On Wed, Jun 26, 2019 at 2:24 AM Peter Zijlstra <[email protected]> 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
