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

[Ruoyu Zhong via cfe-commits]

https://github.com/ZhongRuoyu updated https://github.com/llvm/llvm-project/pull/160903

>From 181c4d65060405cdba484c18ae09b3fe1255414d Mon Sep 17 00:00:00 2001
From: Ruoyu Zhong <zhongruoyu at outlook.com>
Date: Fri, 26 Sep 2025 23:17:32 +0800
Subject: [PATCH] [Clang] only convert dependency filename to native form when
 MS-compatible
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

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 <zhongruoyu at outlook.com>
---
 clang/include/clang/Frontend/Utils.h          |  4 +++
 clang/lib/Frontend/DependencyFile.cpp         | 25 +++++++++++++------
 .../dependency-gen-posix-ms-compatible.c      | 17 +++++++++++++
 3 files changed, 39 insertions(+), 7 deletions(-)
 create mode 100644 clang/test/Frontend/dependency-gen-posix-ms-compatible.c

diff --git a/clang/include/clang/Frontend/Utils.h b/clang/include/clang/Frontend/Utils.h
index 49fd920d1ec43..8fe63d7c2141b 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..fc5b7949afa22 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 +314,20 @@ 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 +411,7 @@ void DependencyFileGenerator::outputDependencyFile(llvm::raw_ostream &OS) {
       Columns = 2;
     }
     OS << ' ';
-    PrintFilename(OS, File, OutputFormat);
+    outputDependencyFilename(OS, File);
     Columns += N + 1;
   }
   OS << '\n';
@@ -411,7 +422,7 @@ 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"