<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/118637>118637</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            GSYM: lookup functionality is lacking for decoded FunctionInfo
        </td>
    </tr>

    <tr>
      <th>Labels</th>
      <td>
            new issue
      </td>
    </tr>

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

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

<pre>
    `GsymReader` has a very convenient `lookup(uint64_t Addr)` method, which is an excellent utility for one off lookup, but doesn't suit repeated lookups that well due to it parsing `FunctionInfo` over and over again.

Documentation gives a hint that `getFunctionInfo`/`getFunctionInfoAtIndex` could be used instead:
```
  /// This should be called when a client will store a copy of the complete
  /// FunctionInfo for a given address. For one off lookups, use the lookup()
 /// function below.
  ///
  /// Symbolication server processes might want to parse the entire function
  /// info for a given address and cache it if the process stays around to
  /// service many symbolication addresses, like for parsing profiling
  /// information.
```
, however there is no functionality to easily lookup an address in the resulting `FunctionInfo` (it only has a static `lookup` version, which parses `FunctionInfo` data stream).

I think `llvm::Expected<LookupResult> FunctionInfo::lookup(uint64_t Addr) const` could be a welcome addition, so one doesn't have to dig into `static lookup` implementation to actually use `FunctionInfo` returned from `GsymReader::getFunctionInfo` for symbolization.

I've managed to piece together what I believe to be a correct implementation of `FunctionInfo::lookup(uint64 Addr)`, should I try to upstream it?
</pre>
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