Retain alignment requirements for load->selects modified by DAGCombine

DAGCombine may choose to rewrite graphs where two loads feed a select into
graphs where a select of two addresses feed a load. While it sanity checks the
loads to make sure they are broadly equivalent it currently just uses the
alignment restriction of the left node. In cases where the right node has
stronger alignment requiresment this may lead to bad codegen, such as generating
an aligned load where an unaligned load is required. This patch makes the
combine generate a load with an alignment that is the same as whichever is more
restrictive of the two alignments.

Tests included.

rdar://17762530

llvm-svn: 214322

1 files changed, 6 insertions, 2 deletions

diff --git a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
index 5fb0ea9146e..c7c7f48dc63 100644
--- a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
@@ -11181,13 +11181,17 @@ bool DAGCombiner::SimplifySelectOps(SDNode *TheSelect, SDValue LHS,
     }
 
     SDValue Load;
+    // It is safe to replace the two loads if they have different alignments,
+    // but the new load must be the minimum (most restrictive) alignment of the
+    // inputs.
+    unsigned Alignment = std::min(LLD->getAlignment(),RLD->getAlignment());
     if (LLD->getExtensionType() == ISD::NON_EXTLOAD) {
       Load = DAG.getLoad(TheSelect->getValueType(0),
                          SDLoc(TheSelect),
                          // FIXME: Discards pointer and AA info.
                          LLD->getChain(), Addr, MachinePointerInfo(),
                          LLD->isVolatile(), LLD->isNonTemporal(),
-                         LLD->isInvariant(), LLD->getAlignment());
+                         LLD->isInvariant(), Alignment);
     } else {
       Load = DAG.getExtLoad(LLD->getExtensionType() == ISD::EXTLOAD ?
                             RLD->getExtensionType() : LLD->getExtensionType(),
@@ -11196,7 +11200,7 @@ bool DAGCombiner::SimplifySelectOps(SDNode *TheSelect, SDValue LHS,
                             // FIXME: Discards pointer and AA info.
                             LLD->getChain(), Addr, MachinePointerInfo(),
                             LLD->getMemoryVT(), LLD->isVolatile(),
-                            LLD->isNonTemporal(), LLD->getAlignment());
+                            LLD->isNonTemporal(), Alignment);
     }
 
     // Users of the select now use the result of the load.