diff --git a/clang/include/clang/Basic/CodeGenOptions.h b/clang/include/clang/Basic/CodeGenOptions.h
--- a/clang/include/clang/Basic/CodeGenOptions.h
+++ b/clang/include/clang/Basic/CodeGenOptions.h
@@ -163,6 +163,12 @@
     Never,    // No loop is assumed to be finite.
   };
 
+  enum AssignmentTrackingOpts {
+    Disabled,
+    Enabled,
+    Forced,
+  };
+
   /// The code model to use (-mcmodel).
   std::string CodeModel;
 
diff --git a/clang/include/clang/Basic/CodeGenOptions.def b/clang/include/clang/Basic/CodeGenOptions.def
--- a/clang/include/clang/Basic/CodeGenOptions.def
+++ b/clang/include/clang/Basic/CodeGenOptions.def
@@ -335,8 +335,8 @@
 CODEGENOPT(NoStackArgProbe, 1, 0) ///< Set when -mno-stack-arg-probe is used
 CODEGENOPT(DebugStrictDwarf, 1, 1) ///< Whether or not to use strict DWARF info.
 
-CODEGENOPT(EnableAssignmentTracking, 1,0) ///< Enable the Assignment Tracking
-                                          ///< debug info feature.
+/// Control the Assignment Tracking debug info feature.
+ENUM_CODEGENOPT(AssignmentTrackingMode, AssignmentTrackingOpts, 2, AssignmentTrackingOpts::Disabled)
 
 CODEGENOPT(DebugColumnInfo, 1, 0) ///< Whether or not to use column information
                                   ///< in debug info.
diff --git a/clang/include/clang/Driver/Options.td b/clang/include/clang/Driver/Options.td
--- a/clang/include/clang/Driver/Options.td
+++ b/clang/include/clang/Driver/Options.td
@@ -5796,11 +5796,11 @@
            "and non-deleting destructors. (No effect on Microsoft ABI)">,
   MarshallingInfoFlag<CodeGenOpts<"CtorDtorReturnThis">>;
 
-defm experimental_assignment_tracking :
-  BoolOption<"f", "experimental-assignment-tracking",
-  CodeGenOpts<"EnableAssignmentTracking">, DefaultFalse,
-  PosFlag<SetTrue, [CC1Option]>, NegFlag<SetFalse>, BothFlags<[CoreOption]>>,
-  Group<f_Group>;
+def experimental_assignment_tracking_EQ : Joined<["-"], "fexperimental-assignment-tracking=">,
+  Group<f_Group>, CodeGenOpts<"EnableAssignmentTracking">,
+  NormalizedValuesScope<"CodeGenOptions::AssignmentTrackingOpts">,
+  Values<"disabled,enabled,forced">, NormalizedValues<["Disabled","Enabled","Forced"]>,
+  MarshallingInfoEnum<CodeGenOpts<"AssignmentTrackingMode">, "Disabled">;
 
 } // let Flags = [CC1Option, NoDriverOption]
 
diff --git a/clang/lib/CodeGen/BackendUtil.cpp b/clang/lib/CodeGen/BackendUtil.cpp
--- a/clang/lib/CodeGen/BackendUtil.cpp
+++ b/clang/lib/CodeGen/BackendUtil.cpp
@@ -840,11 +840,29 @@
   SI.registerCallbacks(PIC, &MAM);
   PassBuilder PB(TM.get(), PTO, PGOOpt, &PIC);
 
-  if (CodeGenOpts.EnableAssignmentTracking) {
+  // Handle the assignment tracking feature options.
+  switch (CodeGenOpts.getAssignmentTrackingMode()) {
+  case CodeGenOptions::AssignmentTrackingOpts::Forced:
     PB.registerPipelineStartEPCallback(
         [&](ModulePassManager &MPM, OptimizationLevel Level) {
           MPM.addPass(AssignmentTrackingPass());
         });
+    break;
+  case CodeGenOptions::AssignmentTrackingOpts::Enabled:
+    // Disable assignment tracking in LTO builds for now as the performance
+    // cost is too high. Disable for LLDB tuning due to llvm.org/PR43126.
+    if (!CodeGenOpts.PrepareForThinLTO && !CodeGenOpts.PrepareForLTO &&
+        CodeGenOpts.getDebuggerTuning() != llvm::DebuggerKind::LLDB) {
+      PB.registerPipelineStartEPCallback(
+          [&](ModulePassManager &MPM, OptimizationLevel Level) {
+            // Only use assignment tracking if optimisations are enabled.
+            if (Level != OptimizationLevel::O0)
+              MPM.addPass(AssignmentTrackingPass());
+          });
+    }
+    break;
+  case CodeGenOptions::AssignmentTrackingOpts::Disabled:
+    break;
   }
 
