SILMem2Reg: don't create an "undef" as a replacement of a load of an empty tuple.

Instead create the empty tuple value.
In general, it's not a good idea to create undef values in SIL.

Author: eeckstein

lib/SILOptimizer/Transforms/SILMem2Reg.cpp

@@ -576,6 +576,18 @@ StackAllocationPromoter::promoteAllocationInBlock(SILBasicBlock *BB) {
   return LastStore;
 }
 
+/// Create a tuple value for an empty tuple or a tuple of empty tuples.
+SILValue createValueForEmptyTuple(SILType ty, SILInstruction *insertionPoint) {
+  auto tupleTy = ty.castTo<TupleType>();
+  SmallVector<SILValue, 4> elements;
+  for (unsigned idx = 0, end = tupleTy->getNumElements(); idx < end; ++ idx) {
+    SILType elementTy = ty.getTupleElementType(idx);
+    elements.push_back(createValueForEmptyTuple(elementTy, insertionPoint));
+  }
+  SILBuilder builder(insertionPoint);
+  return builder.createTuple(insertionPoint->getLoc(), ty, elements);
+}
+
 void MemoryToRegisters::removeSingleBlockAllocation(AllocStackInst *ASI) {
   LLVM_DEBUG(llvm::dbgs() << "*** Promoting in-block: " << *ASI);
 
@@ -596,7 +608,7 @@ void MemoryToRegisters::removeSingleBlockAllocation(AllocStackInst *ASI) {
       if (!RunningVal) {
         // Loading without a previous store is only acceptable if the type is
         // Void (= empty tuple) or a tuple of Voids.
-        RunningVal = SILUndef::get(ASI->getElementType(), *ASI->getFunction());
+        RunningVal = createValueForEmptyTuple(ASI->getElementType(), Inst);
       }
       replaceLoad(cast<LoadInst>(Inst), RunningVal, ASI);
       ++NumInstRemoved;

test/SILOptimizer/mem2reg.sil

@@ -438,8 +438,8 @@ bb3(%20 : $((), ())):
 }
 
 // CHECK-LABEL: sil @load_tuple_of_void
-// CHECK-NOT: alloc_stack
-// CHECK:   return undef : $((), ())
+// CHECK:   [[T:%[0-9]+]] = tuple (%{{[0-9]+}} : $(), %{{[0-9]+}} : $())
+// CHECK:   return [[T]] : $((), ())
 // CHECK: } // end sil function 'load_tuple_of_void'
 sil @load_tuple_of_void : $@convention(thin) () -> ((), ()) {
 bb0:

test/SILOptimizer/mem2reg_ossa.sil

@@ -478,7 +478,8 @@ bb3(%20 : $((), ())):
 
 // CHECK-LABEL: sil [ossa] @load_tuple_of_void :
 // CHECK-NOT: alloc_stack
-// CHECK:   return undef : $((), ())
+// CHECK:   [[T:%[0-9]+]] = tuple (%{{[0-9]+}} : $(), %{{[0-9]+}} : $())
+// CHECK:   return [[T]] : $((), ())
 // CHECK: } // end sil function 'load_tuple_of_void'
 sil [ossa] @load_tuple_of_void : $@convention(thin) () -> ((), ()) {
 bb0: