Re: [PATCH v3 4/4] ARM: pass -march= only to compiler

[Nick Desaulniers <ndesaulniers@google.com>]

Lore thread, for context:
https://lore.kernel.org/llvm/20220516210954.1660716-1-ndesaulniers@google.com/

On Tue, May 17, 2022 at 6:26 PM kernel test robot <lkp@intel.com> wrote:
>
> Hi Nick,
>
> I love your patch! Yet something to improve:
>
> [auto build test ERROR on 0ac824f379fba2c2b17b75fd5ada69cd68c66348]
>
> url:    https://github.com/intel-lab-lkp/linux/commits/Nick-Desaulniers/pass-march-only-to-compiler/20220517-051756
> base:   0ac824f379fba2c2b17b75fd5ada69cd68c66348
> config: arm-randconfig-r026-20220516 (https://download.01.org/0day-ci/archive/20220518/202205180917.RNpZaxIl-lkp@intel.com/config)

^ looks like this is a THUMB2 build.

> compiler: clang version 15.0.0 (https://github.com/llvm/llvm-project 853fa8ee225edf2d0de94b0dcbd31bea916e825e)
> reproduce (this is a W=1 build):
>         wget https://raw.githubusercontent.com/intel/lkp-tests/master/sbin/make.cross -O ~/bin/make.cross
>         chmod +x ~/bin/make.cross
>         # install arm cross compiling tool for clang build
>         # apt-get install binutils-arm-linux-gnueabi
>         # https://github.com/intel-lab-lkp/linux/commit/6da98100eed87e4316be5ec584fe415134f25a3e
>         git remote add linux-review https://github.com/intel-lab-lkp/linux
>         git fetch --no-tags linux-review Nick-Desaulniers/pass-march-only-to-compiler/20220517-051756
>         git checkout 6da98100eed87e4316be5ec584fe415134f25a3e
>         # save the config file
>         mkdir build_dir && cp config build_dir/.config
>         COMPILER_INSTALL_PATH=$HOME/0day COMPILER=clang make.cross W=1 O=build_dir ARCH=arm SHELL=/bin/bash
>
> If you fix the issue, kindly add following tag as appropriate
> Reported-by: kernel test robot <lkp@intel.com>
>
> All errors (new ones prefixed by >>):
>
> >> arch/arm/crypto/poly1305-core.S:16:1: error: target does not support ARM mode
>    .code 32
>    ^

It looks like arch/arm/crypto/poly1305-core.S has a preprocessor guard
for __thumb2__.

My change changes the command line from:

from:
$(CC) ... -march=armv7-m ...
to:
$(CC ... -Wa,-march=armv7-m ...

(where ... contains `-mthumb -Wa,-mthumb` for BOTH).

$ arm-linux-gnueabi-gcc -march=armv7-m -mthumb -dM -E - < /dev/null | grep thumb
#define __thumb2__ 1
#define __thumb__ 1
$ arm-linux-gnueabi-gcc -Wa,-march=armv7-m -mthumb -dM -E - <
/dev/null | grep thumb
#define __thumb__ 1
$ clang --target=arm-linux-gnueabi -march=armv7-m -mthumb -dM -E - <
/dev/null | grep thumb
#define __thumb2__ 1
#define __thumb__ 1
$ clang --target=arm-linux-gnueabi -Wa,-march=armv7-m -mthumb -dM -E -
< /dev/null | grep thumb
#define __thumb__ 1
$

(so it seems that the preprocessor definition of `__thumb2__` is
dependent on `-march=`, not `-Wa,-march=`).

David, we might have a very subtle bug in clang:

$ clang --target=arm-linux-gnueabi -mthumb -Wa,-mthumb -march=armv7-m
-Wa,-march=armv7-m -x assembler-with-cpp
-Wunused-command-line-argument - < /dev/null -dM -E
<prints a bunch of preprocessor defines, but no instance of
-Wunused-command-line-argument>
$ clang --target=arm-linux-gnueabi -mthumb -Wa,-mthumb -march=armv7-m
-Wa,-march=armv7-m -x assembler-with-cpp
-Wunused-command-line-argument - < /dev/null -dM -c
clang-15: warning: argument unused during compilation:
'-march=armv7-m' [-Wunused-command-line-argument]

That seems weird because -c vs -E have different behaviors regarding
-Wunused-command-line-argument, and it seems like for `-x
assembler-with-cpp` that the -march= flag without -Wa, prefix *is*
expected to control the behavior of certain preprocessor defines like
`__thumb2__`.

Perhaps a more straightforward test case:
```asm
@ x.S
@ clang --target=arm-linux-gnueabi -mthumb -Wa,-mthumb -march=armv7-m \
@   -Wa,-march=armv7-m -Wunused-command-line-argument x.S -c
.syntax unified
.text
foo:
  movs r0, #__thumb2__
```

$ clang --target=arm-linux-gnueabi -mthumb -Wa,-mthumb -march=armv7-m
-Wa,-march=armv7-m -Wunused-command-line-argument x.S -c
clang-15: warning: argument unused during compilation:
'-march=armv7-m' [-Wunused-command-line-argument]

$ llvm-objdump -dr x.o | tail -n 2
00000000 <foo>:
       0: 01 20        movs r0, #1

Obviously that warning about `-Wunused-command-line-argument` is
incorrect; the value of `__thumb2__` comes from the preprocessor and
is controlled by `-march=armv7-m`. It can't be simultaneously "unused"
and used to define particular preprocessor directives.

Using the above command line invocation without `-march=armv7-m`
produces the assembler-time failure:
/tmp/x-03ab60.s:10:3: error: unsupported relocation on symbol
  movs r0, #__thumb2__
  ^
because __thumb2__ is not defined.

(perhaps `#ifundef __thumb2__ #error "oops" #endif` would be simpler yet).


>
> --
> 0-DAY CI Kernel Test Service
> https://01.org/lkp
>


--
Thanks,
~Nick Desaulniers