Re: [PATCH 2/3] BaseTools/tools_def ARM CLANG35: work around -mno-movt option name change

[Laszlo Ersek <lersek@redhat.com>]

On 12/12/18 12:51, Ard Biesheuvel wrote:
> On Wed, 12 Dec 2018 at 12:50, Laszlo Ersek <lersek@redhat.com> wrote:
>>
>> On 12/12/18 11:33, Ard Biesheuvel wrote:
>>> PE/COFF only has a very limited id space for runtime relocations, and
>>> so it defines only a single relocation for movw/movt instruction pairs,
>>> which can be combined to load a 32-bit symbol reference into a register.
>>> For this to work as expected, these instructions must always appear in
>>> the same order and adjacently, and this is something few compilers take
>>> into account, unless they target PE/COFF explicitly (and this is not the
>>> case for our ELF based toolchains)
>>>
>>> For Clang 3.6 and later, we can pass the -mno-movt option to suppress
>>> movw/movt pairs entirely, which works around the issue. Unfortunately,
>>> for Clang 3.5, the option is called differently (-mllvm -arm-use-movt=0)
>>> and mutually incompatible between 3.5 and 3.6.
>>>
>>> Since it is desirable for the CLANG35 toolchain to be usable on newer
>>> versions of Clang as well (given that it is the only non-LTO alternative
>>> to CLANG38), let's work around this issue in a way that permits versions
>>> 3.5 and newer of Clang to be used with the CLANG35 profile.
>>>
>>> So pass the -mkernel flag instead (and drop the -mno-unaligned-access
>>> in *_CLANG35_ARM_CC_XIPFLAGS which now becomes redundant, and which
>>> Clang complains about). This also inhibits movw/movt generation, along
>>> with some other changes (e.g., long calls) which do affect code generation
>>> but not in a undesirable manner.
>>>
>>> Contributed-under: TianoCore Contribution Agreement 1.1
>>> Signed-off-by: Ard Biesheuvel <ard.biesheuvel@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 ac2b95e0f5ba..2ba833e1fb06 100755
>>> --- a/BaseTools/Conf/tools_def.template
>>> +++ b/BaseTools/Conf/tools_def.template
>>> @@ -5249,7 +5249,7 @@ DEFINE CLANG35_AARCH64_CC_FLAGS  = DEF(GCC_AARCH64_CC_FLAGS) DEF(CLANG35_AARCH64
>>>  *_CLANG35_ARM_ASM_FLAGS          = DEF(GCC_ASM_FLAGS) DEF(CLANG35_ARM_TARGET) $(ARCHASM_FLAGS) $(PLATFORM_FLAGS) -Qunused-arguments
>>>  *_CLANG35_ARM_DLINK_FLAGS        = DEF(CLANG35_ARM_TARGET) DEF(GCC_ARM_DLINK_FLAGS)
>>>  *_CLANG35_ARM_DLINK2_FLAGS       = DEF(GCC_DLINK2_FLAGS_COMMON) -Wl,--defsym=PECOFF_HEADER_SIZE=0x220
>>> -*_CLANG35_ARM_PLATFORM_FLAGS     = -march=armv7-a
>>> +*_CLANG35_ARM_PLATFORM_FLAGS     = -march=armv7-a -mkernel -Qunused-arguments
>>>  *_CLANG35_ARM_PP_FLAGS           = DEF(GCC_PP_FLAGS) DEF(CLANG35_ARM_TARGET) $(ARCHCC_FLAGS) $(PLATFORM_FLAGS)
>>>  *_CLANG35_ARM_RC_FLAGS           = DEF(GCC_ARM_RC_FLAGS)
>>>  *_CLANG35_ARM_VFRPP_FLAGS        = DEF(GCC_VFRPP_FLAGS) DEF(CLANG35_ARM_TARGET) $(ARCHCC_FLAGS) $(PLATFORM_FLAGS)
>>>
>>
>> The commit message speaks about adding -mkernel, and removing
>> -mno-unaligned-access (elsewhere). In the patch, I only see -mkernel
>> (plus an un-announced -Qunused-arguments option).
>>
>> Is the commit message slightly out-of-date relative to the code? (Or
>> perhaps the other way around?)
>>
> 
> Ah yes. The  -mno-unaligned-access became redundant, but instead of
> removing it, i ended up adding the -Qunused-arguments to stop Clang
> from complaining about it.
> 

OK, thanks! With the commit message updated accordingly:

Acked-by: Laszlo Ersek <lersek@redhat.com>