Re: [stable:linux-4.14.y 4459/9999] lib/mpi/mpih-div.c:142:20: error: invalid use of a cast in a inline asm context requiring an lvalue: remove the cast or build with -fheinous-gnu-extensions

[Nathan Chancellor <nathan@kernel.org>]

On Fri, Aug 04, 2023 at 05:32:55AM +0800, kernel test robot wrote:
> Hi Fangrui,
> 
> First bad commit (maybe != root cause):
> 
> tree:   https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux-stable.git linux-4.14.y
> head:   60a6e3043cc8b918c989707a5eba5fd6830a08a4
> commit: 71749b1181eca12af96654a781cabcd058c37c8f [4459/9999] Makefile: Fix GCC_TOOLCHAIN_DIR prefix for Clang cross compilation
> config: mips-randconfig-r035-20230731 (https://download.01.org/0day-ci/archive/20230804/202308040553.nMR9oB8v-lkp@intel.com/config)
> compiler: clang version 17.0.0 (https://github.com/llvm/llvm-project.git 4a5ac14ee968ff0ad5d2cc1ffa0299048db4c88a)
> reproduce: (https://download.01.org/0day-ci/archive/20230804/202308040553.nMR9oB8v-lkp@intel.com/reproduce)
> 
> If you fix the issue in a separate patch/commit (i.e. not just a new version of
> the same patch/commit), kindly add following tags
> | Reported-by: kernel test robot <lkp@intel.com>
> | Closes: https://lore.kernel.org/oe-kbuild-all/202308040553.nMR9oB8v-lkp@intel.com/
> 
> All errors (new ones prefixed by >>):
> 
> >> lib/mpi/mpih-div.c:142:20: error: invalid use of a cast in a inline asm context requiring an lvalue: remove the cast or build with -fheinous-gnu-extensions
>      142 |                                         umul_ppmm(n1, n0, d0, q);
>          |                                         ~~~~~~~~~~~~~~^~~~~~~~~~
>    lib/mpi/longlong.h:652:20: note: expanded from macro 'umul_ppmm'
>      652 |         : "=l" ((USItype)(w0)), \
>          |                 ~~~~~~~~~~^~~

This is the same as [1] but the upstream solution (b0c091ae04f6) could
cause breakage in 4.14 for compilers older than GCC 4.4; 4.14 claims to
support GCC 3.2 as a minimum version. An alternative solution if we care
about fixing this issue in 4.14 without any regression possiblity would
be something like:

diff --git a/lib/mpi/longlong.h b/lib/mpi/longlong.h
index 6c5229f98c9e..42076c124e23 100644
--- a/lib/mpi/longlong.h
+++ b/lib/mpi/longlong.h
@@ -639,7 +639,7 @@ do { \
 	**************  MIPS  *****************
 	***************************************/
 #if defined(__mips__) && W_TYPE_SIZE == 32
-#if (__GNUC__ >= 5) || (__GNUC__ >= 4 && __GNUC_MINOR__ >= 4)
+#if (__GNUC__ >= 5) || (__GNUC__ >= 4 && __GNUC_MINOR__ >= 4) || defined(__clang__)
 #define umul_ppmm(w1, w0, u, v)			\
 do {						\
 	UDItype __ll = (UDItype)(u) * (v);	\

[1]: https://lore.kernel.org/20230801202602.GA2589710@dev-arch.thelio-3990X/

Cheers,
Nathan