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

[mstorsjo]

Summary

Later we might want to make it settable at runtime - but even then,
we want to be able to set what the built-in default should be.

Default to preferring forward slashes when built for MinGW.

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

[mstorsjo]

This is the last step needed to actually take the forward slash form of paths into use. There's still a number of test failures if built in that mode (I'll send more patches to try to remedy that afterwards).

@rnk 's desire was to be able to change it at runtime - I'm not currently planning on taking this feature that far, but this is at least one step along the way to that goal (even with runtime options, one can want to configure the default). If changing the preference at runtime, one has to redo parts of what D111880 does (running make_preferred() on a number of paths) after setting the new preference (e.g. when Argv[0] has been set early, based on the built-in preferred format, one would have to update that and all other paths derived from it).

[aaron.ballman]

llvm/CMakeLists.txt

363

Should this also happen for cygwin?

[mstorsjo]

llvm/CMakeLists.txt

363

I think cygwin shouldn't matter here - cygwin mostly presents itself as a unix; it doesn't define _WIN32 when compiling etc, so on cygwin you'd get the posix path style.

[aaron.ballman]

llvm/CMakeLists.txt

363

Ooohh! So because _WIN32 isn't defined, we will prefer forward slashes already? (What confused me is this looks like it prefers backslash on *all* Windows targets except mingw and it seems like we want the same for cygwin.)

[mstorsjo]

llvm/CMakeLists.txt

363

Yes - but not Windows-style paths with forward slashes - it gets plain posix paths only.

As cygwin doesn't define _WIN32 and thus doesn't present itself as Windows at all, LLVM built in cygwin thinks it's executing on a regular unix, so path::Style::native does evaluate to path::Style::posix, and it's executing code from llvm/lib/Support/Unix instead of llvm/lib/Support/Windows, etc.

So to avoid implying that it has an effect, I wouldn't add it to the if condition here, but I guess it would warrant a comment. Maybe something along these lines:

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()

[aaron.ballman]

LGTM modulo the comment request.

llvm/CMakeLists.txt

363

Thank you for the explanation! I think that comment would help others reading the CMake without that context in mind.

[mstorsjo]

Thanks for the review! Amended the patch with the comment as discussed; will land the series of these three patches tomorrow.

llvm/CMakeLists.txt

 
 option(LLVM_ENABLE_CRASH_OVERRIDES "Enable crash overrides." ON)
 if(LLVM_ENABLE_CRASH_OVERRIDES)
   set(ENABLE_CRASH_OVERRIDES 1)
 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")
 
 set(LLVM_TARGET_ARCH "host"
   CACHE STRING "Set target to use for LLVM JIT or use \"host\" for automatic detection.")
 
 option(LLVM_ENABLE_TERMINFO "Use terminfo database if available." ON)

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

 #cmakedefine01 ENABLE_BACKTRACES
 
 /* Define to 1 to enable crash overrides, and to 0 otherwise. */
 #cmakedefine01 ENABLE_CRASH_OVERRIDES
 
 /* 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}
 
 #define BACKTRACE_HEADER <${BACKTRACE_HEADER}>
 
 /* Define to 1 if you have the <CrashReporterClient.h> header file. */
 #cmakedefine HAVE_CRASHREPORTERCLIENT_H
 

llvm/lib/Support/Path.cpp

 //===-- Path.cpp - Implement OS Path Concept ------------------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 //
 //  This file implements the operating system Path API.
 //
 //===----------------------------------------------------------------------===//
 
 #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"
 #include "llvm/Support/ErrorHandling.h"
 #include "llvm/Support/FileSystem.h"
 #include "llvm/Support/Process.h"
 #include "llvm/Support/Signals.h"
 #include <cctype>
namespace {
   using llvm::sys::path::is_separator;
   using llvm::sys::path::Style;
 
   inline Style real_style(Style style) {
     if (style != Style::native)
       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) {
     if (is_style_windows(style))
       return "\\/";
     return "/";
   }