   // Enable verify-debuginfo-preserve-each for new PM.
diff --git a/clang/test/CodeGen/assignment-tracking/assignment-tracking.cpp b/clang/test/CodeGen/assignment-tracking/assignment-tracking.cpp
--- a/clang/test/CodeGen/assignment-tracking/assignment-tracking.cpp
+++ b/clang/test/CodeGen/assignment-tracking/assignment-tracking.cpp
@@ -1,5 +1,5 @@
 // RUN: %clang_cc1 -triple x86_64-none-linux-gnu -debug-info-kind=standalone -O0 \
-// RUN:     -emit-llvm  -fexperimental-assignment-tracking %s -o -               \
+// RUN:     -emit-llvm  -fexperimental-assignment-tracking=forced %s -o -        \
 // RUN: | FileCheck %s --implicit-check-not="call void @llvm.dbg"
 
 // Based on llvm/test/DebugInfo/Generic/track-assignments.ll - check that using
diff --git a/clang/test/CodeGen/assignment-tracking/flag.cpp b/clang/test/CodeGen/assignment-tracking/flag.cpp
--- a/clang/test/CodeGen/assignment-tracking/flag.cpp
+++ b/clang/test/CodeGen/assignment-tracking/flag.cpp
@@ -1,24 +1,64 @@
 //// Explicitly enabled:
-// RUN: %clang_cc1 -triple x86_64-none-linux-gnu -debug-info-kind=standalone -O0 \
-// RUN:     -emit-llvm  -fexperimental-assignment-tracking %s -o -               \
+// RUN: %clang_cc1 -triple x86_64-none-linux-gnu -debug-info-kind=standalone   \
+// RUN:     -emit-llvm  -fexperimental-assignment-tracking=enabled %s -o - -O1 \
 // RUN: | FileCheck %s --check-prefixes=ENABLE
+
+//// Explicitly disabled:
+// RUN: %clang_cc1 -triple x86_64-none-linux-gnu -debug-info-kind=standalone   \
+// RUN:     -emit-llvm  %s -o - -fexperimental-assignment-tracking=disabled -O1\
+// RUN: | FileCheck %s --check-prefixes=DISABLE
+
 //// Disabled by default:
-// RUN: %clang_cc1 -triple x86_64-none-linux-gnu -debug-info-kind=standalone -O0 \
-// RUN:     -emit-llvm  %s -o -                                                  \
+// RUN: %clang_cc1 -triple x86_64-none-linux-gnu -debug-info-kind=standalone   \
+// RUN:     -emit-llvm  %s -o - -O1                                            \
 // RUN: | FileCheck %s --check-prefixes=DISABLE
-//// Explicitly disabled:
-// RUN: %clang_cc1 -triple x86_64-none-linux-gnu -debug-info-kind=standalone -O0 \
-// RUN:     -emit-llvm  %s -o - -fno-experimental-assignment-tracking            \
+
+//// Disabled at O0 unless forced.
+// RUN: %clang_cc1 -triple x86_64-none-linux-gnu -debug-info-kind=standalone   \
+// RUN:     -emit-llvm %s -o - -fexperimental-assignment-tracking=enabled      \
+// RUN:     -O0                                                                \
+// RUN: | FileCheck %s --check-prefixes=DISABLE
+// RUN: %clang_cc1 -triple x86_64-none-linux-gnu -debug-info-kind=standalone   \
+// RUN:     -emit-llvm %s -o - -fexperimental-assignment-tracking=forced       \
+// RUN:     -O0                                                                \
+// RUN: | FileCheck %s --check-prefixes=ENABLE
+
+//// Disabled for LTO and thinLTO unless forced.
+// RUN: %clang_cc1 -triple x86_64-none-linux-gnu -debug-info-kind=standalone   \
+// RUN:     -emit-llvm %s -o - -fexperimental-assignment-tracking=enabled      \
+// RUN:     -O1 -flto=full                                                     \
 // RUN: | FileCheck %s --check-prefixes=DISABLE
+// RUN: %clang_cc1 -triple x86_64-none-linux-gnu -debug-info-kind=standalone   \
+// RUN:     -emit-llvm %s -o - -fexperimental-assignment-tracking=enabled      \
+// RUN:     -O1 -flto=thin                                                     \
+// RUN: | FileCheck %s --check-prefixes=DISABLE
+// RUN: %clang_cc1 -triple x86_64-none-linux-gnu -debug-info-kind=standalone   \
+// RUN:     -emit-llvm %s -o - -fexperimental-assignment-tracking=forced       \
+// RUN:     -O1 -flto=full                                                     \
+// RUN: | FileCheck %s --check-prefixes=ENABLE
+// RUN: %clang_cc1 -triple x86_64-none-linux-gnu -debug-info-kind=standalone   \
+// RUN:     -emit-llvm %s -o - -fexperimental-assignment-tracking=forced       \
+// RUN:     -O1 -flto=thin                                                     \
+// RUN: | FileCheck %s --check-prefixes=ENABLE
 
