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

[Matt Arsenault via llvm-commits]

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@@ -4577,16 +4587,22 @@ void DAGTypeLegalizer::ExpandIntRes_ShiftThroughStack(SDNode *N, SDValue &Lo,
   Ch = DAG.getStore(Ch, dl, Init, StackPtr, StackPtrInfo, StackSlotAlignment);
 
   // 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 OffsetInBits = DAG.getNode(ISD::SHL, dl, ShAmtVT,
+                                     DAG.getNode(ISD::SRL, dl, ShAmtVT, ShAmt, DAG.getConstant(Log2_32(ShiftUnitInBits), dl, ShAmtVT), Flags),
+                                     DAG.getConstant(Log2_32(ShiftUnitInBits), dl, ShAmtVT));
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arsenm wrote:

Use some temporary variables and reduce indentation 

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