Re: [PATCH v2 2/2] ArmPlatformPkg: Fix comparison of constants warning

["Philippe Mathieu-Daudé" <philmd@redhat.com>]

Hi Sami,

On 5/18/20 2:46 PM, Sami Mujawar wrote:
> The VS2017 compiler reports 'warning C6326: potential
> comparison of a constant with another constant' when
> a fixed PCD value is compared with a constant value.
> 
> The faulting code is as marked by '-->' below:
> 
> --> if (FixedPcdGet32 (PL011UartInteger) != 0) {
>        Integer = FixedPcdGet32 (PL011UartInteger);
>        Fractional = FixedPcdGet32 (PL011UartFractional);
>      } else {
>      ...
> 
> The macro FixedPcdGet32 (PL011UartInteger) evaluates
> to a macro _PCD_VALUE_PL011UartInteger that is defined
> by the build system to represent the UART Integer
> value. Therefore, the VS2017 compiler reports the above
> warning.
> 
> In this case the warning reported by the Visual Studio
> compiler does not evaluate to an issue. However, it can
> be useful to detect potential issues in other scenarios.
> Other compilers may either be incapable of detecting and
> reporting comparison with constant warnings or may be
> good at reducing false positives. So, it is definitely
> useful to keep this warning enabled, and disabling it
> case by case is a suitable option.
> 
> Therefore, disable this warning for Visual studio
> compilers using the pragma suppress directive that:
> 'Pushes the current state of the pragma on the stack,
> disables the specified warning for the next line, and
> then pops the warning stack so that the pragma state
> is reset.'
> 
> Signed-off-by: Sami Mujawar <sami.mujawar@arm.com>
> ---
> 
> Notes:
>      v2:
>       - Update patch to selectively suppress comparison of         [SAMI]
>         constant warning and submit as a separate series.
>      
>      v1:
>       - Fix comparison of constant warning reported by VS2017      [SAMI]
>       - Various feedbacks can be seen at:
>         https://edk2.groups.io/g/devel/topic/32999801#46278
> 
>   ArmPlatformPkg/Library/PL011UartLib/PL011UartLib.c | 10 +++++++++-
>   1 file changed, 9 insertions(+), 1 deletion(-)
> 
> diff --git a/ArmPlatformPkg/Library/PL011UartLib/PL011UartLib.c b/ArmPlatformPkg/Library/PL011UartLib/PL011UartLib.c
> index 2d3c279cce49304959953ec4a34b50e09a7d0045..3c915e1e8de22a0b0b4cc46d495a5a6cbc784013 100644
> --- a/ArmPlatformPkg/Library/PL011UartLib/PL011UartLib.c
> +++ b/ArmPlatformPkg/Library/PL011UartLib/PL011UartLib.c
> @@ -2,7 +2,7 @@
>     Serial I/O Port library functions with no library constructor/destructor
>   
>     Copyright (c) 2008 - 2010, Apple Inc. All rights reserved.<BR>
> -  Copyright (c) 2011 - 2019, ARM Ltd. All rights reserved.<BR>
> +  Copyright (c) 2011 - 2020, ARM Ltd. All rights reserved.<BR>
>   
>     SPDX-License-Identifier: BSD-2-Clause-Patent
>   
> @@ -174,6 +174,14 @@ PL011UartInitializePort (
>     //
>   
>     // If PL011 Integer value has been defined then always ignore the BAUD rate
> +#if defined(_MSC_EXTENSIONS)
> +  // Suppress 'warning C6326' reported by Visual Studio compiler using
> +  // the suppress pragma directive that: 'Pushes the current state of
> +  // the pragma on the stack, disables the specified warning for the
> +  // next line, and then pops the warning stack so that the pragma state
> +  // is reset.'

We don't need to document how #pragma works each time we use it in the 
source code...

What about a simpler comment, referring the particular Visual Studio 
version:

//

// Disable 'potential comparison of a constant with another constant'
// warning with VS2017 compiler static code analysis option is enabled
//

> +#pragma warning(suppress:6326)
> +#endif
>     if (FixedPcdGet32 (PL011UartInteger) != 0) {
>       Integer = FixedPcdGet32 (PL011UartInteger);
>       Fractional = FixedPcdGet32 (PL011UartFractional);
>