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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 - clang++ fails to optimize variadic template constructor with fold expression"
href="https://bugs.llvm.org/show_bug.cgi?id=37449">37449</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>clang++ fails to optimize variadic template constructor with fold expression
</td>
</tr>
<tr>
<th>Product</th>
<td>clang
</td>
</tr>
<tr>
<th>Version</th>
<td>trunk
</td>
</tr>
<tr>
<th>Hardware</th>
<td>PC
</td>
</tr>
<tr>
<th>OS</th>
<td>Windows NT
</td>
</tr>
<tr>
<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>-New Bugs
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>vittorio.romeo@outlook.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr></table>
<p>
<div>
<pre>Consider the following wrapper around `std::vector`:
#include <vector>
struct init { };
template <typename T>
struct vec : std::vector<T>
{
using std::vector<T>::vector;
template <typename... Xs>
vec(init, Xs&&... xs)
{
this->reserve(sizeof...(Xs));
(this->emplace_back(std::forward<Xs>(xs)), ...);
}
};
When compiling the following code `clang++-trunk -Ofast -std=c++2a`...
void foo()
{
#ifndef VARIADIC
vec<std::string> v;
v.reserve(2);
v.emplace_back("hello");
v.emplace_back("world");
#else
vec<std::string> v{init{}, "hello", "world"};
#endif
}
...the variadic version produces roughly 5x more assembly instructions and is
around 46% slower at run-time.
Marking `vec`'s constructor as `__attribute__((always_inline))` makes both
versions produce the same assembly.
Live assembly comparison on godbolt.org:
<a href="https://godbolt.org/g/YwM86C">https://godbolt.org/g/YwM86C</a>
Live run-time benchmark on quick-bench.com:
<a href="http://quick-bench.com/C5_4Bm5Crsqg1ykE0YmoHFOwmLs">http://quick-bench.com/C5_4Bm5Crsqg1ykE0YmoHFOwmLs</a>
I think that this is a very reasonable/common usage of a variadic template
constructor + fold expression - it is surprising to me as a developer that
clang++ doesn't optimize this. This *should* be a better alternative in terms
of run-time performance to `std::initalizer_list`, which prevents moves and has
other significant drawbacks - it is disappointing to see `always_inline` being
required here.</pre>
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