[lld] [lld][ELF] Only convert dependdency filename to native form on Windows (PR #160927)

[Ruoyu Zhong via llvm-commits]

https://github.com/ZhongRuoyu created https://github.com/llvm/llvm-project/pull/160927

Currently, `llvm::sys::path::native` is used unconditionally when generating dependency filenames. This is correct on Windows, where backslashes are valid path separators, but can be incorrect on non-Windows platforms, because in that case backslashes in the filenames are converted to forward slashes, while they should be treated as ordinary characters.

This fixes the following inconsistency between `ld.lld` and `ld.bfd`:

```console
$ cat test.s
.globl _start
_start:
$ cc -c test.s -o "back\\slash.o"
$ ld.lld -o test "back\\slash.o" --dependency-file=/dev/stdout
test: \
 back/slash.o

back/slash.o:
$ ld.bfd -o test "back\\slash.o" --dependency-file=/dev/stdout
test: \
  back\slash.o

back\slash.o:
```

In addition, while we're here, use 2-space indentation when printing out dependency filenames (consistent with Clang and `ld.bfd`), and escape the target filename too.


>From 93d3cb431c4165164ea26caefb146f78dd2088e8 Mon Sep 17 00:00:00 2001
From: Ruoyu Zhong <zhongruoyu at outlook.com>
Date: Sat, 27 Sep 2025 00:52:40 +0800
Subject: [PATCH] [lld][ELF] Only convert dependdency filename to native form
 on Windows

Currently, llvm::sys::path::native is used unconditionally when
generating dependency filenames. This is correct on Windows, where
backslashes are valid path separators, but can be incorrect on
non-Windows platforms, because in that case backslashes in the
filenames are converted to forward slashes, while they should be treated
as ordinary characters.

This fixes the following inconsistency between ld.lld and ld.bfd:

    $ cat test.s
    .globl _start
    _start:
    $ cc -c test.s -o "back\\slash.o"
    $ ld.lld -o test "back\\slash.o" --dependency-file=/dev/stdout
    test: \
     back/slash.o

    back/slash.o:
    $ ld.bfd -o test "back\\slash.o" --dependency-file=/dev/stdout
    test: \
      back\slash.o

    back\slash.o:

In addition, while we're here, use 2-space indentation when printing out
dependency filenames (consistent with Clang and ld.bfd), and escape the
filename of the target (output file) too.

Signed-off-by: Ruoyu Zhong <zhongruoyu at outlook.com>
---
 lld/ELF/Driver.cpp                                  | 11 ++++++++---
 .../ELF/dependency-file-backslash-as-normal-char.s  | 13 +++++++++++++
 .../dependency-file-backslash-as-path-separator.s   | 13 +++++++++++++
 3 files changed, 34 insertions(+), 3 deletions(-)
 create mode 100644 lld/test/ELF/dependency-file-backslash-as-normal-char.s
 create mode 100644 lld/test/ELF/dependency-file-backslash-as-path-separator.s

diff --git a/lld/ELF/Driver.cpp b/lld/ELF/Driver.cpp
index 1beab8d33f4ba..8483e01364982 100644
--- a/lld/ELF/Driver.cpp
+++ b/lld/ELF/Driver.cpp
@@ -2482,10 +2482,14 @@ static void writeDependencyFile(Ctx &ctx) {
   // We use the same escape rules as Clang/GCC which are accepted by Make/Ninja:
   // * A space is escaped by a backslash which itself must be escaped.
   // * A hash sign is escaped by a single backslash.
-  // * $ is escapes as $$.
+  // * $ is escaped as $$.
   auto printFilename = [](raw_fd_ostream &os, StringRef filename) {
     llvm::SmallString<256> nativePath;
+#ifdef _WIN32
     llvm::sys::path::native(filename.str(), nativePath);
+#else
+    nativePath = filename;
+#endif
     llvm::sys::path::remove_dots(nativePath, /*remove_dot_dot=*/true);
     for (unsigned i = 0, e = nativePath.size(); i != e; ++i) {
       if (nativePath[i] == '#') {
@@ -2502,9 +2506,10 @@ static void writeDependencyFile(Ctx &ctx) {
     }
   };
 
-  os << ctx.arg.outputFile << ":";
+  printFilename(os, ctx.arg.outputFile);
+  os << ":";
   for (StringRef path : ctx.arg.dependencyFiles) {
-    os << " \\\n ";
+    os << " \\\n  ";
     printFilename(os, path);
   }
   os << "\n";
diff --git a/lld/test/ELF/dependency-file-backslash-as-normal-char.s b/lld/test/ELF/dependency-file-backslash-as-normal-char.s
new file mode 100644
index 0000000000000..278e06d41d683
--- /dev/null
+++ b/lld/test/ELF/dependency-file-backslash-as-normal-char.s
@@ -0,0 +1,13 @@
+# REQUIRES: x86, !system-windows
+# RUN: mkdir -p %t
+# RUN: llvm-mc -filetype=obj -triple=x86_64 %s -o "%t/back\\slash.o"
+# RUN: ld.lld -o %t/foo.exe "%t/back\\slash.o" --dependency-file=%t/foo.d
+# RUN: FileCheck --match-full-lines -DFILE=%t %s < %t/foo.d
+
+# CHECK:      [[FILE]]/foo.exe: \
+# CHECK-NEXT:   [[FILE]]/back\slash.o
+# CHECK-EMPTY:
+# CHECK-NEXT: [[FILE]]/back\slash.o:
+
+.global _start
+_start:
diff --git a/lld/test/ELF/dependency-file-backslash-as-path-separator.s b/lld/test/ELF/dependency-file-backslash-as-path-separator.s
new file mode 100644
index 0000000000000..6e738626c4127
--- /dev/null
+++ b/lld/test/ELF/dependency-file-backslash-as-path-separator.s
@@ -0,0 +1,13 @@
+# REQUIRES: x86, system-windows
+# RUN: mkdir -p %t/back
+# RUN: llvm-mc -filetype=obj -triple=x86_64 %s -o "%t/back\\slash.o"
+# RUN: ld.lld -o %t/foo.exe "%t/back\\slash.o" --dependency-file=%t/foo.d
+# RUN: FileCheck --match-full-lines -DFILE=%t %s < %t/foo.d
+
+# CHECK:      [[FILE]]\foo.exe: \
+# CHECK-NEXT:   [[FILE]]\back\slash.o
+# CHECK-EMPTY:
+# CHECK-NEXT: [[FILE]]\back\slash.o:
+
+.global _start
+_start: