[LLVMdev] DAGCombiner: (S|U)REM

[Dietmar Ebner]

hi,

thanks for your answer. 

On Mon, 2007-06-18 at 23:25 -0700, Chris Lattner wrote:
> On Mon, 18 Jun 2007, Chris Lattner wrote:
> > On Thu, 14 Jun 2007, Dietmar Ebner wrote:
> >>  currently, the DAGCombiner unconditionally converts
> >>  (DAGCombiner::visit(U|S)REM) expressions of the form X % C for constants
> >>  C into X-X/C*C. this makes sense in certain cases where the div/mul
> >>  logic will simplify X/C*X but is counterproductive in general,
> >>  especially if the multiply is expensive. also, this doesn't allow
> >>  targets to custom lower rem operations. shouldn't this transformation be
> >>  left for the legalizer (where it's implemented anyway)?
> >
> > Yes, the legalizer should do this transformation.
> 
> Actually, perhaps not.  The reason that dag combine does this is that it 
> knows tricky ways to turn division by a constant into multiplication by a 
> constant (plus magic).  Doing this xform allows the rem case to take 
> advantage of this.  That said, I'm sure there are some targets and some 
> cases where this is counter productive.  Are you hitting one?
yes. i'm compiling for a target without native div/rem instructions. 
when compiling expressions like (a % 3), the dagcombiner turns them into
(a-(a/3)*3) which is not simplified later on. this is quite expensive
since the multiply is not pipelined and the div requires a libcall while
the same could have been achieved with a (S|U)REM_I32 libcall.

the problem is that the involved "magic" int TargetLowering has certain
requirements such as MULHU support, which is not available on my
architecture. i guess the best way is to check this conditions beforehand
in the DAGCombiner, right?

cheers, 

-
dietmar