Re: [Qemu-devel] [PATCH 28/32] ppc: Avoid double translation for lvx/lvxl/stvx/stvxl

[Benjamin Herrenschmidt]

On Fri, 2016-07-29 at 16:42 +1000, Benjamin Herrenschmidt wrote:
> On Fri, 2016-07-29 at 09:46 +0530, Richard Henderson wrote:
> > 
> > The form of declaration you're using takes care of that.  In order
> > to
> > not clobber, you have to use DEF_HELPER_FLAGS with *NO_RWG*.
> 
> BTW. Is that stuff (and the various flags here) somewhat documented
> anywhere ? :-)

More specifically, looking at the flags defined:

#define TCG_CALL_NO_READ_GLOBALS    0x0010
/* Helper does not write globals */
#define TCG_CALL_NO_WRITE_GLOBALS   0x0020
/* Helper can be safely suppressed if the return value is not used. */
#define TCG_CALL_NO_SIDE_EFFECTS    0x0040

I assume by "globals" that means things defined
with tcg_global_mem_new_i32() correct ?

So I can't do something like set TCG_CALL_NO_RWG (nor TCG_CALL_NO_WG)
on something like dcbz because it can take an exception, correct ?

Now if a helper doens't access env->gpr/spr/... but instead take
everything as in/out arguments and cannot take an exception, we can use
these, right ?

I notice that sadly, all of the vector ops are helper with full
clobbers, because I assume, the "avr" is passed as pointer due to the
lack of an int128 type in TCG correct ?

Is the only option here to go down the int128 path or would there be a
reasonable way to add a mechanism for more fine grained clobbers ?

Another reason why I think that might be handy is that a whole lot of
helpers basically have full clobbers simply because they update a CR
bit (or fscr for FP ops). It would be nice to be able to mark just
that clobbered.

Maybe by declaring up to a handful of "globals" special, assigning them
an index and having a special clobber bits corresponding to them in the
flags ?

Cheers,
Ben.