[clang-tidy] new check: bugprone-too-small-loop-variable

The new checker searches for those for loops which has a loop variable with a "too small" type which means this type can't represent all values which are part of the iteration range.

For example:

```
int main() {
  long size = 300000;
  for( short int i = 0; i < size; ++i) {}
}
```

The short type leads to infinite loop here because it can't store all values in the `[0..size]` interval. In a real use case, size means a container's size which depends on the user input. Which means for small amount of objects the algorithm works, but with a larger user input the software will freeze.

The idea of the checker comes from the LibreOffice project, where the same check was implemented as a clang compiler plugin, called `LoopVarTooSmall` (LLVM licensed).
The idea is the same behind this check, but the code is different because of the different framework.

Patch by ztamas.

Reviewers: alexfh, hokein, aaron.ballman, JonasToth, xazax.hun, whisperity

Reviewed By: JonasToth, whisperity

Differential Revision: https://reviews.llvm.org/D53974

3 files changed, 37 insertions, 0 deletions

diff --git a/clang-tools-extra/docs/ReleaseNotes.rst b/clang-tools-extra/docs/ReleaseNotes.rst
index 45dffb92978..17d6238b818 100644
--- a/clang-tools-extra/docs/ReleaseNotes.rst
+++ b/clang-tools-extra/docs/ReleaseNotes.rst
@@ -110,6 +110,13 @@ Improvements to clang-tidy
   Flags uses of ``absl::StrCat()`` to append to a ``std::string``. Suggests
   ``absl::StrAppend()`` should be used instead.
 
+- New :doc:`bugprone-too-small-loop-variable
+  <clang-tidy/checks/bugprone-too-small-loop-variable>` check.
+
+  Detects those ``for`` loops that have a loop variable with a "too small" type
+  which means this type can't represent all values which are part of the
+  iteration range.
+
 - New :doc:`cppcoreguidelines-macro-usage
   <clang-tidy/checks/cppcoreguidelines-macro-usage>` check.
 
diff --git a/clang-tools-extra/docs/clang-tidy/checks/bugprone-too-small-loop-variable.rst b/clang-tools-extra/docs/clang-tidy/checks/bugprone-too-small-loop-variable.rst
new file mode 100644
index 00000000000..7688a3a567e
--- /dev/null
+++ b/clang-tools-extra/docs/clang-tidy/checks/bugprone-too-small-loop-variable.rst
@@ -0,0 +1,29 @@
+.. title:: clang-tidy - bugprone-too-small-loop-variable
+
+bugprone-too-small-loop-variable
+================================
+
+Detects those ``for`` loops that have a loop variable with a "too small" type
+which means this type can't represent all values which are part of the
+iteration range.
+
+.. code-block:: c++
+
+  int main() {
+    long size = 294967296l;
+    for (short i = 0; i < size; ++i) {}
+  }
+
+This ``for`` loop is an infinite loop because the ``short`` type can't represent
+all values in the ``[0..size]`` interval.
+
+In a real use case size means a container's size which depends on the user input.
+
+.. code-block:: c++
+
+  int doSomething(const std::vector& items) {
+    for (short i = 0; i < items.size(); ++i) {}
+  }
+
+This algorithm works for small amount of objects, but will lead to freeze for a
+a larger user input.
diff --git a/clang-tools-extra/docs/clang-tidy/checks/list.rst b/clang-tools-extra/docs/clang-tidy/checks/list.rst
index f33130b5e46..1d5468f47b9 100644
--- a/clang-tools-extra/docs/clang-tidy/checks/list.rst
+++ b/clang-tools-extra/docs/clang-tidy/checks/list.rst
@@ -59,6 +59,7 @@ Clang-Tidy Checks
    bugprone-swapped-arguments
    bugprone-terminating-continue
    bugprone-throw-keyword-missing
+   bugprone-too-small-loop-variable
    bugprone-undefined-memory-manipulation
    bugprone-undelegated-constructor
    bugprone-unused-raii