[Clang] only convert dependency filename to native form when MS-compatible #160903

ZhongRuoyu · 2025-09-26T15:22:09Z

Currently, llvm::sys::path::native is called unconditionally when generating dependency filenames. This is correct on Windows, where backslashes are valid path separators, but can be incorrect on non-Windows platforms when the Clang invocation is not MS-compatible (-fno-ms-compatibility), because in that case backslashes in the filename are converted by llvm::sys::path::native to forward slashes, while they should be treated as ordinary characters.

This fixes the following inconsistency on non-Windows platforms (notice how the dependency output always converts the backslash to a forward slash, assuming it's a path separator, while the actual include directive treats it as an ordinary character when -fno-ms-compatibility is set):

$ tree
.
└── foo
    ├── a
    │   └── b.h
    └── a\b.h

3 directories, 2 files

$ cat foo/a/b.h
#warning a/b.h
$ cat foo/a\\b.h
#warning a\\b.h

$ echo '#include "foo/a\\b.h"' | clang -c -xc - -fms-compatibility -o /dev/null
In file included from <stdin>:1:
./foo/a/b.h:1:2: warning: a/b.h [-W#warnings]
    1 | #warning a/b.h
      |  ^
1 warning generated.
$ echo '#include "foo/a\\b.h"' | clang -xc - -fms-compatibility -M -MG
-.o: foo/a/b.h

$ echo '#include "foo/a\\b.h"' | clang -c -xc - -fno-ms-compatibility -o /dev/null
In file included from <stdin>:1:
./foo/a\b.h:1:2: warning: a\\b.h [-W#warnings]
    1 | #warning a\\b.h
      |  ^
1 warning generated.
$ echo '#include "foo/a\\b.h"' | clang -xc - -fno-ms-compatibility -M -MG
-.o: foo/a/b.h

See also:

If these PRs are accepted then I'll try to extract the logic into a new llvm/support helper class. These changes are presented as separate PRs because each of them change the behavior slightly differently.

github-actions · 2025-09-26T15:22:35Z

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llvmbot · 2025-09-26T15:23:09Z

@llvm/pr-subscribers-clang

Author: Ruoyu Zhong (ZhongRuoyu)

Changes

Currently, llvm::sys::path::native is called unconditionally when generating dependency filenames. This is correct on Windows, where backslashes are valid path separators, but can be incorrect on non-Windows platforms when the Clang invocation is not MS-compatible (-fno-ms-compatibility), because in that case backslashes in the filename are converted by llvm::sys::path::native to forward slashes, while they should be treated as ordinary characters.

This fixes the following inconsistency on non-Windows platforms (notice how the dependency output always converts the backslash to a forward slash, assuming it's a path separator, while the actual include directive treats it as an ordinary character when -fno-ms-compatibility is set):

$ tree
.
└── foo
    ├── a
    │   └── b.h
    └── a\b.h

3 directories, 2 files

$ cat foo/a/b.h
#warning a/b.h
$ cat foo/a\\b.h
#warning a\\b.h

$ echo '#include "foo/a\\b.h"' | clang -c -xc - -fms-compatibility -o /dev/null
In file included from &lt;stdin&gt;:1:
./foo/a/b.h:1:2: warning: a/b.h [-W#warnings]
    1 | #warning a/b.h
      |  ^
1 warning generated.
$ echo '#include "foo/a\\b.h"' | clang -xc - -fms-compatibility -M -MG
-.o: foo/a/b.h

$ echo '#include "foo/a\\b.h"' | clang -c -xc - -fno-ms-compatibility -o /dev/null
In file included from &lt;stdin&gt;:1:
./foo/a\b.h:1:2: warning: a\\b.h [-W#warnings]
    1 | #warning a\\b.h
      |  ^
1 warning generated.
$ echo '#include "foo/a\\b.h"' | clang -xc - -fno-ms-compatibility -M -MG
-.o: foo/a/b.h

See also: #160834.

Full diff: https://github.com/llvm/llvm-project/pull/160903.diff

3 Files Affected:

(modified) clang/include/clang/Frontend/Utils.h (+4)
(modified) clang/lib/Frontend/DependencyFile.cpp (+19-7)
(added) clang/test/Frontend/dependency-gen-posix-ms-compatible.c (+17)

diff --git a/clang/include/clang/Frontend/Utils.h b/clang/include/clang/Frontend/Utils.h
index f86c2f5074de0..6ffc200e61c23 100644
--- a/clang/include/clang/Frontend/Utils.h
+++ b/clang/include/clang/Frontend/Utils.h
@@ -118,6 +118,9 @@ class DependencyFileGenerator : public DependencyCollector {
   void outputDependencyFile(llvm::raw_ostream &OS);
 
 private:
+  void outputDependencyFilename(llvm::raw_ostream &OS,
+                                StringRef Filename) const;
+
   void outputDependencyFile(DiagnosticsEngine &Diags);
 
   std::string OutputFile;
@@ -128,6 +131,7 @@ class DependencyFileGenerator : public DependencyCollector {
   bool SeenMissingHeader;
   bool IncludeModuleFiles;
   DependencyOutputFormat OutputFormat;
+  bool MSCompatible;
   unsigned InputFileIndex;
 };
 
diff --git a/clang/lib/Frontend/DependencyFile.cpp b/clang/lib/Frontend/DependencyFile.cpp
index 15fa7de35df97..90d2918000806 100644
--- a/clang/lib/Frontend/DependencyFile.cpp
+++ b/clang/lib/Frontend/DependencyFile.cpp
@@ -223,7 +223,7 @@ DependencyFileGenerator::DependencyFileGenerator(
       PhonyTarget(Opts.UsePhonyTargets),
       AddMissingHeaderDeps(Opts.AddMissingHeaderDeps), SeenMissingHeader(false),
       IncludeModuleFiles(Opts.IncludeModuleFiles),
-      OutputFormat(Opts.OutputFormat), InputFileIndex(0) {
+      OutputFormat(Opts.OutputFormat), MSCompatible(false), InputFileIndex(0) {
   for (const auto &ExtraDep : Opts.ExtraDeps) {
     if (addDependency(ExtraDep.first))
       ++InputFileIndex;
@@ -235,6 +235,8 @@ void DependencyFileGenerator::attachToPreprocessor(Preprocessor &PP) {
   if (AddMissingHeaderDeps)
     PP.SetSuppressIncludeNotFoundError(true);
 
+  MSCompatible = PP.getLangOpts().MicrosoftExt;
+
   DependencyCollector::attachToPreprocessor(PP);
 }
 
@@ -312,11 +314,21 @@ void DependencyFileGenerator::finishedMainFile(DiagnosticsEngine &Diags) {
 /// https://msdn.microsoft.com/en-us/library/dd9y37ha.aspx for NMake info,
 /// https://msdn.microsoft.com/en-us/library/windows/desktop/aa365247(v=vs.85).aspx
 /// for Windows file-naming info.
-static void PrintFilename(raw_ostream &OS, StringRef Filename,
-                          DependencyOutputFormat OutputFormat) {
-  // Convert filename to platform native path
+///
+/// Whether backslashes in the filename are treated as path separators or
+/// ordinary characters depends on whether the Clang invocation is MS-compatible
+/// (-fms-compatibility). If it is, backslashes are treated as path separators,
+/// and are converted to the native path separator (backslash on Windows,
+/// forward slash on other platforms); if not, backslashes are treated as
+/// ordinary characters, and are not converted.
+void DependencyFileGenerator::outputDependencyFilename(
+    raw_ostream &OS, StringRef Filename) const {
   llvm::SmallString<256> NativePath;
-  llvm::sys::path::native(Filename.str(), NativePath);
+  if (MSCompatible) {
+    llvm::sys::path::native(Filename.str(), NativePath);
+  } else {
+    NativePath = Filename;
+  }
 
   if (OutputFormat == DependencyOutputFormat::NMake) {
     // Add quotes if needed. These are the characters listed as "special" to
@@ -400,7 +412,7 @@ void DependencyFileGenerator::outputDependencyFile(llvm::raw_ostream &OS) {
       Columns = 2;
     }
     OS << ' ';
-    PrintFilename(OS, File, OutputFormat);
+    outputDependencyFilename(OS, File);
     Columns += N + 1;
   }
   OS << '\n';
@@ -411,7 +423,7 @@ void DependencyFileGenerator::outputDependencyFile(llvm::raw_ostream &OS) {
     for (auto I = Files.begin(), E = Files.end(); I != E; ++I) {
       if (Index++ == InputFileIndex)
         continue;
-      PrintFilename(OS, *I, OutputFormat);
+      outputDependencyFilename(OS, *I);
       OS << ":\n";
     }
   }
diff --git a/clang/test/Frontend/dependency-gen-posix-ms-compatible.c b/clang/test/Frontend/dependency-gen-posix-ms-compatible.c
new file mode 100644
index 0000000000000..208240b03a844
--- /dev/null
+++ b/clang/test/Frontend/dependency-gen-posix-ms-compatible.c
@@ -0,0 +1,17 @@
+// REQUIRES: !system-windows
+// RUN: rm -rf %t.dir
+// RUN: mkdir -p %t.dir/include/foo
+// RUN: echo > %t.dir/include/foo/bar.h
+// RUN: echo > %t.dir/include/foo\\bar.h
+
+// RUN: %clang -MD -MF - %s -fsyntax-only -fms-compatibility -I %t.dir/include | FileCheck -check-prefix=CHECK-ONE %s
+// CHECK-ONE: foo/bar.h
+// CHECK-ONE-NOT: foo\bar.h
+// CHECK-ONE-NOT: foo\\bar.h
+
+// RUN: %clang -MD -MF - %s -fsyntax-only -fno-ms-compatibility -I %t.dir/include | FileCheck -check-prefix=CHECK-TWO %s
+// CHECK-TWO: foo\bar.h
+// CHECK-TWO-NOT: foo/bar.h
+// CHECK-TWO-NOT: foo\\bar.h
+
+#include "foo\bar.h"

…tible Currently, llvm::sys::path::native is called unconditionally when generating dependency filenames. This is correct on Windows, where backslashes are valid path separators, but can be incorrect on non-Windows platforms when the Clang invocation is not MS-compatible (-fno-ms-compatibility), because in that case backslashes in the filename are converted by llvm::sys::path::native to forward slashes, while they should be treated as ordinary characters. This fixes the following inconsistency on non-Windows platforms (notice how the dependency output always converts the backslash to a forward slash, assuming it's a path separator, while the actual include directive treats it as an ordinary character when -fno-ms-compatibility is set): $ tree . └── foo ├── a │ └── b.h └── a\b.h 3 directories, 2 files $ cat foo/a/b.h #warning a/b.h $ cat foo/a\\b.h #warning a\\b.h $ echo '#include "foo/a\\b.h"' | clang -c -xc - -fms-compatibility -o /dev/null In file included from <stdin>:1: ./foo/a/b.h:1:2: warning: a/b.h [-W#warnings] 1 | #warning a/b.h | ^ 1 warning generated. $ echo '#include "foo/a\\b.h"' | clang -xc - -fms-compatibility -M -MG -.o: foo/a/b.h $ echo '#include "foo/a\\b.h"' | clang -c -xc - -fno-ms-compatibility -o /dev/null In file included from <stdin>:1: ./foo/a\b.h:1:2: warning: a\\b.h [-W#warnings] 1 | #warning a\\b.h | ^ 1 warning generated. $ echo '#include "foo/a\\b.h"' | clang -xc - -fno-ms-compatibility -M -MG -.o: foo/a/b.h Signed-off-by: Ruoyu Zhong <[email protected]>

llvmbot added the clang Clang issues not falling into any other category label Sep 26, 2025

This was referenced Sep 26, 2025

[lld][ELF] Only convert dependency filename to native form on Windows #160927

Open

[TableGen] correctly escape dependency filenames #160834

Open

ZhongRuoyu force-pushed the clang-dep-filename-ms-compatibility branch from 8159427 to 181c4d6 Compare October 19, 2025 18:33

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