[llvm] 8aff88f - [LegalizeDAG] Propagate alignment in ExpandExtractFromVectorThroughStack

[Benjamin Kramer via llvm-commits]

Author: Benjamin Kramer
Date: 2022-07-19T13:13:55+02:00
New Revision: 8aff88fd3a5f6f974f4ce62923981764fafbb820

URL: https://github.com/llvm/llvm-project/commit/8aff88fd3a5f6f974f4ce62923981764fafbb820
DIFF: https://github.com/llvm/llvm-project/commit/8aff88fd3a5f6f974f4ce62923981764fafbb820.diff

LOG: [LegalizeDAG] Propagate alignment in ExpandExtractFromVectorThroughStack

Unlike the name suggests this can reuse any store as a base for a
memory-based vector extract. If that store is underaligned the loads
created to extract will have an invalid alignment. Since most CPUs are
forgiving wrt alignment this is almost never an issue, on x86 this is
only reproducible by extracting a 128 bit vector out of a wider vector.

I tried making a test case in the context of
https://reviews.llvm.org/D127982 but it's really really fragile, as the
output pretty much looks like a missed optimization.

Added: 
    

Modified: 
    llvm/lib/CodeGen/SelectionDAG/LegalizeDAG.cpp

Removed: 
    


################################################################################
diff  --git a/llvm/lib/CodeGen/SelectionDAG/LegalizeDAG.cpp b/llvm/lib/CodeGen/SelectionDAG/LegalizeDAG.cpp
index 8bdc9410d131..56d35dfe8701 100644
--- a/llvm/lib/CodeGen/SelectionDAG/LegalizeDAG.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/LegalizeDAG.cpp
@@ -1404,17 +1404,21 @@ SDValue SelectionDAGLegalize::ExpandExtractFromVectorThroughStack(SDValue Op) {
   }
 
   SDValue NewLoad;
+  Align ElementAlignment =
+      std::min(cast<StoreSDNode>(Ch)->getAlign(),
+               DAG.getDataLayout().getPrefTypeAlign(
+                   Op.getValueType().getTypeForEVT(*DAG.getContext())));
 
   if (Op.getValueType().isVector()) {
     StackPtr = TLI.getVectorSubVecPointer(DAG, StackPtr, VecVT,
                                           Op.getValueType(), Idx);
-    NewLoad =
-        DAG.getLoad(Op.getValueType(), dl, Ch, StackPtr, MachinePointerInfo());
+    NewLoad = DAG.getLoad(Op.getValueType(), dl, Ch, StackPtr,
+                          MachinePointerInfo(), ElementAlignment);
   } else {
     StackPtr = TLI.getVectorElementPointer(DAG, StackPtr, VecVT, Idx);
     NewLoad = DAG.getExtLoad(ISD::EXTLOAD, dl, Op.getValueType(), Ch, StackPtr,
-                             MachinePointerInfo(),
-                             VecVT.getVectorElementType());
+                             MachinePointerInfo(), VecVT.getVectorElementType(),
+                             ElementAlignment);
   }
 
   // Replace the chain going out of the store, by the one out of the load.