[llvm] [Codegen][LegalizeIntegerTypes] Improve shift through stack (PR #96151)

[Matt Arsenault via llvm-commits]

================
@@ -4574,20 +4584,29 @@ void DAGTypeLegalizer::ExpandIntRes_ShiftThroughStack(SDNode *N, SDValue &Lo,
     Init = DAG.getNode(ISD::BUILD_PAIR, dl, StackSlotVT, AllZeros, Shiftee);
   }
   // And spill it into the stack slot.
-  Ch = DAG.getStore(Ch, dl, Init, StackPtr, StackPtrInfo, StackSlotAlignment);
+  Ch = DAG.getStore(Ch, dl, Init, StackPtr, StackPtrInfo);
 
   // Now, compute the full-byte offset into stack slot from where we can load.
-  // We have shift amount, which is in bits, but in multiples of byte.
-  // So just divide by CHAR_BIT.
+  // We have shift amount, which is in bits. Offset should point to an aligned
+  // address.
   SDNodeFlags Flags;
-  if (ShiftByByteMultiple)
+
+  if (IsOneStepShift)
     Flags.setExact(true);
-  SDValue ByteOffset = DAG.getNode(ISD::SRL, dl, ShAmtVT, ShAmt,
-                                   DAG.getConstant(3, dl, ShAmtVT), Flags);
+  SDValue SrlTmp = DAG.getNode(
+      ISD::SRL, dl, ShAmtVT, ShAmt,
+      DAG.getConstant(Log2_32(ShiftUnitInBits), dl, ShAmtVT), Flags);
+  SDValue OffsetInBits =
+      DAG.getNode(ISD::SHL, dl, ShAmtVT, SrlTmp,
+                  DAG.getConstant(Log2_32(ShiftUnitInBits), dl, ShAmtVT));
+
+  Flags.setExact(true);
+  SDValue Offset = DAG.getNode(ISD::SRL, dl, ShAmtVT, OffsetInBits,
+                               DAG.getConstant(3, dl, ShAmtVT), Flags);
----------------
arsenm wrote:

This is a sneaky hardcoded byte size. Is there a way to make it more obvious this is a byte shift? 

https://github.com/llvm/llvm-project/pull/96151