[llvm-bugs] [Bug 51113] New: Declaring an array of struct without explicit constructor causes excessive memory consumption

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=51113

            Bug ID: 51113
           Summary: Declaring an array of struct without explicit
                    constructor causes excessive memory consumption
           Product: clang
           Version: unspecified
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: Frontend
          Assignee: unassignedclangbugs at nondot.org
          Reporter: mivik at qq.com
                CC: llvm-bugs at lists.llvm.org, neeilans at live.com,
                    richard-llvm at metafoo.co.uk

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

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