BUILD_BUG_ON(!__builtin_constant_p(feature)) breaks bcc trace tool

[Michael Ellerman]

Anton Blanchard <anton at samba.org> writes:
> We added:
>
> BUILD_BUG_ON(!__builtin_constant_p(feature)) 
>
> to cpu_has_feature() and mmu_has_feature() in order to catch usage
> issues (such as cpu_has_feature(cpu_has_feature(X)). Unfortunately LLVM
> isn't smart enough to resolve this, and it errors out.
>
> I work around it in my clang/LLVM builds of the kernel, but I have just
> discovered that it causes a lot of issues for the bcc (eBPF) trace tool
> (which uses LLVM).

I didn't understand that part, but Aneesh explained to me that it's
because bcc pulls in the kernel-internal headers.

I guess as a quick fix we just have to #ifdef it, can you confirm this
works?


diff --git a/arch/powerpc/include/asm/cpu_has_feature.h b/arch/powerpc/include/asm/cpu_has_feature.h
index b312b152461b..6e834caa3720 100644
--- a/arch/powerpc/include/asm/cpu_has_feature.h
+++ b/arch/powerpc/include/asm/cpu_has_feature.h
@@ -23,7 +23,9 @@ static __always_inline bool cpu_has_feature(unsigned long feature)
 {
 	int i;
 
+#ifndef __clang__ /* clang can't cope with this */
 	BUILD_BUG_ON(!__builtin_constant_p(feature));
+#endif
 
 #ifdef CONFIG_JUMP_LABEL_FEATURE_CHECK_DEBUG
 	if (!static_key_initialized) {
diff --git a/arch/powerpc/include/asm/mmu.h b/arch/powerpc/include/asm/mmu.h
index a34c764ca8dd..233a7e8cc8e3 100644
--- a/arch/powerpc/include/asm/mmu.h
+++ b/arch/powerpc/include/asm/mmu.h
@@ -160,7 +160,9 @@ static __always_inline bool mmu_has_feature(unsigned long feature)
 {
 	int i;
 
+#ifndef __clang__ /* clang can't cope with this */
 	BUILD_BUG_ON(!__builtin_constant_p(feature));
+#endif
 
 #ifdef CONFIG_JUMP_LABEL_FEATURE_CHECK_DEBUG
 	if (!static_key_initialized) {


cheers