Re: [PATCH] perf/x86/intel: Mark expected switch fall-throughs

[Nick Desaulniers]

On Wed, Jun 26, 2019 at 2:24 AM Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
>
> On Tue, Jun 25, 2019 at 11:47:06PM +0200, Thomas Gleixner wrote:
> > > On Tue, Jun 25, 2019 at 09:53:09PM +0200, Thomas Gleixner wrote:
>
> > > > but it also makes objtool unhappy:
> > > >
> > > >  arch/x86/events/intel/core.o: warning: objtool: intel_pmu_nhm_workaround()+0xb3: unreachable instruction
> > > >  kernel/fork.o: warning: objtool: free_thread_stack()+0x126: unreachable instruction
> > > >  mm/workingset.o: warning: objtool: count_shadow_nodes()+0x11f: unreachable instruction
> > > >  arch/x86/kernel/cpu/mtrr/generic.o: warning: objtool: get_fixed_ranges()+0x9b: unreachable instruction
> > > >  arch/x86/kernel/platform-quirks.o: warning: objtool: x86_early_init_platform_quirks()+0x84: unreachable instruction
> > > >  drivers/iommu/irq_remapping.o: warning: objtool: irq_remap_enable_fault_handling()+0x1d: unreachable instruction
>
> > I just checked two of them in the disassembly. In both cases it's jump
> > label related. Here is one:
> >
> >       asm volatile("1: rdmsr\n"
> >  410:   b9 59 02 00 00          mov    $0x259,%ecx
> >  415:   0f 32                   rdmsr
> >  417:   49 89 c6                mov    %rax,%r14
> >  41a:   48 89 d3                mov    %rdx,%rbx
> >       return EAX_EDX_VAL(val, low, high);
> >  41d:   48 c1 e3 20             shl    $0x20,%rbx
> >  421:   48 09 c3                or     %rax,%rbx
> >  424:   0f 1f 44 00 00          nopl   0x0(%rax,%rax,1)
> >  429:   eb 0f                   jmp    43a <get_fixed_ranges+0xaa>
> >       do_trace_read_msr(msr, val, 0);
> >  42b:   bf 59 02 00 00          mov    $0x259,%edi   <------- "unreachable"
> >  430:   48 89 de                mov    %rbx,%rsi
> >  433:   31 d2                   xor    %edx,%edx
> >  435:   e8 00 00 00 00          callq  43a <get_fixed_ranges+0xaa>
> >  43a:   44 89 35 00 00 00 00    mov    %r14d,0x0(%rip)        # 441 <get_fixed_ranges+0xb1>
> >
> > Interestingly enough there are some more hunks of the same pattern in that
> > function which look all the same. Those are not upsetting objtool. Josh
> > might give an hint where to stare at.
>
> That's pretty atrocious code-gen :/ Does LLVM support things like label
> attributes? Back when we did jump labels GCC didn't, or rather, it
> ignored it completely when combined with asm goto (and it might still).
>
> That is, would something like this:
>
> diff --git a/arch/x86/include/asm/jump_label.h b/arch/x86/include/asm/jump_label.h
> index 06c3cc22a058..1761b1e76ddc 100644
> --- a/arch/x86/include/asm/jump_label.h
> +++ b/arch/x86/include/asm/jump_label.h
> @@ -32,7 +32,7 @@ static __always_inline bool arch_static_branch(struct static_key *key, bool bran
>                 : :  "i" (key), "i" (branch) : : l_yes);
>
>         return false;
> -l_yes:
> +l_yes: __attribute__((cold));
>         return true;
>  }
>
> @@ -49,7 +49,7 @@ static __always_inline bool arch_static_branch_jump(struct static_key *key, bool
>                 : :  "i" (key), "i" (branch) : : l_yes);
>
>         return false;
> -l_yes:
> +l_yes: __attribute__((hot));
>         return true;
>  }
>
> Help LLVM?

So Clang definitely complains about putting attribute hot/cold on
labels: https://godbolt.org/z/N-Z33Q
In my test case I wasn't able to influence code gen with them though
in GCC at -O2 or -O0.  Maybe GCC has a test case that shows how they
should work?
-- 
Thanks,
~Nick Desaulniers