Add documentation for -Rpass*

[dnovillo]

Summary

This adds documentation for -Rpass, -Rpass-missed and -Rpass-analysis.
It also adds release notes for 3.5.

[rsmith]

LGTM

[dnovillo]

Closed by commit rL209841 (authored by @dnovillo).

cfe/trunk/docs/ReleaseNotes.rst

 respectively.  `-integrated-as` and `-no-integrated-as` are now considered
 legacy flags (but are available as an alias to prevent breaking existing users),
 and users are encouraged to switch to the equivalent new feature flag.
 
 Deprecated flags `-faddress-sanitizer`, `-fthread-sanitizer`,
 `-fcatch-undefined-behavior` and `-fbounds-checking` were removed in favor of
 `-fsanitize=` family of flags.
 
+It is now possible to get optimization reports from the major transformation
+passes via three new flags: `-Rpass`, `-Rpass-missed` and `-Rpass-analysis`.
+These flags take a POSIX regular expression which indicates the name
+of the pass (or passes) that should emit optimization remarks.
+
 C Language Changes in Clang
 ---------------------------
 
 ...
 
 C11 Feature Support
 ^^^^^^^^^^^^^^^^^^^
 

cfe/trunk/docs/UsersManual.rst

 
 .. option:: -fno-crash-diagnostics
 
   Disable auto-generation of preprocessed source files during a clang crash.
 
 The -fno-crash-diagnostics flag can be helpful for speeding the process
 of generating a delta reduced test case.
 
+Options to Emit Optimization Reports
+------------------------------------
+
+Optimization reports trace, at a high-level, all the major decisions
+done by compiler transformations. For instance, when the inliner
+decides to inline function ``foo()`` into ``bar()``, or the loop unroller
+decides to unroll a loop N times, or the vectorizer decides to
+vectorize a loop body.
+
+Clang offers a family of flags which the optimizers can use to emit
+a diagnostic in three cases:
+
+1. When the pass makes a transformation (:option:`-Rpass`).
+
+2. When the pass fails to make a transformation (:option:`-Rpass-missed`).
+
+3. When the pass determines whether or not to make a transformation
+   (:option:`-Rpass-analysis`).
+
+NOTE: Although the discussion below focuses on :option:`-Rpass`, the exact
+same options apply to :option:`-Rpass-missed` and :option:`-Rpass-analysis`.
+
+Since there are dozens of passes inside the compiler, each of these flags
+take a regular expression that identifies the name of the pass which should
+emit the associated diagnostic. For example, to get a report from the inliner,
+compile the code with:
+
+.. code-block:: console
+
+   $ clang -O2 -Rpass=inline code.cc -o code
+   code.cc:4:25: remark: foo inlined into bar [-Rpass=inline]
+   int bar(int j) { return foo(j, j - 2); }
+                           ^
+
+Note that remarks from the inliner are identified with `[-Rpass=inline]`.
+To request a report from every optimization pass, you should use
+:option:`-Rpass=.*` (in fact, you can use any valid POSIX regular
+expression). However, do not expect a report from every transformation
+made by the compiler. Optimization remarks do not really make sense
+outside of the major transformations (e.g., inlining, vectorization,
+loop optimizations) and not every optimization pass supports this
+feature.
+
+Current limitations
+^^^^^^^^^^^^^^^^^^^
+
+1. For :option:`-Rpass` to provide source location information, you
+   need to enable debug line tables and column information. That is,
+   you need to add :option:`-gmlt` (or any of the debug-generating
+   flags) and :option:`-gcolumn-info`. If you omit these options,
+   every remark will be accompanied by a note stating that line number
+   information is missing.
+
+2. Optimization remarks that refer to function names will display the
+   mangled name of the function. Since these remarks are emitted by the
+   back end of the compiler, it does not know anything about the input
+   language, nor its mangling rules.
+
+3. Some source locations are not displayed correctly. The front end has
+   a more detailed source location tracking than the locations included
+   in the debug info (e.g., the front end can locate code inside macro
+   expansions). However, the locations used by :option:`-Rpass` are
+   translated from debug annotations. That translation can be lossy,
+   which results in some remarks having no location information.
+
+
 Language and Target-Independent Features
 ========================================
 
 Controlling Errors and Warnings
 -------------------------------
 
 Clang provides a number of ways to control which code constructs cause
 it to emit errors and warning messages, and how they are displayed to