Index: clang/include/clang/Basic/CodeGenOptions.def
===================================================================
--- clang/include/clang/Basic/CodeGenOptions.def
+++ clang/include/clang/Basic/CodeGenOptions.def
@@ -506,6 +506,10 @@
 /// non-deleting destructors. (No effect on Microsoft ABI.)
 CODEGENOPT(CtorDtorReturnThis, 1, 0)
 
+/// Whether to import function definitions from inter module function
+/// definitions.
+CODEGENOPT(ImportInterModuleFunctionDefs, 1, 0)
+
 #undef CODEGENOPT
 #undef ENUM_CODEGENOPT
 #undef VALUE_CODEGENOPT
Index: clang/include/clang/Driver/Options.td
===================================================================
--- clang/include/clang/Driver/Options.td
+++ clang/include/clang/Driver/Options.td
@@ -545,6 +545,8 @@
 
 defvar std = !strconcat("LangStandard::getLangStandardForKind(", lang_std.KeyPath, ")");
 
+defvar opt_level = CodeGenOpts<"OptimizationLevel">;
+
 /////////
 // Options
 
@@ -2355,6 +2357,11 @@
 def fmodule_output : Flag<["-"], "fmodule-output">, Flags<[NoXarchOption, CC1Option]>,
   HelpText<"Save intermediate module file results when compiling a standard C++ module unit.">;
 
+defm import_inter_module_function_defs : BoolOption<"f", "import-inter-module-function-defs",
+  CodeGenOpts<"ImportInterModuleFunctionDefs">, Default<!strconcat(opt_level.KeyPath, "!= 0")>,
+  PosFlag<SetTrue, [], "Import function definitions from other module units">,
+  NegFlag<SetFalse, [CC1Option], "Don't import function definitions from other module units">>;
+
 def fmodules_prune_interval : Joined<["-"], "fmodules-prune-interval=">, Group<i_Group>,
   Flags<[CC1Option]>, MetaVarName<"<seconds>">,
   HelpText<"Specify the interval (in seconds) between attempts to prune the module cache">,
Index: clang/lib/CodeGen/CodeGenModule.cpp
===================================================================
--- clang/lib/CodeGen/CodeGenModule.cpp
+++ clang/lib/CodeGen/CodeGenModule.cpp
@@ -3541,7 +3541,8 @@
   if (getFunctionLinkage(GD) != llvm::Function::AvailableExternallyLinkage)
     return true;
   const auto *F = cast<FunctionDecl>(GD.getDecl());
-  if (CodeGenOpts.OptimizationLevel == 0 && !F->hasAttr<AlwaysInlineAttr>())
+  if (!CodeGenOpts.ImportInterModuleFunctionDefs &&
+      !F->hasAttr<AlwaysInlineAttr>())
     return false;
 
   if (F->hasAttr<DLLImportAttr>() && !F->hasAttr<AlwaysInlineAttr>()) {
Index: clang/lib/Driver/ToolChains/Clang.cpp
===================================================================
--- clang/lib/Driver/ToolChains/Clang.cpp
+++ clang/lib/Driver/ToolChains/Clang.cpp
@@ -3850,6 +3850,10 @@
   Args.ClaimAllArgs(options::OPT_fmodule_output);
   Args.ClaimAllArgs(options::OPT_fmodule_output_EQ);
 
+  if (Args.hasFlag(options::OPT_fno_import_inter_module_function_defs,
+                   options::OPT_fimport_inter_module_function_defs, false))
+    CmdArgs.push_back("-fno-import-inter-module-function-defs");
+
   return HaveModules;
 }
 
Index: clang/test/Driver/module-no-import-from-other-tu.cppm
===================================================================
--- /dev/null
+++ clang/test/Driver/module-no-import-from-other-tu.cppm
@@ -0,0 +1,6 @@
+// RUN: %clang -O3 -fno-import-inter-module-function-defs -std=c++20 \
+// RUN:     %s -### 2>&1 | FileCheck %s
+
+export module x;
+
+// CHECK: -fno-import-inter-module-function-defs
Index: clang/test/Modules/inter-module-imports-function-defs.cppm
===================================================================
--- /dev/null
+++ clang/test/Modules/inter-module-imports-function-defs.cppm
@@ -0,0 +1,57 @@
+// RUN: rm -rf %t
+// RUN: split-file %s %t
+// RUN: cd %t
+//
+// RUN: %clang_cc1 -std=c++20 -triple %itanium_abi_triple %t/a.cppm \
+// RUN:     -emit-module-interface -o %t/a.pcm
+// RUN: %clang_cc1 -std=c++20 -triple %itanium_abi_triple %t/b.cppm \
+// RUN:     -emit-module-interface -fprebuilt-module-path=%t -o %t/b.pcm
+// RUN: %clang_cc1 -std=c++20 -triple %itanium_abi_triple %t/c.cppm \
+// RUN:     -emit-module-interface -fprebuilt-module-path=%t -o %t/c.pcm
+// RUN: %clang_cc1 -std=c++20 -triple %itanium_abi_triple %t/c.pcm -S \
+// RUN:     -emit-llvm -disable-llvm-passes -o - | FileCheck %t/c.cppm --check-prefix=NO-IMPORT
+//
+// RUN: %clang_cc1 -std=c++20 -triple %itanium_abi_triple -O3 %t/a.cppm \
+// RUN:     -emit-module-interface -o %t/a.pcm
+// RUN: %clang_cc1 -std=c++20 -triple %itanium_abi_triple -O3 %t/b.cppm \
+// RUN:     -emit-module-interface -fprebuilt-module-path=%t -o %t/b.pcm
+// RUN: %clang_cc1 -std=c++20 -triple %itanium_abi_triple -O3 %t/c.cppm \
+// RUN:     -emit-module-interface -fprebuilt-module-path=%t -o %t/c.pcm
+// RUN: %clang_cc1 -std=c++20 -triple %itanium_abi_triple -O3 %t/c.pcm \
+// RUN:     -S -emit-llvm -disable-llvm-passes -o - | FileCheck %t/c.cppm --check-prefix=IMPORT
+//
+// RUN: %clang_cc1 -std=c++20 -triple %itanium_abi_triple -O3 \
+// RUN:     %t/a.cppm -emit-module-interface -o %t/a.pcm
+// RUN: %clang_cc1 -std=c++20 -triple %itanium_abi_triple -O3 \
+// RUN:     %t/b.cppm -emit-module-interface -fprebuilt-module-path=%t -o %t/b.pcm
+// RUN: %clang_cc1 -std=c++20 -triple %itanium_abi_triple -O3 \
+// RUN:     %t/c.cppm -emit-module-interface -fprebuilt-module-path=%t -o %t/c.pcm
+// RUN: %clang_cc1 -std=c++20 -triple %itanium_abi_triple -O3 \
+// RUN:     -fno-import-inter-module-function-defs %t/c.pcm \
+// RUN:     -S -emit-llvm -disable-llvm-passes -o - | FileCheck %t/c.cppm --check-prefix=NO-IMPORT
+
+//--- a.cppm
+export module a;
+export int a() {
+    return 43;
+}
+
+//--- b.cppm
+export module b;
+export import a;
+export int b() {
+    return 43 + a();
+}
+
+//--- c.cppm
+export module c;
+export import b;
+export int c() {
+    return 43 + b() + a();
+}
+
+// NO-IMPORT: declare{{.*}}@_ZW1b1bv
+// NO-IMPORT: declare{{.*}}@_ZW1a1av
+
+// IMPORT: define available_externally{{.*}}@_ZW1b1bv
+// IMPORT: define available_externally{{.*}}@_ZW1a1av