Special vselect code generation case using TableGen

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1

Can I specify in TableGen a match pattern where I replace
d VSELECT with 4 machine instructions?
   But the reason I'm not using it is:
     - the fact I don't know how to handle the type of the predicate vector
     - the fact I need to chose various WHEREEQ/CRY/LT .

NOTE: From all back ends only LEG uses explicit pattern i.e., Pattern<>.
     def : Pattern<(i32 (load_sym tglobaladdr:$addr)), [(MOVi32 $addr)]>;

   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

2

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 How to Write an LLVM Back end - alexsusu, 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

3

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