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

[via llvm-commits]

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@@ -4530,14 +4530,35 @@ void DAGTypeLegalizer::ExpandIntRes_ShiftThroughStack(SDNode *N, SDValue &Lo,
   SDValue ShAmt = N->getOperand(1);
   EVT ShAmtVT = ShAmt.getValueType();
 
-  // This legalization is optimal when the shift is by a multiple of byte width,
-  //   %x * 8 <-> %x << 3   so 3 low bits should be be known zero.
-  bool ShiftByByteMultiple =
-      DAG.computeKnownBits(ShAmt).countMinTrailingZeros() >= 3;
+  EVT LoadStoreVT = VT;
+  do {
+    LoadStoreVT = TLI.getTypeToTransformTo(*DAG.getContext(), LoadStoreVT);
+  } while (!TLI.isTypeLegal(LoadStoreVT));
+
+  const unsigned KnownTrailingZeros =
+      DAG.computeKnownBits(ShAmt).countMinTrailingZeros();
+
+  const Align LoadStoreAlign = [&]() -> Align {
+    unsigned IsFast = 0;
+    const bool AllowsFastMisalignedMemoryAccesses =
+        TLI.allowsMisalignedMemoryAccesses(
+            LoadStoreVT, /*AddrSpace=*/DAG.getDataLayout().getAllocaAddrSpace(),
+            /*Alignment=*/Align(LoadStoreVT.getStoreSize()),
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futog wrote:

Sorry but I don't get it... This function is for checking if I should use natural alignment or not. If the target supports fast unaligned memory access and the shift is a multiple of CHAR_BIT, I don't use natural alignment.

You mean that I should drop this approach and always use `DAG.getReducedAlign(LoadStoreVT, /*UseABI=*/false);`?

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