Replace traditional `#ifndef`/`#define`/`#endif` include guards with
`#pragma` once.
`#pragma once` is a widely supported preprocessor directive that
prevents header files from being included multiple times. It is
supported by all toolchains used to build edk2: GCC, Clang/LLVM, and
MSVC.
Compared to macro-based include guards, `#pragma once`:
- Eliminates the risk of macro name collisions or copy/paste errors
where two headers inadvertently use the same guard macro.
- Eliminate inconsistency in the way include guard macros are named
(e.g., some files use `__FILE_H__`, others use `FILE_H_`, etc.).
- Reduces boilerplate (three lines replaced by one).
- Avoids polluting the macro namespace with guard symbols.
- Can improve build times as the preprocessor can skip re-opening the
file entirely, rather than re-reading it to find the matching
`#endif` ("multiple-include optimization").
- Note that some compilers may already optimize traditional include
guards, by recognzining the idiomatic pattern.
This change is made acknowledging that overall portability of the
code will technically be reduced, as `#pragma once` is not part of the
C/C++ standards.
However, this is considered acceptable given:
1. edk2 already defines a subset of supported compilers in
BaseTools/Conf/tools_def.template, all of which have supported
`#pragma once` for over two decades.
2. There have been concerns raised to the project about inconsistent
include guard naming and potential macro collisions.
Approximate compiler support dates:
- MSVC: Supported since Visual C++ 4.2 (1996)
- GCC: Supported since 3.4 (2004)
(http://gnu.ist.utl.pt/software/gcc/gcc-3.4/changes.html)
- Clang (LLVM based): Since initial release in 2007
Signed-off-by: Michael Kubacki <michael.kubacki@microsoft.com>
The DynamicTablesPkg allows to generate firmware tables based on
information provided by a user or another source of information
(a device tree for instance).
Some information might be implicitly generated by generators.
For instance, for ACPI tables, AML names or Uids are created by
generators, but not provided by the user or another source of
information.
Some generators might need to cross-reference object/generated data
that was previous generated for an object by another generator.
For instance, there are three different generators creating serial
ports in the AML namespace. These 3 generators must ensure not to use
an Id which was not already created by another generator.
Another example would be a generator needing to reference the AML
path of a serial port. As the AML path is dynamically generated, this
is currently not possible to do.
Add a MetaDataObjLib library to keep track of the meta-data previously
generated for an object.
Signed-off-by: Pierre Gondois <pierre.gondois@arm.com>
