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This allows command line tools to use syntaxes like the following: llvm-foo.exe command1 -o1 -o2 llvm-foo.exe command2 -p1 -p2 Where command1 and command2 contain completely different sets of valid options. This is backwards compatible with previous uses of llvm cl which did not support subcommands, as any option which specifies no optional subcommand (e.g. all existing code) goes into a special "top level" subcommand that expects dashed options to appear immediately after the program name. For example, code which is subcommand unaware would generate a command line such as the following, where no subcommand is specified: llvm-foo.exe -q1 -q2 The top level subcommand can co-exist with actual subcommands, as it is implemented as an actual subcommand which is searched if no explicit subcommand is specified. So llvm-foo.exe as specified above could be written so as to support all three aforementioned command lines simultaneously. There is one additional "special" subcommand called AllSubCommands, which can be used to inject an option into every subcommand. This is useful to support things like help, so that commands such as: llvm-foo.exe --help llvm-foo.exe command1 --help llvm-foo.exe command2 --help All work and display the help for the selected subcommand without having to explicitly go and write code to handle each one separately. This patch is submitted without an example of anything actually using subcommands, but a followup patch will convert the llvm-pdbdump tool to use subcommands. Reviewed By: beanz Differential Revision: http://reviews.llvm.org/D21485 llvm-svn: 274054
Design Of lib/System
====================
The software in this directory is designed to completely shield LLVM from any
and all operating system specific functionality. It is not intended to be a
complete operating system wrapper (such as ACE), but only to provide the
functionality necessary to support LLVM.
The software located here, of necessity, has very specific and stringent design
rules. Violation of these rules means that cracks in the shield could form and
the primary goal of the library is defeated. By consistently using this library,
LLVM becomes more easily ported to new platforms since the only thing requiring
porting is this library.
Complete documentation for the library can be found in the file:
llvm/docs/SystemLibrary.html
or at this URL:
http://llvm.org/docs/SystemLibrary.html
While we recommend that you read the more detailed documentation, for the
impatient, here's a high level summary of the library's requirements.
1. No system header files are to be exposed through the interface.
2. Std C++ and Std C header files are okay to be exposed through the interface.
3. No exposed system-specific functions.
4. No exposed system-specific data.
5. Data in lib/System classes must use only simple C++ intrinsic types.
6. Errors are handled by returning "true" and setting an optional std::string
7. Library must not throw any exceptions, period.
8. Interface functions must not have throw() specifications.
9. No duplicate function impementations are permitted within an operating
system class.
To accomplish these requirements, the library has numerous design criteria that
must be satisfied. Here's a high level summary of the library's design criteria:
1. No unused functionality (only what LLVM needs)
2. High-Level Interfaces
3. Use Opaque Classes
4. Common Implementations
5. Multiple Implementations
6. Minimize Memory Allocation
7. No Virtual Methods