[SystemZ] Replicate the load with most uses in buildVector()

Iterate over all elements and count the number of uses among them for each
used load. Then make sure to REPLICATE the load which has the most uses in
order to minimize the number of needed element insertions.

Review: Ulrich Weigand
https://reviews.llvm.org/D54322

llvm-svn: 346637

1 files changed, 11 insertions, 8 deletions

diff --git a/llvm/lib/Target/SystemZ/SystemZISelLowering.cpp b/llvm/lib/Target/SystemZ/SystemZISelLowering.cpp
index 71b7af2b025..2aff585f9f3 100644
--- a/llvm/lib/Target/SystemZ/SystemZISelLowering.cpp
+++ b/llvm/lib/Target/SystemZ/SystemZISelLowering.cpp
@@ -4490,18 +4490,21 @@ static SDValue buildVector(SelectionDAG &DAG, const SDLoc &DL, EVT VT,
     // avoid a false dependency on any previous contents of the vector
     // register.
 
-    // Use a VLREP if at least one element is a load.
-    unsigned LoadElIdx = UINT_MAX;
+    // Use a VLREP if at least one element is a load. Make sure to replicate
+    // the load with the most elements having its value.
+    std::map<const SDNode*, unsigned> UseCounts;
+    SDNode *LoadMaxUses = nullptr;
     for (unsigned I = 0; I < NumElements; ++I)
       if (Elems[I].getOpcode() == ISD::LOAD &&
           cast<LoadSDNode>(Elems[I])->isUnindexed()) {
-        LoadElIdx = I;
-        break;
+        SDNode *Ld = Elems[I].getNode();
+        UseCounts[Ld]++;
+        if (LoadMaxUses == nullptr || UseCounts[LoadMaxUses] < UseCounts[Ld])
+          LoadMaxUses = Ld;
       }
-    if (LoadElIdx != UINT_MAX) {
-      Result = DAG.getNode(SystemZISD::REPLICATE, DL, VT, Elems[LoadElIdx]);
-      Done[LoadElIdx] = true;
-      ReplicatedVal = Elems[LoadElIdx];
+    if (LoadMaxUses != nullptr) {
+      ReplicatedVal = SDValue(LoadMaxUses, 0);
+      Result = DAG.getNode(SystemZISD::REPLICATE, DL, VT, ReplicatedVal);
     } else {
       // Try to use VLVGP.
       unsigned I1 = NumElements / 2 - 1;