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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 - Code generation regression with trunk"
href="https://bugs.llvm.org/show_bug.cgi?id=34554">34554</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>Code generation regression with trunk
</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>Linux
</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>LLVM Codegen
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>matt@godbolt.org
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr></table>
<p>
<div>
<pre>Hi all,
I'm not sure how best to explain what's going on, but this code:
#include <array>
#include <algorithm>
constexpr auto get_array() {
return std::array<char, 42>{"This is my string that might have a char."};
}
bool has_char(const char c) {
static constexpr auto a = get_array();
return std::find(std::begin(a), std::end(a), c) != std::end(a);
}
( <a href="https://godbolt.org/g/qW58Cc">https://godbolt.org/g/qW58Cc</a> )
When compiled with `-std=c++17 -O3` on trunk (r312894) generates a jump table,
each of which corresponds to a long-winded "return true" or "return false", of
the form:
.LBB0_2:
mov eax, has_char(char)::a+42
mov ecx, has_char(char)::a+42
cmp rax, rcx
setne al
ret
This is equivalent to `xor eax, eax; ret`. On clang 5.0.0 and with -O2,
different code is generated:
.LBB0_8:
mov eax, has_char(char)::a+41
jmp .LBB0_19
....
.LBB0_19:
mov ecx, has_char(char)::a+42
cmp rax, rcx
setne al
ret
It seems like the code generator is only one step from inferring that each leaf
of the jump table can be replaced with xor eax,eax;ret, or `mov eax,1;ret`, but
falls at the final hurdle.</pre>
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