Re: linux-next: Tree for Oct 31 (vboxguest)

[Masahiro Yamada]

On Mon, Nov 5, 2018 at 9:54 AM Changbin Du <changbin.du@xxxxxxxxx> wrote:
>
> On Sun, Nov 04, 2018 at 11:43:44AM +0900, Masahiro Yamada wrote:
> > On Sat, Nov 3, 2018 at 12:55 AM Arnd Bergmann <arnd@xxxxxxxx> wrote:
> > >
> > > On 11/2/18, Masahiro Yamada <yamada.masahiro@xxxxxxxxxxxxx> wrote:
> > > > On Thu, Nov 1, 2018 at 11:32 PM Changbin Du <changbin.du@xxxxxxxxx> wrote:
> > > >> On Thu, Nov 01, 2018 at 12:32:48PM +0900, Masahiro Yamada wrote:
> > > >
> > > > How about clang?
> > > >
> > > > For clang, -Og might be equivalent to -O1 at this moment, but I am not
> > > > sure.
> > > >
> > > > In my understanding, Clang does not inline functions marked with 'static
> > > > inline'
> > > > for -Og (or -O1) optimization level.
> > > >
> > > > Theoretically, 'inline' keyword is a just hint for the compiler, after all.
> > >
> > > I think this means that we cannot build the kernel in that configuration,
> > > at least with CONFIG_OPTIMIZE_INLINING=y. Without that option,
> > > every 'inline' becomes 'always_inline'.
> > >
> >
> > Sorry, I missed that fact.
> >
> >
> > At this moment of time, it is OK given the following:
> >
> >  - CONFIG_OPTIMIZE_INLINING is defined only for x86
> >  - Clang cannot build x86 due to missing asm-goto
> >
> >
> > However, Clang with -Og
> > does not inline even such simple code like this:
> >
> >
> > -----test code------
> > static inline int foo(int x)
> > {
> >         return x;
> > }
> >
> > int bar(int x)
> > {
> >         return foo(x);
> > }
> > -------------------
> >
> >
> >
> > $ clang -Og -c -o bar.o  bar.c
> > $ objdump -d  bar.o
> > bar.o:     file format elf64-x86-64
> >
> >
> > Disassembly of section .text:
> >
> > 0000000000000000 <bar>:
> >    0: eb 0e                jmp    10 <foo>
> >    2: 66 2e 0f 1f 84 00 00 nopw   %cs:0x0(%rax,%rax,1)
> >    9: 00 00 00
> >    c: 0f 1f 40 00          nopl   0x0(%rax)
> >
> > 0000000000000010 <foo>:
> >   10: 89 f8                mov    %edi,%eax
> >   12: c3                    retq
> >
> >
> I see. Thanks for your explanation. So I think we should disable
> CONFIG_DEBUG_EXPERIENCE for clang. Do you know how to distinguish
> different copmilers in Kconfig?
>


You can add 'depends on CC_IS_GCC'
to make sure this option is visible only when you use GCC.


config CC_OPTIMIZE_FOR_DEBUGGING
        bool "Optimize for better debugging experience (-Og)"
        depends on CC_IS_GCC
        depends on $(cc-option,-Og)
        select NO_AUTO_INLINE



--
Best Regards
Masahiro Yamada