Turn off PCM validation by default.

lldb/include/lldb/Target/Target.h

@@ -188,6 +188,8 @@ class TargetProperties : public Properties {
 
   bool GetSwiftAllowExplicitModules() const;
 
+  AutoBool GetSwiftPCMValidation() const;
+
   Args GetSwiftPluginServerForPath() const;
 
   bool GetSwiftAutoImportFrameworks() const;

lldb/source/Plugins/TypeSystem/Swift/SwiftASTContext.cpp

@@ -57,7 +57,9 @@
 #include "clang/Basic/TargetOptions.h"
 #include "clang/Driver/Driver.h"
 #include "clang/Frontend/CompilerInstance.h"
+#include "clang/Lex/Preprocessor.h"
 
+#include "clang/Lex/PreprocessorOptions.h"
 #include "llvm/ADT/ArrayRef.h"
 #include "llvm/ADT/STLExtras.h"
 #include "llvm/ADT/SmallVector.h"
@@ -9210,6 +9212,38 @@ bool SwiftASTContext::GetCompileUnitImportsImpl(
   if (cu_imports.size() == 0)
     return true;
 
+  // Set PCM validation. This is not a great place to do this, but it
+  // needs to happen after ClangImporter was created and
+  // m_has_explicit_modules has been initialized.
+  {
+    // Read the setting.
+    AutoBool validate_pcm_setting = AutoBool::Auto;
+    TargetSP target_sp = GetTargetWP().lock();
+    if (target_sp)
+      validate_pcm_setting = target_sp->GetSwiftPCMValidation();
+
+    // If the setting is explicit, honor it.
+    bool validate_pcm = validate_pcm_setting != AutoBool::False;
+    if (validate_pcm_setting == AutoBool::Auto) {
+      // Disable validation for explicit modules.
+      validate_pcm = m_has_explicit_modules ? false : true;
+      // Enable validation in asserts builds.
+#ifndef NDEBUG
+      validate_pcm = true;
+#endif
+    }
+
+    auto &pp_opts = m_clangimporter->getClangPreprocessor()
+      .getPreprocessorOpts();
+    pp_opts.DisablePCHOrModuleValidation =
+        validate_pcm ? clang::DisableValidationForModuleKind::None
+                     : clang::DisableValidationForModuleKind::All;
+    pp_opts.ModulesCheckRelocated = validate_pcm;
+
+    LOG_PRINTF(GetLog(LLDBLog::Types), "PCM validation is %s",
+               validate_pcm ? "disabled" : "enabled");
+  }
+
   LOG_PRINTF(GetLog(LLDBLog::Types), "Importing dependencies of current CU");
 
   // Turn off implicit clang modules while importing CU dependencies.

lldb/source/Target/Target.cpp

@@ -4688,6 +4688,13 @@ static constexpr OptionEnumValueElement g_memory_module_load_level_values[] = {
     },
 };
 
+static constexpr OptionEnumValueElement g_swift_pcm_validation_values[] = {
+    {llvm::to_underlying(AutoBool::Auto), "auto",
+     "Turned on when explicit modules are enabled"},
+    {llvm::to_underlying(AutoBool::True), "true", "Enable validation."},
+    {llvm::to_underlying(AutoBool::False), "false", "Disable validation."},
+};
+
 
 #define LLDB_PROPERTIES_target
 #include "TargetProperties.inc"
@@ -4927,6 +4934,19 @@ bool TargetProperties::GetSwiftAllowExplicitModules() const {
   return true;
 }
 
+AutoBool TargetProperties::GetSwiftPCMValidation() const {
+  const Property *exp_property =
+      m_collection_sp->GetPropertyAtIndex(ePropertyExperimental);
+  OptionValueProperties *exp_values =
+      exp_property->GetValue()->GetAsProperties();
+  if (exp_values)
+    return exp_values
+        ->GetPropertyAtIndexAs<AutoBool>(ePropertySwiftPCMValidation)
+      .value_or(AutoBool::Auto);
+
+  return AutoBool::Auto;
+}
+
 Args TargetProperties::GetSwiftPluginServerForPath() const {
   const uint32_t idx = ePropertySwiftPluginServerForPath;
 

lldb/source/Target/TargetProperties.td

@@ -26,6 +26,11 @@ let Definition = "target_experimental" in {
   def SwiftAllowExplicitModules: Property<"swift-allow-explicit-modules", "Boolean">,
     DefaultTrue,
     Desc<"Allows explicit module flags to be passed through to ClangImporter.">;
+  def SwiftPCMValidation: Property<"swift-pcm-validation", "Enum">,
+    Global,
+    DefaultEnumValue<"llvm::to_underlying(AutoBool::Auto)">,
+    EnumValues<"OptionEnumValues(g_swift_pcm_validation_values)">,
+    Desc<"Enable validation when loading Clang PCM files (-fvalidate-pch, -fmodules-check-relocated).">;
 }
 
 let Definition = "target" in {

lldb/test/API/lang/swift/clangimporter/fmodule_flags/TestSwiftFModuleFlags.py

@@ -24,3 +24,4 @@ def test(self):
 #       CHECK-NOT: -fno-implicit-modules
 #       CHECK-NOT: -fno-implicit-module-maps
 #       CHECK: -DMARKER2
+#       CHECK: PCM validation is