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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 - Declaring an array of struct without explicit constructor causes excessive memory consumption"
href="https://bugs.llvm.org/show_bug.cgi?id=51113">51113</a>
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<th>Summary</th>
<td>Declaring an array of struct without explicit constructor causes excessive memory consumption
</td>
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<th>Product</th>
<td>clang
</td>
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<th>Version</th>
<td>unspecified
</td>
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<th>Hardware</th>
<td>PC
</td>
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<th>OS</th>
<td>Linux
</td>
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<th>Status</th>
<td>NEW
</td>
</tr>
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<th>Severity</th>
<td>enhancement
</td>
</tr>
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<th>Priority</th>
<td>P
</td>
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<th>Component</th>
<td>Frontend
</td>
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<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>mivik@qq.com
</td>
</tr>
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<th>CC</th>
<td>llvm-bugs@lists.llvm.org, neeilans@live.com, richard-llvm@metafoo.co.uk
</td>
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<pre>Declaring an array of struct without explicit constructor can cause abnormally
excessive memory consumption.
Here's the code I compiled:
struct data {
int a, b, c, d;
} arr[10000000];
int main() {}
This array should take only (4 bytes * 4) * 10000000 = 152MB, but clang used
around 3GB to compile it. Here's the log:
$ /usr/bin/time -v clang++ sample.cpp -std=c++11
Command being timed: "clang++ sample.cpp -std=c++11"
User time (seconds): 3.11
System time (seconds): 1.00
Percent of CPU this job got: 99%
Elapsed (wall clock) time (h:mm:ss or m:ss): 0:04.12
Average shared text size (kbytes): 0
Average unshared data size (kbytes): 0
Average stack size (kbytes): 0
Average total size (kbytes): 0
Maximum resident set size (kbytes): 3730012
Average resident set size (kbytes): 0
Major (requiring I/O) page faults: 0
Minor (reclaiming a frame) page faults: 930464
Voluntary context switches: 7
Involuntary context switches: 28
Swaps: 0
File system inputs: 0
File system outputs: 40
Socket messages sent: 0
Socket messages received: 0
Signals delivered: 0
Page size (bytes): 4096
Exit status: 0
The abnormal memory consumption can be observed at "Maximum resident set size".
However, adding a constructor to the struct will let the problem disappear.
Also, I noticed that the problem occurs only when c++11 or higher standards are
specified, and when I use "-std=c++98" this problem does not produce. So I
guess this might has something to do with constexpr analyzing.
Clang Version:
Ubuntu clang version 12.0.0-3ubuntu1~20.04.3
Target: x86_64-pc-linux-gnu
Thread model: posix
InstalledDir: /usr/bin
Found candidate GCC installation: /usr/bin/../lib/gcc/x86_64-linux-gnu/8
Found candidate GCC installation: /usr/bin/../lib/gcc/x86_64-linux-gnu/9
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/8
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/9
Selected GCC installation: /usr/bin/../lib/gcc/x86_64-linux-gnu/9
Candidate multilib: .;@m64
Selected multilib: .;@m64</pre>
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