[clang][DependencyScanner] Remove all warning flags when suppressing warnings (#71612)

Since system modules don't emit most warnings, remove the warning flags
to increase module reuse.

clang/include/clang/Tooling/DependencyScanning/DependencyScanningService.h

@@ -51,8 +51,11 @@ enum class ScanningOptimizations {
   /// Remove unused header search paths including header maps.
   HeaderSearch = 1,
 
-  LLVM_MARK_AS_BITMASK_ENUM(HeaderSearch),
-  All = HeaderSearch,
+  /// Remove warnings from system modules.
+  SystemWarnings = 2,
+
+  LLVM_MARK_AS_BITMASK_ENUM(SystemWarnings),
+  All = HeaderSearch | SystemWarnings,
   Default = All
 };
 

clang/lib/Tooling/DependencyScanning/ModuleDepCollector.cpp

@@ -52,6 +52,28 @@ static void optimizeHeaderSearchOpts(HeaderSearchOptions &Opts,
     Opts.UserEntries.push_back(Entries[Idx]);
 }
 
+static void optimizeDiagnosticOpts(DiagnosticOptions &Opts,
+                                   bool IsSystemModule) {
+  // If this is not a system module or -Wsystem-headers was passed, don't
+  // optimize.
+  if (!IsSystemModule)
+    return;
+  bool Wsystem_headers = false;
+  for (StringRef Opt : Opts.Warnings) {
+    bool isPositive = !Opt.consume_front("no-");
+    if (Opt == "system-headers")
+      Wsystem_headers = isPositive;
+  }
+  if (Wsystem_headers)
+    return;
+
+  // Remove all warning flags. System modules suppress most, but not all,
+  // warnings.
+  Opts.Warnings.clear();
+  Opts.UndefPrefixes.clear();
+  Opts.Remarks.clear();
+}
+
 static std::vector<std::string> splitString(std::string S, char Separator) {
   SmallVector<StringRef> Segments;
   StringRef(S).split(Segments, Separator, /*MaxSplit=*/-1, /*KeepEmpty=*/false);
@@ -532,6 +554,10 @@ ModuleDepCollectorPP::handleTopLevelModule(const Module *M) {
             if (any(MDC.OptimizeArgs & ScanningOptimizations::HeaderSearch))
               optimizeHeaderSearchOpts(BuildInvocation.getMutHeaderSearchOpts(),
                                        *MDC.ScanInstance.getASTReader(), *MF);
+            if (any(MDC.OptimizeArgs & ScanningOptimizations::SystemWarnings))
+              optimizeDiagnosticOpts(
+                  BuildInvocation.getMutDiagnosticOpts(),
+                  BuildInvocation.getFrontendOpts().IsSystemModule);
           });
 
   MDC.associateWithContextHash(CI, MD);

