[Driver] Ignore non-clang pch files when -include a.h probes a.h.gch (#69204)

Instead of deprecating the "gch probe" as in
f726da1193, this makes clang ignore files
which are not clang pch files (See discussion on PR #67084).

This fixes the issues mentioned in the former patch, with GCC-generated
.gch files getting in the way when using clang, while maintaining the
probing behavior for builds which rely on that.

clang/docs/ReleaseNotes.rst

@@ -43,8 +43,8 @@ C/C++ Language Potentially Breaking Changes
 
 - The default extension name for PCH generation (``-c -xc-header`` and ``-c
   -xc++-header``) is now ``.pch`` instead of ``.gch``.
-- ``-include a.h`` probing ``a.h.gch`` is deprecated. Change the extension name
-  to ``.pch`` or use ``-include-pch a.h.gch``.
+- ``-include a.h`` probing ``a.h.gch`` will now ignore ``a.h.gch`` if it is not
+  a clang pch file or a directory containing any clang pch file.
 - Fixed a bug that caused ``__has_cpp_attribute`` and ``__has_c_attribute``
   return incorrect values for some C++-11-style attributes. Below is a complete
   list of behavior changes.

clang/include/clang/Basic/DiagnosticDriverKinds.td

@@ -441,11 +441,14 @@ def warn_drv_overriding_option : Warning<
 def warn_drv_treating_input_as_cxx : Warning<
   "treating '%0' input as '%1' when in C++ mode, this behavior is deprecated">,
   InGroup<Deprecated>;
-def warn_drv_include_probe_gch : Warning<
-  "'%0' probing .gch is deprecated. Use '-include-pch %1' or switch to .pch instead">,
-  InGroup<DeprecatedIncludeGch>;
 def warn_drv_pch_not_first_include : Warning<
   "precompiled header '%0' was ignored because '%1' is not first '-include'">;
+def warn_drv_pch_ignoring_gch_file : Warning<
+  "precompiled header '%0' was ignored because it is not a clang PCH file">,
+  InGroup<IgnoredGCH>;
+def warn_drv_pch_ignoring_gch_dir : Warning<
+  "precompiled header directory '%0' was ignored because it contains no clang PCH files">,
+  InGroup<IgnoredGCH>;
 def warn_missing_sysroot : Warning<"no such sysroot directory: '%0'">,
   InGroup<DiagGroup<"missing-sysroot">>;
 def warn_incompatible_sysroot : Warning<"using sysroot for '%0' but targeting '%1'">,

clang/include/clang/Basic/DiagnosticGroups.td

@@ -210,7 +210,6 @@ def DeprecatedWritableStr : DiagGroup<"deprecated-writable-strings",
                                       [CXX11CompatDeprecatedWritableStr]>;
 def DeprecatedPragma : DiagGroup<"deprecated-pragma">;
 def DeprecatedType : DiagGroup<"deprecated-type">;
-def DeprecatedIncludeGch : DiagGroup<"deprecated-include-gch">;
 // FIXME: Why is DeprecatedImplementations not in this group?
 def Deprecated : DiagGroup<"deprecated", [DeprecatedAnonEnumEnumConversion,
                                           DeprecatedArrayCompare,
@@ -233,8 +232,7 @@ def Deprecated : DiagGroup<"deprecated", [DeprecatedAnonEnumEnumConversion,
                                           DeprecatedType,
                                           DeprecatedVolatile,
                                           DeprecatedWritableStr,
-                                          DeprecatedRedundantConstexprStaticDef,
-                                          DeprecatedIncludeGch
+                                          DeprecatedRedundantConstexprStaticDef
                                           ]>,
                  DiagCategory<"Deprecations">;
 
@@ -444,6 +442,7 @@ def IncrementBool : DiagGroup<"increment-bool", [DeprecatedIncrementBool]>;
 def InfiniteRecursion : DiagGroup<"infinite-recursion">;
 def PureVirtualCallFromCtorDtor: DiagGroup<"call-to-pure-virtual-from-ctor-dtor">;
 def GNUImaginaryConstant : DiagGroup<"gnu-imaginary-constant">;
+def IgnoredGCH : DiagGroup<"ignored-gch">;
 def IgnoredReferenceQualifiers : DiagGroup<"ignored-reference-qualifiers">;
 def IgnoredQualifiers : DiagGroup<"ignored-qualifiers", [IgnoredReferenceQualifiers]>;
 def : DiagGroup<"import">;

clang/lib/Driver/ToolChains/Clang.cpp

@@ -1067,6 +1067,35 @@ static void handleAMDGPUCodeObjectVersionOptions(const Driver &D,
   }
 }
 
