Messages in this thread | | | Date | Mon, 8 Nov 2021 16:40:38 +0100 | From | Vasily Gorbik <> | Subject | Re: [PATCH 1/2] compiler.h: Avoid using inline asm operand modifiers |
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On Fri, Nov 05, 2021 at 09:54:18AM -0700, Josh Poimboeuf wrote: > On Wed, May 19, 2021 at 03:03:08PM +0200, Vasily Gorbik wrote: > > The expansion of annotate_reachable/annotate_unreachable on s390 will > > result in a compiler error if the __COUNTER__ value is high enough. > > For example with "i" (154) the "%c0" operand of annotate_reachable > > will be expanded to -102: > > > > -102: > > .pushsection .discard.reachable > > .long -102b - . > > .popsection > > > > This is a quirk of the gcc backend for s390, it interprets the %c0 > > as a signed byte value. Avoid using operand modifiers in this case > > by simply converting __COUNTER__ to string, with the same result, > > but in an arch assembler independent way. > > > > Signed-off-by: Vasily Gorbik <gor@linux.ibm.com> > > This patch causes these macros to break with Clang and > CONFIG_TRACE_BRANCH_PROFILING. > > I get a lot of warnings like > > arch/x86/kernel/traps.o: warning: objtool: handle_xfd_event()+0x134: unreachable instruction > > Without an asm input, 'volatile' is ignored for some reason and Clang > feels free to move the reachable() annotation away from its intended > location. > > I wonder if we could go back to the original approach of providing > __COUNTER__ as an input to the asm, but then mask it to < 128. > > Does this work on s390? > > diff --git a/include/linux/compiler.h b/include/linux/compiler.h > index 3d5af56337bd..42935500a712 100644 > --- a/include/linux/compiler.h > +++ b/include/linux/compiler.h > @@ -115,24 +115,18 @@ void ftrace_likely_update(struct ftrace_likely_data *f, int val, > * The __COUNTER__ based labels are a hack to make each instance of the macros > * unique, to convince GCC not to merge duplicate inline asm statements. > */ > -#define __stringify_label(n) #n > - > -#define __annotate_reachable(c) ({ \ > - asm volatile(__stringify_label(c) ":\n\t" \ > +#define annotate_reachable() ({ \ > + asm volatile("%c0:\n\t" \ > ".pushsection .discard.reachable\n\t" \ > - ".long " __stringify_label(c) "b - .\n\t" \ > - ".popsection\n\t"); \ > + ".long %c0b - .\n\t" \ > + ".popsection\n\t" : : "i" (__COUNTER__ & 0x7f)); \ > })
hm, could we just add asm input back and not use it? and keep __stringify_label(c) as is? would that work as well?
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