[clang-tidy] New checker for redundant expressions.

Summary:
This checker finds redundant expression on both side of a binary operator.

The current implementation provide a function to check whether expressions
are equivalent. This implementation is able to recognize the common
subset encounter in C++ program. Side-effects like "x++" are not considered
to be equivalent.

There are many False Positives related to macros and to floating point
computations (detecting NaN). The checker is ignoring these cases.

Example:
```
    if( !dst || dst->depth != desired_depth ||
        dst->nChannels != desired_num_channels ||
        dst_size.width != src_size.width ||
        dst_size.height != dst_size.height )    <<--- bug
    {
```

Reviewers: alexfh

Subscribers: danielmarjamaki, fahlgren, jordan_rose, zaks.anna, Eugene.Zelenko, cfe-commits

Differential Revision: http://reviews.llvm.org/D19451

llvm-svn: 267574

2 files changed, 21 insertions, 0 deletions

diff --git a/clang-tools-extra/docs/clang-tidy/checks/list.rst b/clang-tools-extra/docs/clang-tidy/checks/list.rst
index db31a73fbba..113b341add2 100644
--- a/clang-tools-extra/docs/clang-tidy/checks/list.rst
+++ b/clang-tools-extra/docs/clang-tidy/checks/list.rst
@@ -67,6 +67,7 @@ Clang-Tidy Checks
    misc-noexcept-move-constructor
    misc-non-copyable-objects
    misc-pointer-and-integral-operation
+   misc-redundant-expression
    misc-sizeof-container
    misc-sizeof-expression
    misc-static-assert
diff --git a/clang-tools-extra/docs/clang-tidy/checks/misc-redundant-expression.rst b/clang-tools-extra/docs/clang-tidy/checks/misc-redundant-expression.rst
new file mode 100644
index 00000000000..812031d91b2
--- /dev/null
+++ b/clang-tools-extra/docs/clang-tidy/checks/misc-redundant-expression.rst
@@ -0,0 +1,20 @@
+.. title:: clang-tidy - misc-redundant-expression
+
+misc-redundant-expression
+=========================
+
+Detect redundant expressions which are typically errors due to copy-paste.
+
+Depending on the operator expressions may be
+  * redundant,
+  * always be `true`,
+  * always be `false`,
+  * always be a constant (zero or one)
+
+Example:
+.. code:: c++
+
+  ((x+1) | (x+1))             // (x+1) is redundant
+  (p->x == p->x)              // always true
+  (p->x < p->x)               // always false
+  (speed - speed + 1 == 12)   // speed - speed is always zero