Re: [PATCH 1/2]middle-end: Add new tbranch optab to add support for bit-test-and-branch operations

[Richard Biener <rguenther@suse.de>]

On Wed, 2 Nov 2022, Aldy Hernandez wrote:

> On Wed, Nov 2, 2022 at 10:55 AM Tamar Christina <Tamar.Christina@arm.com> wrote:
> >
> > Hi Aldy,
> >
> > I'm trying to use Ranger to determine if a range of an expression is a single bit.
> >
> > If possible in case of a mask then also the position of the bit that's being checked by the mask (or the mask itself).
> 
> Just instantiate a ranger, and ask for the range of an SSA name (or an
> arbitrary tree expression) at a particular gimple statement (or an
> edge):
> 
> gimple_ranger ranger;
> int_range_max r;
> if (ranger.range_of_expr (r, <SSA_NAME>, <STMT>)) {
>   // do stuff with range "r"
>   if (r.singleton_p ()) {
>     wide_int num = r.lower_bound ();
>     // Check the bits in NUM, etc...
>   }
> }
> 
> You can see the full ranger API in gimple-range.h.
> 
> Note that instantiating a new ranger is relatively lightweight, but
> it's not free.  So unless you're calling range_of_expr sporadically,
> you probably want to have one instance for your pass.  You can pass
> around the gimple_ranger around your pass.  Another way of doing this
> is calling enable_rager() at pass start, and then doing:
> 
>   get_range_query (cfun)->range_of_expr (r, <SSA_NAME>, <STMT>));
> 
> gimple-loop-versioning.cc has an example of using enable_ranger /
> disable_ranger.
> 
> I am assuming you are interested in ranges for integers / pointers.
> Otherwise (floats, etc) you'd have to use "Value_Range" instead of
> int_range_max.  I can give you examples on that if necessary.
> 
> Let me know if that helps.

I think you maybe just want get_nonzero_bits?

> Aldy
> 
> >
> > Do you have any pointers/existing code I can look at to do this?
> >
> > Kind regards,
> > Tamar
> >
> > > -----Original Message-----
> > > From: Jeff Law <jeffreyalaw@gmail.com>
> > > Sent: Tuesday, November 1, 2022 5:00 PM
> > > To: Tamar Christina <Tamar.Christina@arm.com>; gcc-patches@gcc.gnu.org
> > > Cc: nd <nd@arm.com>; rguenther@suse.de
> > > Subject: Re: [PATCH 1/2]middle-end: Add new tbranch optab to add support
> > > for bit-test-and-branch operations
> > >
> > >
> > > On 11/1/22 09:53, Tamar Christina wrote:
> > > >>
> > > >>>    from the machine description.
> > > >>>
> > > >>> +@cindex @code{tbranch@var{mode}4} instruction pattern @item
> > > >>> +@samp{tbranch@var{mode}4} Conditional branch instruction
> > > combined
> > > >>> +with a bit test-and-compare instruction. Operand 0 is a comparison
> > > >>> +operator.  Operand 1 is the operand of the comparison. Operand 2 is
> > > >>> +the bit position of Operand 1 to test.
> > > >>> +Operand 3 is the @code{code_label} to jump to.
> > > >> Should we refine/document the set of comparison operators allowed?
> > > >> Is operand 1 an arbitrary RTL expression or more limited?  I'm
> > > >> guessing its relatively arbitrary given how you've massaged the
> > > >> existing branch-on-bit patterns from the aarch backend.
> > > > It can be any expression in theory. However in practical terms we
> > > > usually force the values to registers before calling the expansion.
> > > > My assumption is that this is for CSE purposes but that's only a guess.
> > >
> > > Understood.  And generally yes, forcing expressions into regs is good for CSE.
> > >
> > >
> > > >
> > > >> Do we have enough information lying around from Ranger to avoid the
> > > need
> > > >> to walk the def-use chain to discover that we're masking off all but one
> > > bit?
> > > >>
> > > > That's an interesting thought.  I'll try to see if I can figure out how to query
> > > > Ranger here.  It would be nice to do so here.
> > >
> > > Reach out to Aldy, I suspect he can probably give you the necessary
> > > pseudocode pretty quickly.
> > >
> > >
> > > Jeff
> > >
> >
> 
> 

-- 
Richard Biener <rguenther@suse.de>
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