<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/102768>102768</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            [libc++] LLVM libc++'s fstream is ridiculously slow compared to LLVM libstdc++
        </td>
    </tr>

    <tr>
      <th>Labels</th>
      <td>
            libc++
      </td>
    </tr>

    <tr>
      <th>Assignees</th>
      <td>
      </td>
    </tr>

    <tr>
      <th>Reporter</th>
      <td>
          trcrsired
      </td>
    </tr>
</table>

<pre>
    ```
D:\msys64\home\unlvs\projects\fast_io\benchmark\0002.file_io_tsc\time>clang++ -o fstream fstream.cc -O3 -std=c++26 -I../../include -lntdll -s -flto=thin -fuse-ld=lld --sysroot=D:\toolchains\llvm\x86_64-windows-gnu\x86_64-windows-gnu -rtlib=compiler-rt --unwindlib=libunwind --stdlib=libc++ -lntdll -lc++abi -I../../../include

D:\msys64\home\unlvs\projects\fast_io\benchmark\0002.file_io_tsc\time>fstream
output:9.5154552s
input:2.6723663s

D:\msys64\home\unlvs\projects\fast_io\benchmark\0002.file_io_tsc\time>clang++ -o fstream fstream.cc -O3 -std=c++26 -I../../include -lntdll -s -flto=thin -fuse-ld=lld -lntdll -I../../../include

D:\msys64\home\unlvs\projects\fast_io\benchmark\0002.file_io_tsc\time>fstream
output:0.3279629s
input:0.4409194s

D:\msys64\home\unlvs\projects\fast_io\benchmark\0002.file_io_tsc\time>clang++ -o iobuf_file iobuf_file.cc -O3 -std=c++26 -I../../include -lntdll -s -flto=thin -fuse-ld=lld --sysroot=D:\toolchains\llvm\x86_64-windows-gnu\x86_64-windows-gnu -rtlib=compiler-rt --unwindlib=libunwind --stdlib=libc++ -lntdll -lc++abi -I../../../include

D:\msys64\home\unlvs\projects\fast_io\benchmark\0002.file_io_tsc\time>iobuf_file.exe
output:0.0587952s
input:0.1234146s

D:\msys64\home\unlvs\projects\fast_io\benchmark\0002.file_io_tsc\time>clang++ -o filebuf_file filebuf_file.cc -O3 -std=c++26 -I../../include -lntdll -s -flto=thin -fuse-ld=lld --sysroot=D:\toolchains\llvm\x86_64-windows-gnu\x86_64-windows-gnu -rtlib=compiler-rt --unwindlib=libunwind --stdlib=libc++ -lntdll -lc++abi -I../../../include

D:\msys64\home\unlvs\projects\fast_io\benchmark\0002.file_io_tsc\time>filebuf_file
output:0.3089046s
input:0.1379314s
```



This is at least 20x slower than libstdc++, and I am sure it is not the buffering part but the formatting part that is ridiculously slow since the ```fast_io::filebuf_file``` isn't slow.

https://github.com/cppfastio/fast_io/blob/next/benchmark/0002.file_io_tsc/time/fstream.cc

Benchmark is here.
</pre>
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