Lazily IR-declare global variables

Analogous to #4420.

Author: kinke

gen/declarations.cpp

@@ -253,9 +253,12 @@ class CodegenVisitor : public Visitor {
                            decl->toPrettyChars());
     LOG_SCOPE;
 
-    if (decl->ir->isDefined()) {
+    if (decl->ir->isDefined())
+      return;
+
+    // skip external declarations (IR-declared lazily)
+    if (decl->storage_class & STCextern)
       return;
-    }
 
     if (decl->type->ty == TY::Terror) {
       error(decl->loc, "%s `%s` had semantic errors when compiling",

gen/llvmhelpers.cpp

@@ -957,6 +957,9 @@ DValue *DtoDeclarationExp(Dsymbol *declaration) {
     // static
     if (vd->isDataseg()) {
       Declaration_codegen(vd);
+      if (vd->storage_class & STCextern) {
+        DtoResolveVariable(vd); // make sure there's an IR declaration
+      }
     } else {
       DtoVarDeclaration(vd);
     }

tests/codegen/avr.d

@@ -11,3 +11,5 @@ version (D_SoftFloat) {} else static assert(0);
 int definedGlobal = 123;
 // CHECK: @_D3avr14declaredGlobali = external global i32
 extern int declaredGlobal;
+
+int dummyRef() { return declaredGlobal; } // make sure `declaredGlobal` is IR-declared

tests/codegen/wasi.d

@@ -15,8 +15,6 @@ version (CRuntime_WASI) {} else static assert(0);
 
 // CHECK: @_D4wasi13definedGlobali = global i32 123
 int definedGlobal = 123;
-// CHECK: @_D4wasi14declaredGlobali = external global i32
-extern int declaredGlobal;
 
 
 // make sure the ModuleInfo ref is emitted into the __minfo section:

tests/fail_compilation/global_var_collision.d

@@ -1,9 +1,18 @@
-// RUN: not %ldc -c %s 2>&1 | FileCheck %s
+// It should compile fine when not referencing the colliding external global:
+// RUN: %ldc -c %s -d-version=DontReference
+
+// But fail if referenced:
+// RUN: not %ldc -c %s -verrors-context=false 2>&1 | FileCheck %s
 
 extern(C) extern int myGlobal;
 
-// CHECK:      global_var_collision.d(9): Error: Global variable type does not match previous declaration with same mangled name: `myGlobal`
-// CHECK:      Previous IR type: i32, mutable, thread-local
+version (DontReference) {} else
+{
+    int dummyRef() { return myGlobal; }
+}
+
+// CHECK:      global_var_collision.d([[@LINE+4]]): Error: Global variable type does not match previous declaration with same mangled name: `myGlobal`
+// CHECK-NEXT: Previous IR type: i32, mutable, thread-local
 // CHECK-NEXT: New IR type:      i64, const, non-thread-local
 pragma(mangle, myGlobal.mangleof)
 extern(C) __gshared const long myGlobal2 = 123;