[edk2-devel] [PATCH v2 2/2] ArmPlatformPkg: Fix comparison of constants warning

[Sami Mujawar]

Hi Philippe,

I had put a descriptive message to make people aware that the pragma effects the next line of code only. But I agree we can abbreviate it as per your suggestion.
However, there is an ongoing discussion about an alternative (by which we could avoid the pragma) on another thread at https://edk2.groups.io/g/devel/message/59948.

Regards,

Sami Mujawar

-----Original Message-----
From: Philippe Mathieu-Daudé <philmd at redhat.com> 
Sent: 20 May 2020 11:47 AM
To: Sami Mujawar <Sami.Mujawar at arm.com>; devel at edk2.groups.io
Cc: Alexei Fedorov <Alexei.Fedorov at arm.com>; Ard Biesheuvel <Ard.Biesheuvel at arm.com>; leif at nuviainc.com; Matteo Carlini <Matteo.Carlini at arm.com>; Laura Moretta <Laura.Moretta at arm.com>; nd <nd at arm.com>
Subject: Re: [PATCH v2 2/2] ArmPlatformPkg: Fix comparison of constants warning

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 at 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..3c915e1e8de22a0b0b4cc46d495a
> 5a6cbc784013 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);
> 


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