Promote partial-loads after stores.

Swift SVN r14217

lib/SILPasses/LoadStoreOpts.cpp

@@ -144,6 +144,25 @@ bool performLoadStoreOptimizations(SILBasicBlock *BB, AliasAnalysis *AA) {
         continue;
       }
 
+      // Promote partial loads.
+      // Check that we are loading a struct element:
+      if (auto *SEAI = dyn_cast<StructElementAddrInst>(LI->getOperand())) {
+        // And that the previous store stores into that struct.
+        if (PrevStore && PrevStore->getDest() == SEAI->getOperand()) {
+          // And that the stored value is a struct construction instruction:
+          if (auto *SI = dyn_cast<StructInst>(PrevStore->getSrc())) {
+            DEBUG(llvm::dbgs() << "    Forwarding element store from: " <<
+                  *PrevStore);
+            unsigned FieldNo = SEAI->getFieldNo();
+            SILValue(LI, 0).replaceAllUsesWith(SI->getOperand(FieldNo));
+            recursivelyDeleteTriviallyDeadInstructions(LI, true);
+            Changed = true;
+            NumForwardedLoads++;
+            continue;
+          }
+        }
+      }
+
       // Search the previous loads and replace the current load with one of the
       // previous loads.
       for (auto PrevLI : Loads) {

test/SILPasses/loadstoreopts.sil

@@ -169,3 +169,21 @@ bb0(%0 : $*Agg1):
   return %5 : $Builtin.Int32
 }
 
+struct Wrapper {
+  var value : Int
+}
+//CHECK-LABEL: promote_partial_load
+//CHECK: alloc_stack
+//CHECK-NOT: load
+//CHECK: return %0
+sil @promote_partial_load : $@thin (Int) -> Int {
+bb0(%0 : $Int):
+  %1 = alloc_stack $Wrapper
+  %2 = struct $Wrapper (%0 : $Int)
+  store %2 to %1#1 : $*Wrapper
+  %3 = struct_element_addr %1#1 : $*Wrapper, #value
+  %4 = load %3 : $*Int
+  dealloc_stack %1#0 : $*@local_storage Wrapper
+  return %4 : $Int
+}
+