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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - Merge constant literals"
href="https://bugs.llvm.org/show_bug.cgi?id=43445">43445</a>
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<th>Summary</th>
<td>Merge constant literals
</td>
</tr>
<tr>
<th>Product</th>
<td>clang
</td>
</tr>
<tr>
<th>Version</th>
<td>trunk
</td>
</tr>
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<th>Hardware</th>
<td>PC
</td>
</tr>
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<th>OS</th>
<td>Linux
</td>
</tr>
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<th>Status</th>
<td>NEW
</td>
</tr>
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<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>zamazan4ik@tut.by
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org, neeilans@live.com, richard-llvm@metafoo.co.uk
</td>
</tr></table>
<p>
<div>
<pre>Consider the example:
static const char data1[] = {'t','e','s','t'};
static const char data2[] = "test test";
bool index0(const char* cmp) {
return cmp == data1 || cmp == data2;
}
Right now Clang(trunk) with 'std=c++17 -O3' generates suboptimal assembly:
index0(char const*): # @index0(char const*)
mov eax, offset data1
cmp rdi, rax
sete cl
mov eax, offset data2
cmp rdi, rax
sete al
or al, cl
ret
data1:
.ascii "test"
data2:
.asciz "test test"
A more efficient way to generate the code is to merge `data1` and `data2`:
index0(char const*):
mov eax, offset data
cmp rdi, rax
sete al
ret
data:
.ascii "test test"
Constant literals merging significantly reduces binary size and cache misses.
Godbolt playground: <a href="https://godbolt.org/z/z-rJ6J">https://godbolt.org/z/z-rJ6J</a>
Related GCC issue: <a href="https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91899">https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91899</a></pre>
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