Re: MIPS atomic memory operations (A.K.A PR 33479).

[David Daney <ddaney at avtrex dot com>]

Maciej W. Rozycki wrote:
> On Wed, 19 Sep 2007, Ralf Baechle wrote:
>
> > Please make this loop closure branch a branch-likely.  This is necessary
> > as a errata workaround for some processors.
>
>  Do we emulate them for MIPS I?  We do emulate "ll" and "sc" and adding 
> "sync" is easy

Currently, I (and thus GCC 4.3) am assuming that Linux emulates 'll', 
'sc' and 'sync', If sync is not emulated, we would need to adjust the 
code generation so that it is not emitted on ISAs that don't support it.

> (as a no-op as support for R3000 SMP is unlikely to ever 
> happen).  Adding branches-likely, hmm...  Even though we do have logic to 
> do that as a part of the FP emulator.
>
>  A workaround for a CPU erratum fits within the "-mfix-*" option family 
> quite well though.

Do we know which CPUs require branch-likely?

I would be inclined to agree with adding a "-mfix-??" option.

The only place where GCC's __sync_* primitives are generated without 
explicitly writing them into your program is in GCJ compiled java code 
that uses volatile fields.

If we expect the use of the __sync_* primitives on CPUs that require 
branch-likely to be rare, we shouldn't penalize those trying to rid 
themselves of the beasts.

>   Maciej