Re: [Ksummit-discuss] [TECH TOPIC] Compiler shopping list

[Alexei Starovoitov <alexei.starovoitov@gmail.com>]

On Sun, Jul 24, 2016 at 06:35:49AM +1000, Benjamin Herrenschmidt wrote:
> On Fri, 2016-07-22 at 16:59 +0100, David Woodhouse wrote:
> > I'm not sure Linus proposed that. I certainly did, many times.
> > 
> > With the work I put in to make use of __builtin_bswapXX() we do have
> > a
> > *certain* amount of the functionality that full endianness
> > attribution
> > would give us — the compiler can see and optimise certain
> > load/mask/save operations, and can use movbe and equivalent
> > instructions.

both llvm and gcc already optimize load + builtin_bswap into movbe on x64.

> > But a full implementation that let us just do assignment without
> > jumping through the hoops might still be nice.
> 
> One advantage of that is it might allow to work around a limitation
> with the current __biultin_bswap* and READ_ONCE/ACCESS_ONCE (such
> as used in gup).
> 
> The ACCESS_ONCE magic pretty much forces the compiler to separate
> the load from the swap, it thus prevents us from using the byteswapped-
> load instructions that we have on powerpc, thus degrading to a load
> followed by the 5 or 6 instructions (with back-to-back dependencies)
> needed to do the swap.

yeah, looks like volatile somehow preventing gcc to optimize it,
but that's a compiler missing an optimization. New 'bigendian' attribute
for a variable is not going to help this situation.