[Clang][analyzer] add documentation for optin performance padding (padding checker) #73675

[komalverma04]

Added documentation for optin.performance.Padding.

• Performance package has PaddingChecker checker.
• It checks for excessively padded structs.
  • It has one option that is AllowedPad, an integer option.
  • It's default value is 24 bytes.
    • Reports are generated when padding exceeds AllowedPad.

Purpose

The purpose of this pull request is to improve the clarity and completeness of the documentation for PaddingChecker in the optin.performance.Padding checker.

Closes #73675

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[llvmbot]

@llvm/pr-subscribers-clang-static-analyzer-1

@llvm/pr-subscribers-clang

Author: None (komalverma04)

Changes

Added documentation for optin.performance.Padding.

• Performance package has PaddingChecker checker.
• It checks for excessively padded structs.
  • It has one option that is AllowedPad, an integer option.
  • It's default value is 24 bytes.
    • Reports are generated when padding exceeds AllowedPad.

Purpose

The purpose of this pull request is to improve the clarity and completeness of the documentation for PaddingChecker in the optin.performance.Padding checker.

Closes #73675

---

Full diff: https://github.com/llvm/llvm-project/pull/86411.diff

2 Files Affected:

• (modified) clang/docs/analyzer/checkers.rst (+20-2)
• (modified) clang/include/clang/StaticAnalyzer/Checkers/Checkers.td (+1-1)

diff --git a/clang/docs/analyzer/checkers.rst b/clang/docs/analyzer/checkers.rst
index fe211514914272..64b09bc6ecd1d8 100644
--- a/clang/docs/analyzer/checkers.rst
+++ b/clang/docs/analyzer/checkers.rst
@@ -804,10 +804,28 @@ Check for performance anti-patterns when using Grand Central Dispatch.
 
 .. _optin-performance-Padding:
 
-optin.performance.Padding
-"""""""""""""""""""""""""
+optin.performance.Padding (PaddingChecker)
+"""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""
 Check for excessively padded structs.
 
+.. code-block:: objc
+
+ struct TestStruct {
+      int data1;   // 4 bytes
+      char data2;  // 1 byte
+      char padding[27];  // 27 bytes of padding
+    };  // Total size: 32 bytes 
+  
+  void TestFunction() {
+      struct TestStruct struct1;  // Warning is generated due to excessive padding.
+    }
+
+   // Reports are only generated if the excessive padding exceeds 'AllowedPad' in bytes.
+
+  // AllowedPad: Default Value: 24 bytes
+
+   Usage: `AllowedPad=<value>`
+
 .. _optin-portability-UnixAPI:
 
 optin.portability.UnixAPI
diff --git a/clang/include/clang/StaticAnalyzer/Checkers/Checkers.td b/clang/include/clang/StaticAnalyzer/Checkers/Checkers.td
index 686e5e99f4a62c..c0e4e9a70c2ef3 100644
--- a/clang/include/clang/StaticAnalyzer/Checkers/Checkers.td
+++ b/clang/include/clang/StaticAnalyzer/Checkers/Checkers.td
@@ -908,7 +908,7 @@ def PaddingChecker : Checker<"Padding">,
                   "24",
                   Released>
   ]>,
-  Documentation<NotDocumented>;
+  Documentation<HasDocumentation>;
 
 } // end: "padding"
 

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[komalverma04]

@AtariDreams Thank you for pointing out. I apologise for that mistake. Please review it, if there are more changes.

[steakhal]

I figured, it might be easier to directly propose and apply some of my thoughts. Let me know if you like it.

My goal was:

• be specific what is the impact of not having the optimal layout: memory, cachemisses
• have a single block of examples, demonstrating the bad, the good and an alternative good (bitpacking) case, while have no header dependencies
• rework the example to work with the default configuration of the checker (this makes it more similar to the average user's diagnostics)
• move all the advanced explanations to the end, as most users won't consider setting a custom threshold
• make the definition of "when it warns" more accurate. I've checked the code and it seems that the difference counts compared to the optimal layout.
• discourage the use of pragma pack
• add guide for fixing the issue, along with a practical hint.

Given that it looks now as I would like it, I'll invite @NagyDonat for a round of review. Let me know if you like it too, or have suggestions for making it better.

Lastly, now the generated html would look like this:

[komalverma04]

Okay, i completely agree with you. Thank you so much for helping.

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