On 2017-06-14 03:06, Will Deacon wrote:
Hi Prasad,
On Tue, Jun 13, 2017 at 03:39:37PM -0700, Sodagudi Prasad wrote:
With a variant of a CLANG(based on 4.0) following errors observed on
Linux
4.12-rc5 tag.
How are you building the kernel with clang, btw? I'd be keen to try the
same
thing, but I thought the LLVMLinux project was largely dead at his
point. Do
you still need build system patches, or is this now integrated with
Kbuild?
Hi Will,
Trying clang patches from below tree -
https://android.googlesource.com/kernel/common/+/experimental/android-4.4-llvm
net/built-in.o: In function `__xchg_mb':
arch/arm64/include/asm/cmpxchg.h:99: \
undefined reference to `__compiletime_assert_99'
arch/arm64/include/asm/cmpxchg.h:99: \
undefined reference to `__compiletime_assert_99
Clang does not seems to be marking this macro as inline and causing
above
compilation issue due to BUILD_BUG().
The only BUILD_BUG I see around here is if the size parameter (which is
calculated using sizeof) is not known to be 1,2,4 or 8 at compile time.
It
would be interesting to know which call site is confusing clang in this
way
and what code is actually being emitted here.
If it's just that __xchg_mb isn't being inlined into the __xchg_wrapper
macro, then your patch should be ok, but I'd like to be sure it's not
something else. I'm also surprised you haven't see this crop up in
other
places.
After digging further, we observed that inline definition was changed
recently and causing this issue.
Here is missing part of inline macro definition __attribute__((unused)).
Commit abb2ea7dfd82 ("compiler, clang: suppress warning for unused
static inline functions") have redefined the inline macro as below
#define inline inline __attribute__((unused))
But actual definition of inline macro defined compiler-gcc.h file as
shown below.
#define inline inline __attribute__((always_inline))
notrace
As always_inline attribute is missing in inline definition, compile may
not inline macros __xchg_mb in
arch/arm64/include/asm/cmpxchg.h file and leading to error.
Some compilers may not honor inline as inline if always_inline attribute
is removed because of
-inline-threshold compiler options.
Here is the change to fix this issue-
diff --git a/include/linux/compiler-clang.h
b/include/linux/compiler-clang.h
index d614c5e..a0e6433 100644
--- a/include/linux/compiler-clang.h
+++ b/include/linux/compiler-clang.h
@@ -22,4 +22,4 @@
* directives. Suppress the warning in clang as well.
*/
#undef inline
-#define inline inline __attribute__((unused)) notrace
+#define inline __attribute__((always_inline)) __attribute__((unused))
notrace
-Thanks, Prasad
Will
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