Subject: Re: BUILD_BUG_ON(!__builtin_constant_p(feature)) breaks bcc trace tool

From: Anton Blanchard <[email-masked]>
Date: Sat, 21 Jan 2017 20:49:48 +1100

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