[CommandLineParser] Add DefaultOption flag

Summary: Add DefaultOption flag to CommandLineParser which provides a
default option or alias, but allows users to override it for some
other purpose as needed.

Also, add `-h` as a default alias to `-help`, which can be seamlessly
overridden by applications like llvm-objdump and llvm-readobj which
use `-h` as an alias for other options.

Reviewers: alexfh, klimek

Reviewed By: klimek

Subscribers: MaskRay, mehdi_amini, inglorion, dexonsmith, hiraditya, llvm-commits, jhenderson, arphaman, cfe-commits

Tags: #clang, #llvm

Differential Revision: https://reviews.llvm.org/D59746

llvm-svn: 358337

1 files changed, 10 insertions, 0 deletions

diff --git a/llvm/docs/CommandLine.rst b/llvm/docs/CommandLine.rst
index 46ba6d32bb3..d34fb2c3822 100644
--- a/llvm/docs/CommandLine.rst
+++ b/llvm/docs/CommandLine.rst
@@ -128,6 +128,7 @@ this:
   USAGE: compiler [options]
 
   OPTIONS:
+    -h                - Alias for -help
     -help             - display available options (-help-hidden for more)
     -o <filename>     - Specify output filename
 
@@ -194,6 +195,7 @@ declarations above, the ``-help`` option synopsis is now extended to:
   USAGE: compiler [options] <input file>
 
   OPTIONS:
+    -h                - Alias for -help
     -help             - display available options (-help-hidden for more)
     -o <filename>     - Specify output filename
 
@@ -1251,6 +1253,14 @@ specify boolean properties that modify the option.
   with ``cl::CommaSeparated``, this modifier only makes sense with a `cl::list`_
   option.
 
+.. _cl::DefaultOption:
+
+* The **cl::DefaultOption** modifier is used to specify that the option is a
+  default that can be overridden by application specific parsers. For example,
+  the ``-help`` alias, ``-h``, is registered this way, so it can be overridden
+  by applications that need to use the ``-h`` option for another purpose,
+  either as a regular option or an alias for another option.
+
 .. _response files:
 
 Response files