[llvm-dev] Special vselect code generation case using TableGen

[Alex Susu via llvm-dev]

   Hello.
     I would like to ask a few more things related to the subject of manually generating 
SelectionDAG nodes (in C++, not with TableGen):
       - can I add new use edges to an already created node (that is the existing node to 
use some more values from some other node)?
       - is it OK to add a chain or glue edge between a node with a chain/glue output and 
a node that has no chain/glue input?
       - is it OK to add a chain or glue output to a machine instruction defined in 
TableGen that does not have any output?

   Thank you,
     Alex

On 10/10/2016 9:54 PM, Alex Susu wrote:
>   Hello.
>     I addressed the problem in the end without using TableGen - in
> ISelLowering::LowerOperation(). It turned out that writing code to generate about 10
> SDNode and especially linking them correctly is quite complicated (a few details at
> https://sites.google.com/site/alexsusu/home/backend-llvm, look for "P27. Making VSELECT
> instruction selection work").
>     I guess that one could do in TableGen, but I'm not very good at it and there are some
> difficulties: I need to create an SDNode of a certain target instruction depending on the
> predicate of the SETCC node being fed to the VSELECT.
>
>     I would like to mention that an interesting example is given in the book Mayur Pandey,
> Suyog_Sarda, "LLVM Cookbook", 2015, page 226, section "Lowering to multiple instructions":
> it describes how we can do in InstrInfo correct instruction selection by first lowering a
> MOVi32 to MOVLOi16 and MOVHIi16 .
>
>   Best regards,
>     Alex
>
>
>> Could you do this in ISelLowering/DAGToDAG? This is probably where I would do this. I
>> think it would be cleaner there.
>>
>> On Oct 3, 2016 19:06, "Alex Susu via llvm-dev" <llvm-dev at lists.llvm.org
>> <mailto:llvm-dev at lists.llvm.org>> wrote:
>>
>>       Hello.
>>         I want to implement in TableGen code generation for vselect for the Connex
>>     research SIMD processor. The problem is that my SIMD processor does not have a vselect
>>     instruction and therefore we need to use the other existing instructions. More
>>     exactly, if we have in LLVM IR a program like this:
>>           %pred = cmp eq %op1, %op2
>>           %vres = vselect %pred, %if_set, %if_clear
>>         we can implement it in Connex like this:
>>           EQ op1, op2
>>           Rdst = Rif_clear
>>           WHEREEQ
>>             Rdst = Rif_set
>>           ENDWHERE
>>
>>         Normally TableGen allows specifying single instructions that match part of a given
>>     DAG.
>>         But in include/llvm/Target/TargetSelectionDAG.td it is given  the possibility to
>>     match a given DAG node with several target nodes by using Pattern<> records:
>>         // Selection DAG Pattern Support.
>>         // Patterns are what are actually matched against by the target-flavored
>>         // instruction selection DAG.  Instructions defined by the target implicitly
>>         // define patterns in most cases, but patterns can also be explicitly added when
>>         // an operation is defined by a sequence of instructions (e.g. loading a large
>>         // immediate value on RISC targets that do not support immediates as large as
>>         // their GPRs).
>>         //
>>         class Pattern<dag patternToMatch, list<dag> resultInstrs> {
>>           dag             PatternToMatch  = patternToMatch;
>>           list<dag>       ResultInstrs    = resultInstrs;
>>           list<Predicate> Predicates      = [];  // See class Instruction in Target.td.
>>           int             AddedComplexity = 0;   // See class Instruction in Target.td.
>>         }
>>
>>       // From MispMSAInstrInfo.td: (vselect cond, if_set, if_clear)
>>       def : Pattern<(set MSA128BOpnd:$wd, (vselect (MSA128BOpnd:$pred),
>>     (MSA128BOpnd:$ws_true), MSA128BOpnd:$ws_false)),
>>                     [ // assign value $ws_false to vregx, which keeps the result of
>>     vselect - use CopyToReg
>>                      (WHEREEQ), // no inputs, no outputs
>>                       // assign value $ws_true to vregx,  which keeps the result of
>>     vselect - use CopyToReg
>>                      (ENDWHERE) // no inputs, no outputs
>>                     ]
>>                     >;
>>
>>         I'm having difficulties in specifying this TableGen Pattern.
>>         For example, does the "set" TableGen pattern operation specify CopyToReg?
>>
>>       Thank you,
>>         Alex
>>     _______________________________________________
>>     LLVM Developers mailing list
>>     llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>
>>     http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>     <http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev>
>>