IRGen: don't initialized a global_object static object if not needed.

We don't need to initialize the global object if it's never used for something which can access the object header.

This let's e.g. lookup-tables to be compiled with zero Array-overhead.

rdar://59874359

Author: eeckstein

lib/IRGen/IRGenSIL.cpp

@@ -2634,6 +2634,23 @@ void IRGenSILFunction::visitGlobalAddrInst(GlobalAddrInst *i) {
   setLoweredAddress(i, addr);
 }
 
+/// Returns true if \p val has no other uses than ref_element_addr or
+/// ref_tail_addr.
+static bool hasOnlyProjections(SILValue val) {
+  for (Operand *use : val->getUses()) {
+    SILInstruction *user = use->getUser();
+    if (auto *upCast = dyn_cast<UpcastInst>(user)) {
+      if (!hasOnlyProjections(upCast))
+        return false;
+      continue;
+    }
+    if (isa<RefElementAddrInst>(user) || isa<RefTailAddrInst>(user))
+      continue;
+    return false;
+  }
+  return true;
+}
+
 void IRGenSILFunction::visitGlobalValueInst(GlobalValueInst *i) {
   SILGlobalVariable *var = i->getReferencedGlobal();
   assert(var->isInitializedObject() &&
@@ -2645,17 +2662,19 @@ void IRGenSILFunction::visitGlobalValueInst(GlobalValueInst *i) {
 
   llvm::Value *Ref = IGM.getAddrOfSILGlobalVariable(var, ti,
                                                 NotForDefinition).getAddress();
-
-  auto ClassType = loweredTy.getASTType();
-  llvm::Value *Metadata =
-    emitClassHeapMetadataRef(*this, ClassType, MetadataValueType::TypeMetadata,
-                             MetadataState::Complete);
-  llvm::Value *CastAddr = Builder.CreateBitCast(Ref, IGM.RefCountedPtrTy);
-  llvm::Value *InitRef = emitInitStaticObjectCall(Metadata, CastAddr, "staticref");
-  InitRef = Builder.CreateBitCast(InitRef, Ref->getType());
-
+  // We don't need to initialize the global object if it's never used for
+  // something which can access the object header.
+  if (!hasOnlyProjections(i)) {
+    auto ClassType = loweredTy.getASTType();
+    llvm::Value *Metadata =
+      emitClassHeapMetadataRef(*this, ClassType, MetadataValueType::TypeMetadata,
+                               MetadataState::Complete);
+    llvm::Value *CastAddr = Builder.CreateBitCast(Ref, IGM.RefCountedPtrTy);
+    llvm::Value *InitRef = emitInitStaticObjectCall(Metadata, CastAddr, "staticref");
+    Ref = Builder.CreateBitCast(InitRef, Ref->getType());
+  }
   Explosion e;
-  e.add(InitRef);
+  e.add(Ref);
   setLoweredExplosion(i, e);
 }
 

test/SILOptimizer/static_arrays.swift

@@ -1,4 +1,5 @@
 // RUN: %target-swift-frontend  -primary-file %s -O -sil-verify-all -Xllvm -sil-disable-pass=FunctionSignatureOpts -module-name=test -emit-sil | %FileCheck %s
+// RUN: %target-swift-frontend  -primary-file %s -O -sil-verify-all -Xllvm -sil-disable-pass=FunctionSignatureOpts -module-name=test -emit-ir | %FileCheck %s -check-prefix=CHECK-LLVM
 
 // Also do an end-to-end test to check all components, including IRGen.
 // RUN: %empty-directory(%t) 
@@ -98,9 +99,18 @@
 // CHECK:   return
 public let globalVariable = [ 100, 101, 102 ]
 
-// CHECK-LABEL: sil {{.*}}arrayLookup{{.*}} : $@convention(thin) (Int) -> Int {
-// CHECK:   global_value @{{.*}}arrayLookup{{.*}}
-// CHECK:   return
+// CHECK-LABEL: sil [noinline] @$s4test11arrayLookupyS2iF
+// CHECK:   global_value @$s4test11arrayLookupyS2iFTv_
+// CHECK-NOT: retain
+// CHECK-NOT: release
+// CHECK:   } // end sil function '$s4test11arrayLookupyS2iF'
+
+// CHECK-LLVM-LABEL: define {{.*}} @"$s4test11arrayLookupyS2iF"
+// CHECK-LLVM-NOT:  call
+// CHECK-LLVM:      [[E:%[0-9]+]] = getelementptr {{.*}} @"$s4test11arrayLookupyS2iFTv_"
+// CHECK-LLVM-NEXT: [[L:%[0-9]+]] = load {{.*}} [[E]]
+// CHECK-LLVM-NEXT: ret {{.*}} [[L]]
+// CHECK-LLVM:   }
 @inline(never)
 public func arrayLookup(_ i: Int) -> Int {
   let lookupTable = [10, 11, 12]