Remove possible loop creation in DAGCombiner

[Hal Finkel]

----- Original Message -----
> From: "deadal nix" <deadalnix at gmail.com>
> To: "Hal Finkel" <hfinkel at anl.gov>
> Cc: "llvm-commits" <llvm-commits at cs.uiuc.edu>, "Nadav Rotem" <nrotem at apple.com>, "Owen Anderson" <resistor at mac.com>,
> "Matt Arsenault" <arsenm2 at gmail.com>
> Sent: Wednesday, October 1, 2014 2:23:22 AM
> Subject: Re: Remove possible loop creation in DAGCombiner
> 
> 
> 
> Ok I took the approach of adding a new method for the purpose at hand
> here.
> 
> We can get that in and start spreading the use little by little.
> 

I actually think that is going to be confusing. Can you please audit the other 75 callsites of getSetCCResultType in lib/CodeGen and see how many others should change. If we're going to do this refactoring we should do it right and make sure it will actually work.

 -Hal

> 
> 
> 2014-09-29 11:54 GMT-07:00 Hal Finkel < hfinkel at anl.gov > :
> 
> 
> 
> 
> ----- Original Message -----
> > From: "deadal nix" < deadalnix at gmail.com >
> > To: "Hal Finkel" < hfinkel at anl.gov >
> 
> 
> > Cc: "llvm-commits" < llvm-commits at cs.uiuc.edu >, "Nadav Rotem" <
> > nrotem at apple.com >, "Owen Anderson" < resistor at mac.com >,
> > "Matt Arsenault" < arsenm2 at gmail.com >
> > Sent: Sunday, September 28, 2014 2:42:10 AM
> > Subject: Re: Remove possible loop creation in DAGCombiner
> > 
> > 2014-09-28 0:12 GMT-07:00 Hal Finkel < hfinkel at anl.gov > :
> > 
> > 
> > ----- Original Message -----
> > > From: "deadal nix" < deadalnix at gmail.com >
> > > To: "Hal Finkel" < hfinkel at anl.gov >
> > > Cc: "llvm-commits" < llvm-commits at cs.uiuc.edu >, "Nadav Rotem" <
> > > nrotem at apple.com >, "Owen Anderson" < resistor at mac.com >
> > > Sent: Friday, September 26, 2014 3:46:32 AM
> > > Subject: Re: Remove possible loop creation in DAGCombiner
> > > 
> > > I tried various options:
> > > 
> > > - Not do the transformation when size do not match. This break
> > > several backends as the sext is done from a 1bit value which most
> > > backend do not expect or support. This needs to be transformed
> > > into
> > > a select one way or another.
> > 
> > This seems odd, type legalization should take care of that problem
> > (and DAGCombine should not introduce illegal types after type
> > legalization is complete). What transformation in producing these
> > illegal nodes?
> > 
> > 
> > 
> > 
> > The problem is that the sext from a setcc node is unexpected by
> > several in tree backends. I didn't invetigated in details, simply
> > noticed that there was a lot of broken ciodegen tests .
> 
> Can you please revisit this approach? This really seems like the
> right solution (perhaps unfortunately), even if it does require some
> additional fix somewhere.
> 
> > 
> > 
> > 
> > > - do the select with the setcc size, and then resize the select.
> > > It
> > > does work but produce inefficient code in various situations
> > > (select
> > > a 64 bit and resize the result to 32bits).
> > > 
> > > - trunc but never extend. This yield the same results as the
> > > proposed
> > > diff.
> > 
> > What does "same results" mean?
> > 
> > 
> > 
> > 
> > That the codegen were the same for all tests I looked at.
> > 
> 
> Fair enough, but I have no idea how good the coverage is, so the
> conservative impulse in me says to not change more existing behavior
> than necessary.
> 
> > 
> > 
> > > 
> > > - the proposed diff.
> > > 
> > > Ultimately, the question is do the backend must handle slect with
> > > different size for selected values and predicate. It look like
> > > they
> > > do (and other part of the toolchain generate such code).
> > 
> > As documented, the sizes of the selected values and the predicate
> > may
> > not match. include/llvm/CodeGen/ISDOpcodes.h says:
> > 
> > /// Select(COND, TRUEVAL, FALSEVAL). If the type of the boolean
> > COND
> > is not
> > /// i1 then the high bits must conform to getBooleanContents.
> > SELECT,
> > 
> > and so the SIGN_EXTEND there is generically wrong anyway (it is
> > correct only for ZeroOrNegativeOneBooleanContent).
> > 
> > Then I guess we should simply not do it.
> 
> Maybe ;) -- or do it correctly: sign extend for
> ZeroOrNegativeOneBooleanContent, zero extend for the others.
> 
> 
> 
> > 
> > 
> > 
> > > 
> > > Another thing worth noticing is the use of getSetCCResultType
> > > with
> > > the selected value's type as parameter. This is a different
> > > semantic
> > > than other uses of the method (predicate's argument type is
> > > passed).
> > > It seems that this is an incorrect use of that function to begin
> > > with, as there is no reason that the 2 respond to the same
> > > constraint, and, in the case of the select, this is not given
> > > that
> > > there is a constraint at all. Basing any decision on the result
> > > of
> > > an incorrect use of that function will be a source of trouble.
> > 
> > This is unfortunately somewhat confusing. getSetCCResultType is
> > documented as:
> > 
> > /// Return the ValueType of the result of SETCC operations. Also
> > used
> > to
> > /// obtain the target's preferred type for the condition operand of
> > SELECT and
> > /// BRCOND nodes. In the case of BRCOND the argument passed is
> > MVT::Other
> > /// since there are no other operands to get a type hint from.
> > virtual EVT getSetCCResultType(LLVMContext &Context, EVT VT) const;
> > 
> > so it seems to be a matter of determining what "preference" means
> > here, and whether backends deal reasonably with non-preferred
> > choices.
> > 
> > Thanks again,
> > Hal
> > 
> > 
> > 
> > 
> > 
> > 
> > That do not make a lot of sense to me. There is no reason for a
> > target to have the same preference in both scenarios.
> 
> I agree with this. Please feel free to submit patches to improve
> this.
> 
> Thanks again,
> Hal
> 
> > In fact the
> > one I'm working on will generate a result that has the same size as
> > argument for the setcc, and do not care about the size of the
> > predicate. If I select a 64bits values from a 32bits predicate, I
> > get a loop in the dag:
> > 
> > 
> > (sext i64 (setcc i32:X i32:Y)) =>(select (sext i64 (setcc i32:X
> > i32:Y)) -1 0)
> > 
> > 
> > When you proceed to the substitution, you end up with a loop:
> > 
> > N = (select N -1 0)
> > 
> > 
> > To make the right decision in the case of the select, the backend
> > need to have information about the type of the predicate and the
> > type of the selected values. One of them is insufficient.
> > 
> 
> 
> 
> --
> Hal Finkel
> Assistant Computational Scientist
> Leadership Computing Facility
> Argonne National Laboratory
> 
> 

-- 
Hal Finkel
Assistant Computational Scientist
Leadership Computing Facility
Argonne National Laboratory