Re: [PATCH 1/1] OvmfPkg/IoMmuDxe: shut up "unused-const-variable" gcc-6 warning in RELEASE

[Laszlo Ersek <lersek@redhat.com>]

On 09/06/17 18:56, Ard Biesheuvel wrote:
> On 6 September 2017 at 17:48, Laszlo Ersek <lersek@redhat.com> wrote:
>> Starting with gcc-6, a new warning option called "-Wunused-const-variable"
>> has become available (and enabled by default under our build settings):
>>
>>   https://gcc.gnu.org/onlinedocs/gcc-6.4.0/gcc/Warning-Options.html
>>
>> We should give it the same treatment as Ard gave "unused-but-set-variable"
>> in commit 20d00edf21d2 ("BaseTools/GCC: set -Wno-unused-but-set-variables
>> only on RELEASE builds", 2016-03-24); i.e., we should restrict the warning
>> to the DEBUG build target.
>>
>> However, because the new warning is gcc-6+ only, we cannot add
>> "-Wno-unused-const-variable" to any GCC5 macros in
>> "BaseTools/Conf/tools_def.template". While the GCC6 toolchain and/or the
>> desired handling of the new warning are investigated in
>> <https://bugzilla.tianocore.org/show_bug.cgi?id=700>, suppress the warning
>> for gcc-6+ (in RELEASE builds) as follows:
>>
>> - Replace the "mBusMasterOperationName" array with the
>>   BUS_MASTER_OPERATION_NAME(Expression, ShortName) macro that compares
>>   Expression against EdkiiIoMmuOperationBusMaster<ShortName>, and in case
>>   of a match, evaluates to "ShortName", stringified,
>>
>> - when composing the DEBUG message -- which the preprocessor might
>>   eliminate from RELEASE builds, thereby removing its references to
>>   variables --, build a ladder of comparisons with
>>   BUS_MASTER_OPERATION_NAME().
>>
> 
> I wonder if we should put this in a header file somewhere. I really
> hate that we need to create new GCCx versions just for small
> deviations between versions of the compiler.
> 
> I.e., add this to MdePkg/Include/Base.h
> 
> #ifdef MDEPKG_NDEBUG
>   //
>   // Don't warn about unused but set variables: they may only be
>   // referenced by DEBUG code.
>   //
>   #pragma GCC diagnostic ignored "-Wunused-but-set-variable"
>   #if __GNUC__ >= 6
>     #pragma GCC diagnostic ignored "-Wunused-const-variable"
>   #endif
> #endif

The option in question is "-Wno-unused-const-variable". But, that's a
side question.

If the above is acceptable to the MdePkg maintainers, it is certainly
good enough for me. In that case, I will reassign TianoCore BZ#700 to
MdePkg. (I was careful to write "desired handling of the new warning" in
the commit message, because I expected that the introduction of the GCC6
toolchain just for this might not be universally liked.)

Personally I wouldn't like to submit the MdePkg patch myself, as GCC
pragmas are another delicacy that doesn't fit on my plate right now. (If
I happen to break something with it, I get to fix it too... Like I'm
fixing it now within OvmfPkg.)

The point of this patch is to get OVMF back to a building state ASAP.
Once we fix BZ#700, I'm more than happy to revert the patch. I sought to
write the patch in a way that would be tolerable both short-term and
mid/long-term.

Thanks!
Laszlo


> 
> 
>> Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
>> Cc: Brijesh Singh <brijesh.singh@amd.com>
>> Cc: Jordan Justen <jordan.l.justen@intel.com>
>> Cc: Thomas Lamprecht <t.lamprecht@proxmox.com>
>> Reported-by: Thomas Lamprecht <t.lamprecht@proxmox.com>
>> Contributed-under: TianoCore Contribution Agreement 1.1
>> Signed-off-by: Laszlo Ersek <lersek@redhat.com>
>> ---
>>  OvmfPkg/IoMmuDxe/AmdSevIoMmu.c | 29 +++++++++++---------
>>  1 file changed, 16 insertions(+), 13 deletions(-)
>>
>> diff --git a/OvmfPkg/IoMmuDxe/AmdSevIoMmu.c b/OvmfPkg/IoMmuDxe/AmdSevIoMmu.c
>> index bc57de5b572b..dcd03d8f0474 100644
>> --- a/OvmfPkg/IoMmuDxe/AmdSevIoMmu.c
>> +++ b/OvmfPkg/IoMmuDxe/AmdSevIoMmu.c
>> @@ -45,17 +45,17 @@ STATIC LIST_ENTRY mRecycledMapInfos = INITIALIZE_LIST_HEAD_VARIABLE (
>>  #define COMMON_BUFFER_SIG SIGNATURE_64 ('C', 'M', 'N', 'B', 'U', 'F', 'F', 'R')
>>
>>  //
>> -// ASCII names for EDKII_IOMMU_OPERATION constants, for debug logging.
>> +// The following macro builds the first two operands of a conditional (ternary)
>> +// operator that matches Expression against the EDKII_IOMMU_OPERATION constant
>> +// derived from ShortName. In case of a match, the replacement text evaluates
>> +// to an ASCII string literal that stands for the constant. The replacement
>> +// text stops before the colon (":"). The macro invocation site is supposed to
>> +// provide the colon and the third operand, either as another invocation of the
>> +// same macro, or as a default (fallback) string literal.
>>  //
>> -STATIC CONST CHAR8 * CONST
>> -mBusMasterOperationName[EdkiiIoMmuOperationMaximum] = {
>> -  "Read",
>> -  "Write",
>> -  "CommonBuffer",
>> -  "Read64",
>> -  "Write64",
>> -  "CommonBuffer64"
>> -};
>> +#define BUS_MASTER_OPERATION_NAME(Expression, ShortName) \
>> +          ((Expression) == (EdkiiIoMmuOperationBusMaster ## ShortName)) ? \
>> +          (# ShortName)
>>
>>  //
>>  // The following structure enables Map() and Unmap() to perform in-place
>> @@ -133,9 +133,12 @@ IoMmuMap (
>>      DEBUG_VERBOSE,
>>      "%a: Operation=%a Host=0x%p Bytes=0x%Lx\n",
>>      __FUNCTION__,
>> -    ((Operation >= 0 &&
>> -      Operation < ARRAY_SIZE (mBusMasterOperationName)) ?
>> -     mBusMasterOperationName[Operation] :
>> +    (BUS_MASTER_OPERATION_NAME (Operation, Read) :
>> +     BUS_MASTER_OPERATION_NAME (Operation, Write) :
>> +     BUS_MASTER_OPERATION_NAME (Operation, CommonBuffer) :
>> +     BUS_MASTER_OPERATION_NAME (Operation, Read64) :
>> +     BUS_MASTER_OPERATION_NAME (Operation, Write64) :
>> +     BUS_MASTER_OPERATION_NAME (Operation, CommonBuffer64) :
>>       "Invalid"),
>>      HostAddress,
>>      (UINT64)((NumberOfBytes == NULL) ? 0 : *NumberOfBytes)
>> --
>> 2.14.1.3.gb7cf6e02401b
>>