In LowerShift*Parts, xor with bits-1 instead of -1.

If we start with an i128 shift, the initial shift amount would
usually have zeros in bit 8 and above. xoring the shift amount with
-1 will set those upper bits to 1. If DAGCombiner is able to prove those
bits are now 1, then the shift that uses the xor will be replaced
with undef. Which we don't want.

Reduce the xor constant to VT.bits-1 where VT is half the size of
the larger shift type. This avoids toggling the upper bits. The
hardware shift instruction only uses the lower bits of the shift
amount. I assume the code used NOT because the hardware doesn't
use the upper bits, but that isn't compatible with the LLVM poison
semantics.

Fixes rust-lang#71142.

Author: topperc

src/llvm-project/llvm/lib/Target/Mips/MipsISelLowering.cpp

@@ -2593,12 +2593,13 @@ SDValue MipsTargetLowering::lowerShiftLeftParts(SDValue Op,
   SDValue Shamt = Op.getOperand(2);
   // if shamt < (VT.bits):
   //  lo = (shl lo, shamt)
-  //  hi = (or (shl hi, shamt) (srl (srl lo, 1), ~shamt))
+  //  hi = (or (shl hi, shamt) (srl (srl lo, 1), (xor shamt, (VT.bits-1))))
   // else:
   //  lo = 0
   //  hi = (shl lo, shamt[4:0])
-  SDValue Not = DAG.getNode(ISD::XOR, DL, MVT::i32, Shamt,
-                            DAG.getConstant(-1, DL, MVT::i32));
+  SDValue Not =
+      DAG.getNode(ISD::XOR, DL, MVT::i32, Shamt,
+                  DAG.getConstant(VT.getSizeInBits() - 1, DL, MVT::i32));
   SDValue ShiftRight1Lo = DAG.getNode(ISD::SRL, DL, VT, Lo,
                                       DAG.getConstant(1, DL, VT));
   SDValue ShiftRightLo = DAG.getNode(ISD::SRL, DL, VT, ShiftRight1Lo, Not);
@@ -2623,7 +2624,7 @@ SDValue MipsTargetLowering::lowerShiftRightParts(SDValue Op, SelectionDAG &DAG,
   MVT VT = Subtarget.isGP64bit() ? MVT::i64 : MVT::i32;
 
   // if shamt < (VT.bits):
-  //  lo = (or (shl (shl hi, 1), ~shamt) (srl lo, shamt))
+  //  lo = (or (shl (shl hi, 1), (xor shamt, (VT.bits-1))) (srl lo, shamt))
   //  if isSRA:
   //    hi = (sra hi, shamt)
   //  else:
@@ -2635,8 +2636,9 @@ SDValue MipsTargetLowering::lowerShiftRightParts(SDValue Op, SelectionDAG &DAG,
   //  else:
   //   lo = (srl hi, shamt[4:0])
   //   hi = 0
-  SDValue Not = DAG.getNode(ISD::XOR, DL, MVT::i32, Shamt,
-                            DAG.getConstant(-1, DL, MVT::i32));
+  SDValue Not =
+      DAG.getNode(ISD::XOR, DL, MVT::i32, Shamt,
+                  DAG.getConstant(VT.getSizeInBits() - 1, DL, MVT::i32));
   SDValue ShiftLeft1Hi = DAG.getNode(ISD::SHL, DL, VT, Hi,
                                      DAG.getConstant(1, DL, VT));
   SDValue ShiftLeftHi = DAG.getNode(ISD::SHL, DL, VT, ShiftLeft1Hi, Not);