BUILD_BUG_ON(!__builtin_constant_p(feature)) breaks bcc trace tool
Anton Blanchard
anton at samba.org
Sat Jan 21 20:49:48 AEDT 2017
Hi,
> 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).
>
> How should we work around this? Wrap the checks in !clang perhaps?
Looks like it's a weakness in LLVM with inlining:
#include <assert.h>
#if 1
static inline void foo(unsigned long x)
{
assert(__builtin_constant_p(x));
}
#else
#define foo(X) assert(__builtin_constant_p(X))
#endif
int main(void)
{
foo(1);
return 0;
}
And there is an old bug on it:
https://llvm.org/bugs/show_bug.cgi?id=4898
Anton
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