MdeModulePkg: Replace include guards with #pragma once

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>

2 files changed, 2 insertions, 8 deletions

diff --git a/MdeModulePkg/Library/PiSmmCoreMemoryAllocationLib/PiSmmCoreMemoryAllocationServices.h b/MdeModulePkg/Library/PiSmmCoreMemoryAllocationLib/PiSmmCoreMemoryAllocationServices.h
index 64429e028a..2a1f9734e6 100644
--- a/MdeModulePkg/Library/PiSmmCoreMemoryAllocationLib/PiSmmCoreMemoryAllocationServices.h
+++ b/MdeModulePkg/Library/PiSmmCoreMemoryAllocationLib/PiSmmCoreMemoryAllocationServices.h
@@ -9,8 +9,7 @@
 

 **/

 

-#ifndef _PI_SMM_CORE_MEMORY_ALLOCATION_SERVICES_H_

-#define _PI_SMM_CORE_MEMORY_ALLOCATION_SERVICES_H_

+#pragma once

 

 //

 // It should be aligned with the definition in PiSmmCore.

@@ -181,5 +180,3 @@ EFIAPI
 SmmFreePool (

   IN      VOID  *Buffer

   );

-

-#endif

diff --git a/MdeModulePkg/Library/PiSmmCoreMemoryAllocationLib/PiSmmCoreMemoryProfileServices.h b/MdeModulePkg/Library/PiSmmCoreMemoryAllocationLib/PiSmmCoreMemoryProfileServices.h
index 236a5ead25..a2c75ac5ac 100644
--- a/MdeModulePkg/Library/PiSmmCoreMemoryAllocationLib/PiSmmCoreMemoryProfileServices.h
+++ b/MdeModulePkg/Library/PiSmmCoreMemoryAllocationLib/PiSmmCoreMemoryProfileServices.h
@@ -9,8 +9,7 @@
 

 **/

 

-#ifndef _PI_SMM_CORE_MEMORY_PROFILE_SERVICES_H_

-#define _PI_SMM_CORE_MEMORY_PROFILE_SERVICES_H_

+#pragma once

 

 /**

   Update SMRAM profile information.

@@ -44,5 +43,3 @@ SmmCoreUpdateProfile (
   IN VOID                   *Buffer,

   IN CHAR8                  *ActionString OPTIONAL

   );

-

-#endif