org/show_bug.cgi?id=32522">32522</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>Missed optimization: useless write of zero to memory
</td>
</tr>
<tr>
<th>Product</th>
<td>Polly
</td>
</tr>
<tr>
<th>Version</th>
<td>unspecified
</td>
</tr>
<tr>
<th>Hardware</th>
<td>PC
</td>
</tr>
<tr>
<th>OS</th>
<td>Linux
</td>
</tr>
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<th>Status</th>
<td>NEW
</td>
</tr>
<tr>
<th>Severity</th>
<td>enhancement
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
</tr>
<tr>
<th>Component</th>
<td>Optimizer
</td>
</tr>
<tr>
<th>Assignee</th>
<td>polly-dev@googlegroups.com
</td>
</tr>
<tr>
<th>Reporter</th>
<td>antoshkka@gmail.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr></table>
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<pre>The following code:
///////////////
#include <new>
template <class T>
struct shared_ptr_like {
T* ptr;
shared_ptr_like(shared_ptr_like&& v) noexcept : ptr(v.ptr) { v.ptr = 0; }
~shared_ptr_like() { if (ptr) { delete ptr; } }
};
typedef shared_ptr_like<int> test_type;
void relocate(test_type* new_buffer, test_type* old_buffer) {
new (new_buffer) test_type{ static_cast<test_type&&>(*old_buffer) };
old_buffer->~test_type();
}
///////////////
Generates suboptimal assembly:
relocate(shared_ptr_like<int>*, shared_ptr_like<int>*): #
@relocate(shared_ptr_like<int>*, shared_ptr_like<int>*)
mov rax, qword ptr [rsi]
mov qword ptr [rdi], rax
mov qword ptr [rsi], 0 # Remove this
ret
For that example GCC 7.0.1 20170330 with flags -std=c++17 -O2 generates
following assembly
relocate(shared_ptr_like<int>*, shared_ptr_like<int>*):
mov rax, QWORD PTR [rsi]
mov QWORD PTR [rdi], rax
ret
This is allowed by C++ standard because
* [class.dtor] specifies that "Once a destructor is invoked for an object, the
object no longer exists<...>"
* all the accesses to stored values require an object (if object does not exist
- accessing value is UB) [basic.lval] "If a program attempts to access the
stored value of an object through a glvalue <...>"
Described optimization is essential because code that moves an object to new
location and then destroys it often used in loops (for example
std::vector::reserve does that).
Checked on CLANG 4.0 (tags/RELEASE_400/final 297782) with flags -std=c++1z -O2</pre>
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