[LLVMdev] predicates and conditional execution

[roy rosen]

2011/5/24 Justin Holewinski <justin.holewinski at gmail.com>:
> On Tue, May 24, 2011 at 8:35 AM, roy rosen <roy.1rosen at gmail.com> wrote:
>>
>> 2011/5/24 Justin Holewinski <justin.holewinski at gmail.com>:
>> > On Tue, May 24, 2011 at 3:02 AM, roy rosen <roy.1rosen at gmail.com> wrote:
>> >>
>> >> Hi,
>> >>
>> >> I was wondering if LLVM supports predicates and conditional execution.
>> >> Something like we have in IA64.
>> >> There is a register class of predicates and then every instruction may
>> >> be predicated by a register from this class.
>> >> For example:
>> >>
>> >> cmp_less p, x, y // p is a predicate which gets the result of x < y
>> >> p add x, x, 2 // if p then do the add instruction
>> >>
>> >> Is there support in LLVM to something like that?
>> >>
>> >> Which architecture can show a good example for the implementation of
>> >> that?
>> >
>> > You may want to look at the PTX back-end.  The PTX assembly language
>> > supports exactly what you are describing, and we currently use it to
>> > implement conditional branching.  There is a register class for
>> > predicates
>> > (i1 in LLVM) and all machine instructions have a predicate operand and
>> > predicate filter (PTX supports inverted predicates).  For un-predicated
>> > instructions, the predicate operand is just set to the special
>> > NoRegister
>> > constant.
>> >
>> >>
>> >> Thanks, Roy.
>> >> _______________________________________________
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>> >> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
>> >> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>> >
>> >
>> >
>> > --
>> >
>> > Thanks,
>> > Justin Holewinski
>> >
>>
>> I see that PTX has predicated execution for all instructions.
>> in LLVM you implement conditional execution only for branches?
>> Do you know what it takes to make it work for all instructions?
>
> It already works for all instructions, as long as you supply a predicate
> operand in the machine instruction (not to be confused with an LLVM IR
> instruction).  Currently, this feature is only being used to implement
> conditional branches, but that is just because we have not implemented it
> for other cases.
> The primary difficulty here is deciding when and how to use predicates.
>  LLVM IR does not support predicated execution, so we need to match specific
> LLVM IR patterns into predicated instructions when compiling for PTX.  It is
> somewhat trivial in cases such as conditional branches, where predicates are
> the only way to implement them in PTX, and some LLVM IR constructs such as
> the select instruction.  For more interesting cases, such as converting
> simple if-then-else statements into predicated instruction blocks, we still
> have yet to decide how best to handle this.
> Can you provide a little more information on what you want to do in LLVM?
>  Are you wanting to generate an Itanium back-end, or are you just using that
> as an example?
>>
>> Thanks, Roy.
>
>
>
> --
>
> Thanks,
> Justin Holewinski
>

I am not writing an Itanium backend.
I am writing for a different architecture and am trying to understand
LLVM's capabilities in regards to the special features of my
architecture.

Thanks for the answers, Roy.