Remove possible loop creation in DAGCombiner

[deadal nix]

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 .


> > - 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.


> >
> > - 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.


> >
> > 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. 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.
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