[DAGCombiner] add operation legality checks before creating shift ops (PR43542)

[spatel]

Summary

As discussed on llvm-dev and:
https://bugs.llvm.org/show_bug.cgi?id=43542
...we have transforms that assume shift operations are legal and transforms to use them are profitable, but that may not hold for simple targets.
In this case, the MSP430 target custom lowers shifts by repeating (many) simpler/fixed ops. That can be avoided by keeping this code as setcc/select.

[lebedev.ri]

I've always wondered why in DAGCombine we create nodes rarely consulting with TLI.isOperation*().
This patch explains it :/ Looks ok to me..

[joanlluch]

It works for me.

llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp

     return V;
 
   // fold (select_cc seteq (and x, y), 0, 0, A) -> (and (shr (shl x)) A)
   // where y is has a single bit set.
   // A plaintext description would be, we can turn the SELECT_CC into an AND
   // when the condition can be materialized as an all-ones register.  Any
   // single bit-test can be materialized as an all-ones register with
   // shift-left and shift-right-arith.
+  // TODO: The operation legality checks could be loosened to include "custom",
+  //       but that may cause regressions for targets that do not have shift
+  //       instructions.
   if (CC == ISD::SETEQ && N0->getOpcode() == ISD::AND &&
-      N0->getValueType(0) == VT && isNullConstant(N1) && isNullConstant(N2)) {
+      N0->getValueType(0) == VT && isNullConstant(N1) && isNullConstant(N2) &&
+      TLI.isOperationLegal(ISD::SHL, VT) &&
+      TLI.isOperationLegal(ISD::SRA, VT)) {
     SDValue AndLHS = N0->getOperand(0);
     auto *ConstAndRHS = dyn_cast<ConstantSDNode>(N0->getOperand(1));
     if (ConstAndRHS && ConstAndRHS->getAPIntValue().countPopulation() == 1) {
       // Shift the tested bit over the sign bit.
       const APInt &AndMask = ConstAndRHS->getAPIntValue();
       SDValue ShlAmt =
         DAG.getConstant(AndMask.countLeadingZeros(), SDLoc(AndLHS),
                         getShiftAmountTy(AndLHS.getValueType()));

llvm/test/CodeGen/MSP430/selectcc.ll

 ; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py
 ; RUN: llc -mtriple=msp430-- < %s | FileCheck %s
 
 define i16 @select_to_shifts_i16(i16 %a, i16 %b) {
 ; CHECK-LABEL: select_to_shifts_i16:
 ; CHECK:       ; %bb.0:
-; CHECK-NEXT:    mov.b r12, r12
-; CHECK-NEXT:    swpb r12
-; CHECK-NEXT:    add r12, r12
-; CHECK-NEXT:    add r12, r12
-; CHECK-NEXT:    add r12, r12
-; CHECK-NEXT:    add r12, r12
-; CHECK-NEXT:    add r12, r12
-; CHECK-NEXT:    add r12, r12
-; CHECK-NEXT:    swpb r12
-; CHECK-NEXT:    sxt r12
-; CHECK-NEXT:    rra r12
-; CHECK-NEXT:    rra r12
-; CHECK-NEXT:    rra r12
-; CHECK-NEXT:    rra r12
-; CHECK-NEXT:    rra r12
-; CHECK-NEXT:    rra r12
-; CHECK-NEXT:    rra r12
-; CHECK-NEXT:    and r13, r12
+; CHECK-NEXT:    mov r12, r14
+; CHECK-NEXT:    clr r12
+; CHECK-NEXT:    bit #2, r14
+; CHECK-NEXT:    jeq .LBB0_2
+; CHECK-NEXT:  ; %bb.1:
+; CHECK-NEXT:    mov r13, r12
+; CHECK-NEXT:  .LBB0_2:
 ; CHECK-NEXT:    ret
   %and = and i16 %a, 2
   %tobool = icmp eq i16 %and, 0
   %select = select i1 %tobool, i16 0, i16 %b
   ret i16 %select
 }
 
 define i32 @select_to_shifts_i32(i32 %a, i32 %b) {
 ; CHECK-LABEL: select_to_shifts_i32:
 ; CHECK:       ; %bb.0:
-; CHECK-NEXT:    mov r12, r13
-; CHECK-NEXT:    mov.b r13, r13
-; CHECK-NEXT:    swpb r13
-; CHECK-NEXT:    add r13, r13
-; CHECK-NEXT:    add r13, r13
-; CHECK-NEXT:    add r13, r13
-; CHECK-NEXT:    add r13, r13
-; CHECK-NEXT:    add r13, r13
-; CHECK-NEXT:    add r13, r13
-; CHECK-NEXT:    swpb r13
-; CHECK-NEXT:    sxt r13
-; CHECK-NEXT:    rra r13
-; CHECK-NEXT:    rra r13
-; CHECK-NEXT:    rra r13
-; CHECK-NEXT:    rra r13
-; CHECK-NEXT:    rra r13
-; CHECK-NEXT:    rra r13
-; CHECK-NEXT:    rra r13
-; CHECK-NEXT:    and r13, r14
-; CHECK-NEXT:    and r15, r13
+; CHECK-NEXT:    mov r12, r11
+; CHECK-NEXT:    and #2, r11
+; CHECK-NEXT:    clr r13
+; CHECK-NEXT:    tst r11
+; CHECK-NEXT:    clr r12
+; CHECK-NEXT:    jne .LBB1_3
+; CHECK-NEXT:  ; %bb.1:
+; CHECK-NEXT:    tst r11
+; CHECK-NEXT:    jne .LBB1_4
+; CHECK-NEXT:  .LBB1_2:
+; CHECK-NEXT:    ret
+; CHECK-NEXT:  .LBB1_3:
 ; CHECK-NEXT:    mov r14, r12
+; CHECK-NEXT:    tst r11
+; CHECK-NEXT:    jeq .LBB1_2
+; CHECK-NEXT:  .LBB1_4:
+; CHECK-NEXT:    mov r15, r13
 ; CHECK-NEXT:    ret
   %and = and i32 %a, 2
   %tobool = icmp eq i32 %and, 0
   %select = select i1 %tobool, i32 0, i32 %b
   ret i32 %select
 }