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<body><span class="vcard"><a class="email" href="mailto:richard-llvm@metafoo.co.uk" title="Richard Smith <richard-llvm@metafoo.co.uk>"> <span class="fn">Richard Smith</span></a>
</span> changed
<a class="bz_bug_link
bz_status_RESOLVED bz_closed"
title="RESOLVED DUPLICATE - clang 300x slower to compile recursive constexpr template vs. gcc"
href="https://llvm.org/bugs/show_bug.cgi?id=25729">bug 25729</a>
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<table border="1" cellspacing="0" cellpadding="8">
<tr>
<th>What</th>
<th>Removed</th>
<th>Added</th>
</tr>
<tr>
<td style="text-align:right;">Status</td>
<td>NEW
</td>
<td>RESOLVED
</td>
</tr>
<tr>
<td style="text-align:right;">Resolution</td>
<td>---
</td>
<td>DUPLICATE
</td>
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<b><a class="bz_bug_link
bz_status_RESOLVED bz_closed"
title="RESOLVED DUPLICATE - clang 300x slower to compile recursive constexpr template vs. gcc"
href="https://llvm.org/bugs/show_bug.cgi?id=25729#c2">Comment # 2</a>
on <a class="bz_bug_link
bz_status_RESOLVED bz_closed"
title="RESOLVED DUPLICATE - clang 300x slower to compile recursive constexpr template vs. gcc"
href="https://llvm.org/bugs/show_bug.cgi?id=25729">bug 25729</a>
from <span class="vcard"><a class="email" href="mailto:richard-llvm@metafoo.co.uk" title="Richard Smith <richard-llvm@metafoo.co.uk>"> <span class="fn">Richard Smith</span></a>
</span></b>
<pre>The code is obviously exponential-time in Index:
template <unsigned Index>
constexpr unsigned crc32 (char const * str) {
unsigned const a = crc32 <Index - 1> (str) >> 8;
unsigned const b = crc32 <Index - 1> (str) ^ str [Index - 1];
return a ^ crc_table [b & 0x000000FF];
}
Don't compute crc32<Index - 1>(str) twice, compute it once and reuse the
result.
I don't know why people expect to get asymptotic performance improvements by
slapping 'constexpr' on their functions, and I'm not convinced GCC's
memoization is doing anyone any favours here -- if we or GCC choose to evaluate
the initializer at runtime (as we are permitted to do), it will also be
extremely slow (in a -O0 build at least), due to the use of a ridiculous
exponential-time algorithm.
*** This bug has been marked as a duplicate of <a class="bz_bug_link
bz_status_NEW "
title="NEW --- - memoize constexpr function invocations"
href="show_bug.cgi?id=12850">bug 12850</a> ***</pre>
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