[DAG] computeKnownBits - add basic shift-by-parts handling

Concat KnownBits from ISD::SHL_PARTS / ISD::SRA_PARTS / ISD::SRL_PARTS lo/hi operands and perform the KnownBits calculation by the shift amount on the extended type, before splitting the KnownBits based on the requested lo/hi result.

Author: RKSimon

llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp

@@ -3337,6 +3337,38 @@ KnownBits SelectionDAG::computeKnownBits(SDValue Op, const APInt &DemandedElts,
       Known.Zero |= Known2.Zero;
     }
     break;
+  case ISD::SHL_PARTS:
+  case ISD::SRA_PARTS:
+  case ISD::SRL_PARTS: {
+    assert((Op.getResNo() == 0 || Op.getResNo() == 1) && "Unknown result");
+
+    // Collect lo/hi source values and concatenate.
+    // TODO: Would a KnownBits::concatBits helper be useful?
+    unsigned LoBits = Op.getOperand(0).getScalarValueSizeInBits();
+    unsigned HiBits = Op.getOperand(1).getScalarValueSizeInBits();
+    Known = computeKnownBits(Op.getOperand(0), DemandedElts, Depth + 1);
+    Known2 = computeKnownBits(Op.getOperand(1), DemandedElts, Depth + 1);
+    Known = Known.anyext(LoBits + HiBits);
+    Known.insertBits(Known2, LoBits);
+
+    // Collect shift amount.
+    Known2 = computeKnownBits(Op.getOperand(2), DemandedElts, Depth + 1);
+
+    if (Opcode == ISD::SHL_PARTS)
+      Known = KnownBits::shl(Known, Known2);
+    else if (Opcode == ISD::SRA_PARTS)
+      Known = KnownBits::ashr(Known, Known2);
+    else // if (Opcode == ISD::SRL_PARTS)
+      Known = KnownBits::lshr(Known, Known2);
+
+    // TODO: Minimum shift low/high bits are known zero.
+
+    if (Op.getResNo() == 0)
+      Known = Known.extractBits(LoBits, 0);
+    else
+      Known = Known.extractBits(HiBits, LoBits);
+    break;
+  }
   case ISD::SIGN_EXTEND_INREG: {
     Known = computeKnownBits(Op.getOperand(0), DemandedElts, Depth + 1);
     EVT EVT = cast<VTSDNode>(Op.getOperand(1))->getVT();

llvm/test/CodeGen/X86/shift-parts.ll

@@ -19,7 +19,7 @@ define i32 @int87(i32 %uint64p_8, i1 %cond) nounwind {
 ; CHECK-NEXT:    testb $64, %dl
 ; CHECK-NEXT:    movq %rcx, %rsi
 ; CHECK-NEXT:    cmovneq %rax, %rsi
-; CHECK-NEXT:    orl $0, %esi
+; CHECK-NEXT:    testl %esi, %esi
 ; CHECK-NEXT:    je .LBB0_1
 ; CHECK-NEXT:  # %bb.2: # %if.then
 ; CHECK-NEXT:    movl $1, %eax