-// Check some assignment-tracking stuff appears in the output when the flag
-// -fexperimental-assignment-tracking is used, that it doesn't when
-// -fno-experimental-assignment-tracking is used or neither flag is specified.
+//// Disabled for LLDB debugger tuning unless forced.
+// RUN: %clang_cc1 -triple x86_64-none-linux-gnu -debug-info-kind=standalone   \
+// RUN:     -emit-llvm %s -o - -fexperimental-assignment-tracking=enabled      \
+// RUN:     -O1 -debugger-tuning=lldb                                          \
+// RUN: | FileCheck %s --check-prefixes=DISABLE
+// RUN: %clang_cc1 -triple x86_64-none-linux-gnu -debug-info-kind=standalone   \
+// RUN:     -emit-llvm %s -o - -fexperimental-assignment-tracking=forced       \
+// RUN:     -O1 -debugger-tuning=lldb                                          \
+// RUN: | FileCheck %s --check-prefixes=ENABLE
 
-// ENABLE: DIAssignID
-// ENABLE: dbg.assign
+// Check the assignment-tracking module flag appears in the output when the
+// flag -fexperimental-assignment-tracking is set to 'enabled' (in some cases)
+// or 'forced' (always), and is does not appear when the flag is set to
+// 'disabled' (default).
 
-// DISABLE-NOT: DIAssignID
-// DISABLE-NOT: dbg.assign
+// ENABLE: "debug-info-assignment-tracking"
+// DISABLE-NOT: "debug-info-assignment-tracking"
+//// Check there's actually any output at all.
+// DISABLE: llvm.module.flags
 
 void fun(int a) {}
diff --git a/clang/test/CodeGen/assignment-tracking/memcpy-fragment.cpp b/clang/test/CodeGen/assignment-tracking/memcpy-fragment.cpp
--- a/clang/test/CodeGen/assignment-tracking/memcpy-fragment.cpp
+++ b/clang/test/CodeGen/assignment-tracking/memcpy-fragment.cpp
@@ -1,5 +1,5 @@
 // RUN: %clang_cc1 -triple x86_64-none-linux-gnu -debug-info-kind=standalone -O0 \
-// RUN:     -emit-llvm  -fexperimental-assignment-tracking %s -o -               \
+// RUN:     -emit-llvm  -fexperimental-assignment-tracking=forced %s -o -        \
 // RUN: | FileCheck %s
 
 // Check that the (debug) codegen looks right with assignment tracking
diff --git a/clang/test/CodeGen/assignment-tracking/nested-scope.cpp b/clang/test/CodeGen/assignment-tracking/nested-scope.cpp
--- a/clang/test/CodeGen/assignment-tracking/nested-scope.cpp
+++ b/clang/test/CodeGen/assignment-tracking/nested-scope.cpp
@@ -1,5 +1,5 @@
 // RUN: %clang_cc1 -triple x86_64-none-linux-gnu -debug-info-kind=standalone -O0 \
-// RUN:     -emit-llvm  -fexperimental-assignment-tracking %s -o -               \
+// RUN:     -emit-llvm  -fexperimental-assignment-tracking=forced %s -o -        \
 // RUN: | FileCheck %s
 
 // Check that dbg.assign intrinsics get a !dbg with with the same scope as