FatPkg: Refine casting expression result to bigger size

There are cases that the operands of an expression are all with rank less
than UINT64/INT64 and the result of the expression is explicitly cast to
UINT64/INT64 to fit the target size.

An example will be:
UINT32 a,b;
// a and b can be any unsigned int type with rank less than UINT64, like
// UINT8, UINT16, etc.
UINT64 c;
c = (UINT64) (a + b);

Some static code checkers may warn that the expression result might
overflow within the rank of "int" (integer promotions) and the result is
then cast to a bigger size.

The commit refines codes by the following rules:
1). When the expression is possible to overflow the range of unsigned int/
int:
c = (UINT64)a + b;

2). When the expression will not overflow within the rank of "int", remove
the explicit type casts:
c = a + b;

3). When the expression will be cast to pointer of possible greater size:
UINT32 a,b;
VOID *c;
c = (VOID *)(UINTN)(a + b); --> c = (VOID *)((UINTN)a + b);

4). When one side of a comparison expression contains only operands with
rank less than UINT32:
UINT8 a;
UINT16 b;
UINTN c;
if ((UINTN)(a + b) > c) {...} --> if (((UINT32)a + b) > c) {...}

For rule 4), if we remove the 'UINTN' type cast like:
if (a + b > c) {...}
The VS compiler will complain with warning C4018 (signed/unsigned
mismatch, level 3 warning) due to promoting 'a + b' to type 'int'.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Hao Wu <hao.a.wu@intel.com>
Reviewed-by: Ruiyu Ni <ruiyu.ni@intel.com>

1 files changed, 2 insertions, 2 deletions

diff --git a/FatPkg/FatPei/FatLiteAccess.c b/FatPkg/FatPei/FatLiteAccess.c
index 110634508b..a92c5bff99 100644
--- a/FatPkg/FatPei/FatLiteAccess.c
+++ b/FatPkg/FatPei/FatLiteAccess.c
@@ -1,7 +1,7 @@
 /** @file

   FAT file system access routines for FAT recovery PEIM

 

-Copyright (c) 2006 - 2013, Intel Corporation. All rights reserved.<BR>

+Copyright (c) 2006 - 2017, Intel Corporation. All rights reserved.<BR>

 

 This program and the accompanying materials are licensed and made available

 under the terms and conditions of the BSD License which accompanies this

@@ -393,7 +393,7 @@ FatReadFile (
   } else {

 

     if ((File->Attributes & FAT_ATTR_DIRECTORY) == 0) {

-      Size = Size < (File->FileSize - File->CurrentPos) ? Size : (UINTN) (File->FileSize - File->CurrentPos);

+      Size = Size < (File->FileSize - File->CurrentPos) ? Size : (File->FileSize - File->CurrentPos);

     }

     //

     // This is a normal cluster based file