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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>
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Resource data are defined in the ACPI 6.3 specification,
s6.4 "Resource Data Types for ACPI". They can be created
using the ASL ResourceTemplate () statement, cf s19.3.3
"ASL Resource Templates".
Resource data can be of the small or large type and are
defined by their encoding. The resource data is stored
in the Bytelist of a BufferOp node. The Bytelist of a
BufferOp node is represented by an AML Data node in
the AML tree.
The resource data parser, examines the Bytelist (Data
node buffer) to detect the presence of resource data.
If the Bytelist data matches the encoding for resource
data types, the resource data parser fragments the
Bytelist containing the resource data buffer into
resource data elements represented as individual Data
nodes and stores them in the variable arguments list
of the BufferOp object nodes.
Example: ASL code and the corresponding AML tree
representation for the resource data.
ASL Code
--------
Name (_CRS, ResourceTemplate() {
QWordMemory (...)
Interrupt (...)
}
AML Tree
--------
(NameOp)
\
|-[_CRS]-[BufferOp] # Fixed Arguments
|-{NULL} \ # Variable Argument
\ list
|-[BuffSize] # Fixed Arguments
|-{(Rd1)->(Rd2)->(EndTag)} # Variable Argument
list
Where:
Rd1 - QWordMemory resource data element.
Rd2 - Interrupt resource data element.
EndTag - Resource data end tag.
Signed-off-by: Pierre Gondois <pierre.gondois@arm.com>
Signed-off-by: Sami Mujawar <sami.mujawar@arm.com>
Reviewed-by: Alexei Fedorov <Alexei.Fedorov@arm.com>
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