<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/87010>87010</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            clang fails to eliminate redundant bounds check from std::vector::push_back
        </td>
    </tr>

    <tr>
      <th>Labels</th>
      <td>
            clang
      </td>
    </tr>

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

    <tr>
      <th>Reporter</th>
      <td>
          efriedma-quic
      </td>
    </tr>
</table>

<pre>
    Consider the following:

```
#include <vector>
void f1(std::vector<int>&x) { x.push_back(1);}
```

Compile with `-O2 -stdlib=libc++`, and looks at the resulting assembly.  (https://godbolt.org/z/d3GzvEGY4)

There are two bounds checks on the size of the vector: the first in the implementation of std::vector itself, which checks that the length of the vector isn't too large, and calls __throw_length_error if it fails.  Then, there's a check inside the allocator that the allocation isn't too large, and calls __throw_bad_array_new_length if it fails.  But the second check shouldn't be necessary: if the vector isn't too large, the allocation also isn't too large, since "too large" is defined exactly the same way for both checks.

(Briefly discussed in #64132, but I'm breaking it out into its own issue for clarity.)

CC @fhahn @hiraditya @philnik777
</pre>
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