[SDAG] try to fold one-demanded-bit-of-multiply

This is a translation of the transform added to InstCombine with:
D118539

Author: rotateright

llvm/lib/CodeGen/SelectionDAG/TargetLowering.cpp

@@ -2265,6 +2265,19 @@ bool TargetLowering::SimplifyDemandedBits(
     break;
   }
   case ISD::MUL:
+    if (DemandedBits.isPowerOf2()) {
+      // The LSB of X*Y is set only if (X & 1) == 1 and (Y & 1) == 1.
+      // If we demand exactly one bit N and we have "X * (C' << N)" where C' is
+      // odd (has LSB set), then the left-shifted low bit of X is the answer.
+      unsigned CTZ = DemandedBits.countTrailingZeros();
+      ConstantSDNode *C = isConstOrConstSplat(Op.getOperand(1), DemandedElts);
+      if (C && C->getAPIntValue().countTrailingZeros() == CTZ) {
+        EVT ShiftAmtTy = getShiftAmountTy(VT, TLO.DAG.getDataLayout());
+        SDValue AmtC = TLO.DAG.getConstant(CTZ, dl, ShiftAmtTy);
+        SDValue Shl = TLO.DAG.getNode(ISD::SHL, dl, VT, Op.getOperand(0), AmtC);
+        return TLO.CombineTo(Op, Shl);
+      }
+    }
     // For a squared value "X * X", the bottom 2 bits are 0 and X[0] because:
     // X * X is odd iff X is odd.
     // 'Quadratic Reciprocity': X * X -> 0 for bit[1]

llvm/test/CodeGen/AArch64/combine-mul.ll

@@ -66,7 +66,7 @@ define <4 x i32> @combine_mul_self_demandedbits_vector(<4 x i32> %x) {
 define i8 @one_demanded_bit(i8 %x) {
 ; CHECK-LABEL: one_demanded_bit:
 ; CHECK:       // %bb.0:
-; CHECK-NEXT:    neg w8, w0, lsl #6
+; CHECK-NEXT:    lsl w8, w0, #6
 ; CHECK-NEXT:    orr w0, w8, #0xffffffbf
 ; CHECK-NEXT:    ret
   %m = mul i8 %x, 192  ; 0b1100_0000
@@ -77,16 +77,9 @@ define i8 @one_demanded_bit(i8 %x) {
 define <2 x i64> @one_demanded_bit_splat(<2 x i64> %x) {
 ; CHECK-LABEL: one_demanded_bit_splat:
 ; CHECK:       // %bb.0:
-; CHECK-NEXT:    fmov x8, d0
-; CHECK-NEXT:    mov x9, v0.d[1]
-; CHECK-NEXT:    add x8, x8, x8, lsl #2
-; CHECK-NEXT:    lsl x8, x8, #5
-; CHECK-NEXT:    add x9, x9, x9, lsl #2
-; CHECK-NEXT:    fmov d0, x8
-; CHECK-NEXT:    lsl x8, x9, #5
-; CHECK-NEXT:    mov w9, #32
-; CHECK-NEXT:    mov v0.d[1], x8
-; CHECK-NEXT:    dup v1.2d, x9
+; CHECK-NEXT:    mov w8, #32
+; CHECK-NEXT:    shl v0.2d, v0.2d, #5
+; CHECK-NEXT:    dup v1.2d, x8
 ; CHECK-NEXT:    and v0.16b, v0.16b, v1.16b
 ; CHECK-NEXT:    ret
   %m = mul <2 x i64> %x, <i64 160, i64 160> ; 0b1010_0000
@@ -97,8 +90,7 @@ define <2 x i64> @one_demanded_bit_splat(<2 x i64> %x) {
 define i32 @one_demanded_low_bit(i32 %x) {
 ; CHECK-LABEL: one_demanded_low_bit:
 ; CHECK:       // %bb.0:
-; CHECK-NEXT:    neg w8, w0
-; CHECK-NEXT:    and w0, w8, #0x1
+; CHECK-NEXT:    and w0, w0, #0x1
 ; CHECK-NEXT:    ret
   %m = mul i32 %x, -63 ; any odd number will do
   %r = and i32 %m, 1