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

[Tamar Christina <Tamar.Christina@arm.com>]

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).

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
>