clang/test/ClangScanDeps/optimize-system-warnings.m

@@ -0,0 +1,123 @@
+// This test verifies that system module variants are mergable despite having
+// different warning flags, as most warnings are disabled in system modules.
+// This checks for system modules marked as such both via `-isystem` and
+// `[system]`.
+
+// RUN: rm -rf %t
+// RUN: split-file %s %t
+// RUN: sed -e "s|DIR|%/t|g" %t/build/compile-commands.json.in > %t/build/compile-commands.json
+// RUN: clang-scan-deps -compilation-database %t/build/compile-commands.json \
+// RUN:   -j 1 -format experimental-full -optimize-args=system-warnings > %t/deps.db
+// RUN: cat %t/deps.db | sed 's:\\\\\?:/:g' | FileCheck %s -DPREFIX=%/t
+
+// CHECK:      {
+// CHECK-NEXT:   "modules": [
+// CHECK-NEXT:     {
+// CHECK-NEXT:       "clang-module-deps": [],
+// CHECK-NEXT:       "clang-modulemap-file":
+// CHECK-NEXT:       "command-line": [
+// CHECK-NOT:          "-W
+// CHECK:            ],
+// CHECK-NEXT:       "context-hash": "{{.*}}",
+// CHECK-NEXT:       "file-deps": [
+// CHECK:            ],
+// CHECK-NEXT:       "name": "A"
+// CHECK-NEXT:     },
+// CHECK-NEXT:     {
+// CHECK-NEXT:       "clang-module-deps": [],
+// CHECK-NEXT:       "clang-modulemap-file":
+// CHECK-NEXT:       "command-line": [
+// CHECK-NOT:          "-W
+// CHECK:            ],
+// CHECK-NEXT:       "context-hash": "{{.*}}",
+// CHECK-NEXT:       "file-deps": [
+// CHECK:            ],
+// CHECK-NEXT:       "name": "B"
+// CHECK-NEXT:     },
+// CHECK-NEXT:     {
+// CHECK-NEXT:       "clang-module-deps": [],
+// CHECK-NEXT:       "clang-modulemap-file":
+// CHECK-NEXT:       "command-line": [
+// CHECK:              "-Wmaybe-unused
+// CHECK:            ],
+// CHECK-NEXT:       "context-hash": "{{.*}}",
+// CHECK-NEXT:       "file-deps": [
+// CHECK:            ],
+// CHECK-NEXT:       "name": "C"
+// CHECK-NEXT:     }
+// CHECK-NEXT:   ],
+// CHECK-NEXT:   "translation-units": [
+// CHECK:        ]
+// CHECK:      }
+
+// A.m and B.m verify that system modules with different warning flags get
+// merged. C.m verifies that -Wsystem-headers disables the optimization.
+//--- build/compile-commands.json.in
+
+[
+{
+  "directory": "DIR",
+  "command": "clang -c DIR/A.m -isystem modules/A -I modules/B -fmodules -fmodules-cache-path=DIR/module-cache -fimplicit-module-maps",
+  "file": "DIR/A.m"
+},
+{
+  "directory": "DIR",
+  "command": "clang -c DIR/B.m -isystem modules/A -I modules/B -fmodules -fmodules-cache-path=DIR/module-cache -fimplicit-module-maps -Wmaybe-unused",
+  "file": "DIR/B.m"
+},
+{
+  "directory": "DIR",
+  "command": "clang -c DIR/C.m -isystem modules/C              -fmodules -fmodules-cache-path=DIR/module-cache -fimplicit-module-maps -Wmaybe-unused -Wsystem-headers",
+  "file": "DIR/C.m"
+}
+]
+
+//--- modules/A/module.modulemap
+
+module A {
+  umbrella header "A.h"
+}
+
+//--- modules/A/A.h
+
+typedef int A_t;
+
+//--- modules/B/module.modulemap
+
+module B [system] {
+  umbrella header "B.h"
+}
+
+//--- modules/B/B.h
+
+typedef int B_t;
+
+//--- modules/C/module.modulemap
+
+module C [system] {
+  umbrella header "C.h"
+}
+
+//--- modules/C/C.h
+
+typedef int C_t;
+
+//--- A.m
+
+#include <A.h>
+#include <B.h>
+
+A_t a = 0;
+
+//--- B.m
+
+#include <A.h>
+#include <B.h>
+
+A_t b = 0;
+
+//--- C.m
+
+#include <C.h>
+
+C_t c = 0;

clang/tools/clang-scan-deps/ClangScanDeps.cpp

@@ -156,6 +156,7 @@ static void ParseArgs(int argc, char **argv) {
         llvm::StringSwitch<std::optional<ScanningOptimizations>>(Arg)
             .Case("none", ScanningOptimizations::None)
             .Case("header-search", ScanningOptimizations::HeaderSearch)
+            .Case("system-warnings", ScanningOptimizations::SystemWarnings)
             .Case("all", ScanningOptimizations::All)
             .Default(std::nullopt);
     if (!Optimization) {