+static bool hasClangPchSignature(const Driver &D, StringRef Path) {
+  if (llvm::ErrorOr<std::unique_ptr<llvm::MemoryBuffer>> MemBuf =
+          D.getVFS().getBufferForFile(Path))
+    return (*MemBuf)->getBuffer().startswith("CPCH");
+  return false;
+}
+
+static bool gchProbe(const Driver &D, StringRef Path) {
+  llvm::ErrorOr<llvm::vfs::Status> Status = D.getVFS().status(Path);
+  if (!Status)
+    return false;
+
+  if (Status->isDirectory()) {
+    std::error_code EC;
+    for (llvm::vfs::directory_iterator DI = D.getVFS().dir_begin(Path, EC), DE;
+         !EC && DI != DE; DI = DI.increment(EC)) {
+      if (hasClangPchSignature(D, DI->path()))
+        return true;
+    }
+    D.Diag(diag::warn_drv_pch_ignoring_gch_dir) << Path;
+    return false;
+  }
+
+  if (hasClangPchSignature(D, Path))
+    return true;
+  D.Diag(diag::warn_drv_pch_ignoring_gch_file) << Path;
+  return false;
+}
+
 void Clang::AddPreprocessingOptions(Compilation &C, const JobAction &JA,
                                     const Driver &D, const ArgList &Args,
                                     ArgStringList &CmdArgs,
@@ -1283,11 +1312,9 @@ void Clang::AddPreprocessingOptions(Compilation &C, const JobAction &JA,
         FoundPCH = true;
 
       if (!FoundPCH) {
+        // For GCC compat, probe for a file or directory ending in .gch instead.
         llvm::sys::path::replace_extension(P, "gch");
-        if (D.getVFS().exists(P)) {
-          FoundPCH = true;
-          D.Diag(diag::warn_drv_include_probe_gch) << A->getAsString(Args) << P;
-        }
+        FoundPCH = gchProbe(D, P.str());
       }
 
       if (FoundPCH) {

clang/test/PCH/Inputs/gch-probe.h

@@ -0,0 +1 @@
+void g(void);

clang/test/PCH/Inputs/gch-probe.h.gch

@@ -0,0 +1 @@
+This is not a pch file.

clang/test/PCH/gch-probe.c

@@ -0,0 +1,9 @@
+// For GCC compatibility, clang should probe also with the .gch extension.
+// RUN: %clang -x c-header -c %s -o %t.h.gch
+// RUN: %clang -fsyntax-only -include %t.h %s
+
+// gch probing should ignore files which are not clang pch files.
+// RUN: %clang -fsyntax-only -include %S/Inputs/gch-probe.h %s 2>&1 | FileCheck %s
+// CHECK: warning: precompiled header '{{.*}}gch-probe.h.gch' was ignored because it is not a clang PCH file
+
+void f(void);

clang/test/PCH/pch-dir.c

@@ -23,19 +23,25 @@
 // RUN: not %clang_cc1 -include-pch %t.h.gch -DFOO=baz -fsyntax-only %s -print-stats 2> %t.missinglog
 // RUN: FileCheck -check-prefix=CHECK-NO-SUITABLE %s < %t.missinglog
 
+// Don't gch probe directories which contain no pch files.
+// RUN: rm -rf %t.x.h.gch
+// RUN: mkdir -p %t.x.h.gch
+// RUN: not %clang -include %t.x.h -fsyntax-only %s 2>&1 | FileCheck -check-prefix=CHECK-IGNORED-DIR %s
+
 // CHECK-CBAR: int bar
 int FOO;
 
 int get(void) {
 #ifdef __cplusplus
-  // CHECK-CPP: warning: '-include {{.*}}.h' probing .gch is deprecated. Use '-include-pch {{.*}}.h.gch' or switch to .pch instead
   // CHECK-CPP: .h.gch{{[/\\]}}cpp.gch
   return i;
 #else
-  // CHECK-C: warning: '-include {{.*}}.h' probing .gch is deprecated. Use '-include-pch {{.*}}.h.gch' or switch to .pch instead
   // CHECK-C: .h.gch{{[/\\]}}c.gch
   return j;
 #endif
 }
 
 // CHECK-NO-SUITABLE: no suitable precompiled header file found in directory
+
+// CHECK-IGNORED-DIR: precompiled header directory '{{.*}}pch-dir.c.tmp.x.h.gch' was ignored because it contains no clang PCH files
+// CHECK-IGNORED-DIR: '{{.*}}pch-dir.c.tmp.x.h' file not found