Subject: Re: [PATCH RFC 1/4] include/linux/compiler*.h: fix OPTIMIZER_HIDE_VAR

From: "Michael S. Tsirkin" <[email-masked]>
Date: Tue, 8 Jan 2019 13:50:58 -0500

On Tue, Jan 08, 2019 at 09:44:28AM -0800, Nick Desaulniers wrote:
> Thanks for the patch and sorry for the delay; was totally unplugged
> for the holidays.
> On Wed, Jan 2, 2019 at 12:57 PM Michael S. Tsirkin <[email-masked]> wrote:
> >
> > Since commit 815f0ddb346c ("include/linux/compiler*.h: make compiler-*.h
> > mutually exclusive") clang no longer reuses the OPTIMIZER_HIDE_VAR macro
> > from compiler-gcc - instead it gets the version in
> > include/linux/compiler.h.  Unfortunately that version doesn't actually
> > prevent compiler from optimizing out the variable.
> 
> Good catch. Did you find this via eyeballing the code, a test, or some
> other way?

eyeballing

> >
> > Fix up by moving the macro out from compiler-gcc.h to compiler.h.
> > Compilers without incline asm support will keep working
> > since it's protected by an ifdef.
> >
> > Also fix up comments to match reality since we are no longer overriding
> > any macros.
> >
> > Build-tested with gcc and clang.
> >
> > Fixes: 815f0ddb346c ("include/linux/compiler*.h: make compiler-*.h mutually exclusive")
> > Cc: Eli Friedman <[email-masked]>
> > Cc: Joe Perches <[email-masked]>
> > Cc: Nick Desaulniers <[email-masked]>
> > Cc: Linus Torvalds <[email-masked]>
> > Signed-off-by: Michael S. Tsirkin <[email-masked]>
> 
> Also for more context, see:
> commit 7829fb09a2b4 ("lib: make memzero_explicit more robust against
> dead store elimination")

Interesting. That added this text:

 * while gcc behavior gets along with a normal
 * barrier(), llvm needs an explicit input variable to be assumed
 * clobbered.

however:
#define barrier_data(ptr) __asm__ __volatile__("": :"r"(ptr) :"memory")

So no explicit variable is clobbered.
Weird isn't it?

> > ---
> >  include/linux/compiler-clang.h | 5 ++---
> >  include/linux/compiler-gcc.h   | 4 ----
> >  include/linux/compiler-intel.h | 4 +---
> >  include/linux/compiler.h       | 4 +++-
> >  4 files changed, 6 insertions(+), 11 deletions(-)
> >
> > diff --git a/include/linux/compiler-clang.h b/include/linux/compiler-clang.h
> > index 3e7dafb3ea80..7ddaeb5182e3 100644
> > --- a/include/linux/compiler-clang.h
> > +++ b/include/linux/compiler-clang.h
> > @@ -3,9 +3,8 @@
> >  #error "Please don't include <linux/compiler-clang.h> directly, include <linux/compiler.h> instead."
> >  #endif
> >
> > -/* Some compiler specific definitions are overwritten here
> > - * for Clang compiler
> > - */
> > +/* Compiler specific definitions for Clang compiler */
> > +
> >  #define uninitialized_var(x) x = *(&(x))
> >
> >  /* same as gcc, this was present in clang-2.6 so we can assume it works
> > diff --git a/include/linux/compiler-gcc.h b/include/linux/compiler-gcc.h
> > index 2010493e1040..72054d9f0eaa 100644
> > --- a/include/linux/compiler-gcc.h
> > +++ b/include/linux/compiler-gcc.h
> > @@ -58,10 +58,6 @@
> >         (typeof(ptr)) (__ptr + (off));                                  \
> >  })
> >
> > -/* Make the optimizer believe the variable can be manipulated arbitrarily. */
> > -#define OPTIMIZER_HIDE_VAR(var)                                                \
> > -       __asm__ ("" : "=r" (var) : "0" (var))
> > -
> >  /*
> >   * A trick to suppress uninitialized variable warning without generating any
> >   * code
> > diff --git a/include/linux/compiler-intel.h b/include/linux/compiler-intel.h
> > index 517bd14e1222..b17f3cd18334 100644
> > --- a/include/linux/compiler-intel.h
> > +++ b/include/linux/compiler-intel.h
> > @@ -5,9 +5,7 @@
> >
> >  #ifdef __ECC
> >
> > -/* Some compiler specific definitions are overwritten here
> > - * for Intel ECC compiler
> > - */
> > +/* Compiler specific definitions for Intel ECC compiler */
> >
> >  #include <asm/intrinsics.h>
> >
> > diff --git a/include/linux/compiler.h b/include/linux/compiler.h
> > index 06396c1cf127..1ad367b4cd8d 100644
> > --- a/include/linux/compiler.h
> > +++ b/include/linux/compiler.h
> > @@ -152,7 +152,9 @@ void ftrace_likely_update(struct ftrace_likely_data *f, int val,
> >  #endif
> >
> >  #ifndef OPTIMIZER_HIDE_VAR
> > -#define OPTIMIZER_HIDE_VAR(var) barrier()
> > +/* Make the optimizer believe the variable can be manipulated arbitrarily. */
> > +#define OPTIMIZER_HIDE_VAR(var)                                                \
> > +       __asm__ ("" : "=r" (var) : "0" (var))
> >  #endif
> 
> This should be fine, thanks for the cleanup!  For now, we're not yet
> confident to turn on Clang's integrated assembler for the kernel, but
> I'll make sure to revisit this should we, in case Clang is then able
> to optimize this out.
> + Eric, who might know of a better trick for what we're trying to
> accomplish with this macro.
> 
> Reviewed-by: Nick Desaulniers <[email-masked]>
> 
> + Miguel
> Miguel, would you mind taking this into your compiler-attributes tree?
> -- 
> Thanks,
> ~Nick Desaulniers