Re: [PATCH 2/2] BaseTools/tools_def: AARCH64: disable LTO type mismatch warnings

["Gao, Liming" <liming.gao@intel.com>]

Reviewed-by: Liming Gao <liming.gao@intel.com>

>-----Original Message-----
>From: Leif Lindholm [mailto:leif.lindholm@linaro.org]
>Sent: Wednesday, June 21, 2017 4:00 AM
>To: Ard Biesheuvel <ard.biesheuvel@linaro.org>
>Cc: edk2-devel@lists.01.org; Gao, Liming <liming.gao@intel.com>; Zhu,
>Yonghong <yonghong.zhu@intel.com>; lersek@redhat.com
>Subject: Re: [PATCH 2/2] BaseTools/tools_def: AARCH64: disable LTO type
>mismatch warnings
>
>On Tue, Jun 20, 2017 at 08:43:54PM +0200, Ard Biesheuvel wrote:
>> On AARCH64, any code that may execute with the MMU off needs to be
>built
>> with -mstrict-align, given that unaligned accesses are not allowed unless
>> the MMU is enabled. This does not only affect SEC and PEI modules, but
>> also static libraries of the BASE type, which may be linked into such
>> modules, as well as into modules of other types. As it turns out, the
>> presence of -mstrict-align is reflected in the internal representations
>> of the types defined in those libraries.
>>
>> When -fstrict-aliasing is passed to GCC, it assumes that pointers to
>> objects of different types cannot refer to the same memory location, and
>> attempts to exploit this fact when optimizing the code. Since such
>> assumptions are only valid under very strict conditions which are not
>> guaranteed to be met in EDK2, we disable this optimization by passing
>> -fno-strict-aliasing by default.
>
>Just a comment - you don't necessarily have to update this excellent
>commit mesage, but:
>-fstrict-aliasing is GCC default, because it is a restriction in the C
>language. Because it's a bit non-obvious, things can go hilariously
>wrong in very non-obvious ways, and the potential optimization gains
>are ulikely to be generally relevant, -fno-strict-aliasing is a
>sensible thing to always have set (like we do).
>
>> When LTO is in effect, this applies equally to the code generation that
>> may occur at link time, which is why the linker warns about unexpected
>> differences in type definitions between the intermediate representations
>> that are present in the object files being linked. This may result in
>> warnings such as the one below, even if -fno-strict-aliasing is used:
>>
>>   MdePkg/Include/Library/BaseLib.h:1712:1:
>>   warning: type of 'StrToGuid' does not match original declaration
>>   [-Wlto-type-mismatch]
>>    StrToGuid (
>>    ^
>>   MdePkg/Library/BaseLib/SafeString.c:1506:1:
>>   note: 'StrToGuid' was previously declared here
>>    StrToGuid (
>>    ^
>>   MdePkg/Library/BaseLib/SafeString.c:1506:1:
>>   note: code may be misoptimized unless -fno-strict-aliasing is used
>>
>> This warning is inadvertently triggered when linking BASE libraries built
>> with -mstrict-align into modules of types other than SEC or PEI, since the
>> types are subtly different, even though the use of code that maintains
>> strict alignment in a module that does not care about this is unlikely to
>> cause problems. And even if it did, it would still only affect code built
>> with -fstrict-aliasing enabled, which we disable unconditionally. So let's
>> just silence the warning by passing -Wno-lto-type-mismatch.
>>
>> Contributed-under: TianoCore Contribution Agreement 1.0
>> Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
>
>Reviewed-by: Leif Lindholm <leif.lindholm@linaro.org>
>
>> ---
>>  BaseTools/Conf/tools_def.template | 2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/BaseTools/Conf/tools_def.template
>b/BaseTools/Conf/tools_def.template
>> index 7a58ce365ed2..9a3ba9defb12 100755
>> --- a/BaseTools/Conf/tools_def.template
>> +++ b/BaseTools/Conf/tools_def.template
>> @@ -5407,7 +5407,7 @@ RELEASE_GCC5_ARM_DLINK_FLAGS     =
>DEF(GCC5_ARM_DLINK_FLAGS) -flto -Os -L$(WORKS
>>    DEBUG_GCC5_AARCH64_DLINK_XIPFLAGS = -z common-page-size=0x20
>>
>>  RELEASE_GCC5_AARCH64_CC_FLAGS    = DEF(GCC5_AARCH64_CC_FLAGS) -
>flto -Wno-unused-but-set-variable -mcmodel=tiny -fomit-frame-pointer
>> -RELEASE_GCC5_AARCH64_DLINK_FLAGS =
>DEF(GCC5_AARCH64_DLINK_FLAGS) -flto -Os -
>L$(WORKSPACE)/ArmPkg/Library/GccLto -llto-aarch64 -Wl,-plugin-opt=-pass-
>through=-llto-aarch64
>> +RELEASE_GCC5_AARCH64_DLINK_FLAGS =
>DEF(GCC5_AARCH64_DLINK_FLAGS) -flto -Os -
>L$(WORKSPACE)/ArmPkg/Library/GccLto -llto-aarch64 -Wl,-plugin-opt=-pass-
>through=-llto-aarch64 -Wno-lto-type-mismatch
>>
>>    NOOPT_GCC5_AARCH64_CC_FLAGS    = DEF(GCC5_AARCH64_CC_FLAGS) -
>O0 -mcmodel=small
>>    NOOPT_GCC5_AARCH64_DLINK_FLAGS =
>DEF(GCC5_AARCH64_DLINK_FLAGS) -z common-page-size=0x1000 -O0
>> --
>> 2.7.4
>>