[Support] Allow configuring the preferred type of slashes on Windows

Default to preferring forward slashes when built for MinGW, as
many usecases, when e.g. Clang is used as a drop-in replacement
for GCC, requires the compiler to output paths with forward slashes.

Not all tests pass yet, if configuring to prefer forward slashes though.

Differential Revision: https://reviews.llvm.org/D112787

Author: mstorsjo

llvm/CMakeLists.txt

@@ -359,6 +359,14 @@ endif()
 
 option(LLVM_ENABLE_CRASH_DUMPS "Turn on memory dumps on crashes. Currently only implemented on Windows." OFF)
 
+set(WINDOWS_PREFER_FORWARD_SLASH_DEFAULT OFF)
+if (MINGW)
+  # Cygwin doesn't identify itself as Windows, and thus gets path::Style::posix
+  # as native path style, regardless of what this is set to.
+  set(WINDOWS_PREFER_FORWARD_SLASH_DEFAULT ON)
+endif()
+option(LLVM_WINDOWS_PREFER_FORWARD_SLASH "Prefer path names with forward slashes on Windows." ${WINDOWS_PREFER_FORWARD_SLASH_DEFAULT})
+
 option(LLVM_ENABLE_FFI "Use libffi to call external functions from the interpreter" OFF)
 set(FFI_LIBRARY_DIR "" CACHE PATH "Additional directory, where CMake should search for libffi.so")
 set(FFI_INCLUDE_DIR "" CACHE PATH "Additional directory, where CMake should search for ffi.h or ffi/ffi.h")

llvm/include/llvm/Config/config.h.cmake

@@ -19,6 +19,10 @@
 /* Define to 1 to enable crash memory dumps, and to 0 otherwise. */
 #cmakedefine01 LLVM_ENABLE_CRASH_DUMPS
 
+/* Define to 1 to prefer forward slashes on Windows, and to 0 prefer
+   backslashes. */
+#cmakedefine01 LLVM_WINDOWS_PREFER_FORWARD_SLASH
+
 /* Define to 1 if you have the `backtrace' function. */
 #cmakedefine HAVE_BACKTRACE ${HAVE_BACKTRACE}
 

llvm/lib/Support/Path.cpp

@@ -12,6 +12,7 @@
 
 #include "llvm/Support/Path.h"
 #include "llvm/ADT/ArrayRef.h"
+#include "llvm/Config/config.h"
 #include "llvm/Config/llvm-config.h"
 #include "llvm/Support/Endian.h"
 #include "llvm/Support/Errc.h"
@@ -41,7 +42,8 @@ namespace {
       return style;
     if (is_style_posix(style))
       return Style::posix;
-    return Style::windows;
+    return LLVM_WINDOWS_PREFER_FORWARD_SLASH ? Style::windows_slash
+                                             : Style::windows_backslash;
   }
 
   inline const char *separators(Style style) {