Re: [PATCH] scsi: fc: force inlining of wwn conversion functions

[Chris Metcalf]

On 4/28/2016 11:23 AM, Arnd Bergmann wrote:
> On Thursday 28 April 2016 10:58:43 Chris Metcalf wrote:
> > (Resending as text/plain)
> >
> > On 4/27/2016 5:34 PM, Arnd Bergmann wrote:
> > > This won't help on TILE, which is the one architecture that sets
> > > ARCH_SUPPORTS_OPTIMIZED_INLINING but does not set ARCH_USE_BUILTIN_BSWAP.
> > > Chris Metcalf should be able to figure out whether we can just
> > > set ARCH_USE_BUILTIN_BSWAP for tile as well.
> > We certainly could enable ARCH_USE_BUILTIN_BSWAP.  The only problem is
> > that we never added explicit support for bswap16() in gcc, which is
> > efficiently done on tilegx via the "revbytes" instruction and a 48-bit
> > right-shift.  So gcc instead does a generic thing with four
> > instructions in three bundles, so really not as good as our asm/swab.h.
> >
> > I'm not sure how to weigh the implications of converting to
> > builtin_bswap16 (and possibly upstreaming a better implementation to
> > gcc), vs. disabling ARCH_SUPPORTS_OPTIMIZED_INLINING (which no one
> > else but x86 uses anyway), vs. just ignoring the compiler bug and
> > hoping it's not an issue in practice
> How about figuring out whether you hit the gcc bug on tile as a
> first step?

I don't have an affected build of gcc handy (just 4.8 and 4.4).  I will pass
this to our compiler folks and see what they know.

> Another idea would be to adapt this section in include/linux/compiler-gcc.h:
>
> #if !defined(CONFIG_ARCH_SUPPORTS_OPTIMIZED_INLINING) ||                \
>      !defined(CONFIG_OPTIMIZE_INLINING) || (__GNUC__ < 4)
> #define inline          inline          __attribute__((always_inline)) notrace
> #define __inline__      __inline__      __attribute__((always_inline)) notrace
> #define __inline        __inline        __attribute__((always_inline)) notrace
> #else
> /* A lot of inline functions can cause havoc with function tracing */
> #define inline          inline          notrace
> #define __inline__      __inline__      notrace
> #define __inline        __inline        notrace
> #endif
>
> to work around the issue. We already check for gcc before 4.0, and
> we could also check for the affected releases (4.9, 5.x, 6.1) in the
> same place, possibly conditional on ARCH_USE_BUILTIN_BSWAP with
> a comment pointing to the gcc bug tracker.

This does seem like a more robust solution anyway, since more instances of the
bad inline pattern might get introduced in the future in other places.
I wouldn't make it conditional on ARCH_USE_BUILTIN_BSWAP for the same reason.

--
Chris Metcalf, Mellanox Technologies
http://www.mellanox.com