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

[Ard Biesheuvel <ard.biesheuvel@linaro.org>]

On Wed, 12 Dec 2018 at 11:33, Ard Biesheuvel <ard.biesheuvel@linaro.org> 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

Alternatively, we could switch to armv6 code generation here (except
for the .S pieces which us Thumb2 instructions), which also gets rid
of the movw/movt pairs.

>  *_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)
> --
> 2.19.2
>