[llvm-dev] RFC: Support for preferring paths with forward slashes on Windows

Chris Tetreault via llvm-dev
Thu, 14 Oct 2021 16:43:14 -0000

I could be mistaken, but I believe that since the dawn of time, Windows has just
secretly supported forward slashes. A quick google search does not turn up any
Microsoft docs stating that this is true, but I've heard rumors that
it's been this way since DOS. On my Windows 10 machine, Powershell accepts
/, cmd.exe accepts /, and Visual Studio accepts /. Whomever takes it upon
themselves to work on this should test extensively before committing code. I
would probably feel better if somebody could dig up some authoritative source on
this.

Assuming that this is the case, it would probably be nice if any paths we take
in were just immediately canonicalized to use / and all paths just have forward
slash. I know we have a ton of tests that have this `{(/|\\)}` regex in them,
and it would be nice if we could just not do that.

Thanks,
	Chris Tetreault

-----Original Message-----
From: llvm-dev <llvm-dev-bounces at lists.llvm.org> On Behalf Of Martin
Storsjö via llvm-dev
Sent: Thursday, October 14, 2021 5:22 AM
To: llvm-dev at lists.llvm.org
Cc: mati865 at gmail.com; github at jdrake.com
Subject: [llvm-dev] RFC: Support for preferring paths with forward slashes on
Windows

WARNING: This email originated from outside of Qualcomm. Please be wary of any
links or attachments, and do not enable macros.

Hi,

When using Clang on Windows as a drop-in replacement for GCC, one issue that
crops up fairly soon is that not all callers can tolerate paths spelled out with
backslashes.

This is an issue when e.g. libtool parses the output of "$CC -v"
(where clang passes an absolute path to compiler-rt libraries) and uses parts of
that in shell script contexts that don't tolerate backslashes, when some
callers call "$CC --print-search-dirs", etc.

This is also one of the most important things that MSYS2 patches in their
distribution of Clang/LLVM according to their patch tracker [1].

(I've locally worked around this in my distribution without patching, by
filtering clang's stdout in a wrapper, when options like "-v" or
"--print-search-dirs" are detected, but that's essentially the
same as
patching.)

I've finally taken the plunge and tried to implement this properly. I've
got a decent patch set [2] that I could start sending for review, but before
doing that, I'd like to discuss the overall design.


The main idea is that I add a third alternative to path::Style - in addition to
the existing Windows and Posix path styles, I'm adding Windows_forward,
which otherwise parses and handles Windows paths like before (i.e. accepting and
interpreting both separators), but with a different preferred separator (as
returned by get_separator()).

This allows any code on any platform to handle paths in all three forms, just
like in the existing design, when explicitly giving a path::Style argument.

To actually make it have effect, one can make path::Style::native act like
Windows_forward instead of plain Windows. I'm not entirely sure what the
best strategy is for when to do that - one could do it when LLVM itself was
built for a MinGW target (which kind of breaks the assumption that the tools
work pretty much the same as long as one passes the right --target options etc),
or one could maybe set it up as a configure time CMake option? Or even make it a
globally settable option in the process, to allow changing it e.g. depending on
the tool's target configuration?

I also faintly remember that Reid at some point implied that it could be an
option to switch all Windows builds outright to such a behaviour?

Most of the code is entirely independent of the policy decision of when/where to
enable the behaviour - the decision is centralised to one single spot in
LLVMSupport.

In any case, with this design and a quite moderate amount of fixups, most of the
tests in check-all seem to pass, if switching the preference.

There's a couple tests that fail due to checking e.g. the literal paths %s
or %t (as output by llvm-lit, with backslashes) against paths that the tools
output. There's also a dozen or so of tests in Clang (mainly regarding PCH)
that seem to misbehave when the same paths are referred to with varying kinds of
slashes, e.g. stored with a forward slash in the PCH but referred to with
backslashes in arguments to Clang, where paths are essentially equal but the
strings differ. (For actual use with PCH, Clang built this way seems to work -
and MSYS2 have been running with tools patched this way for quite some time, and
I haven't heard about reports about bugs relating to that patch.)

If the design seems sane (have a look at [2] if you want to have a look at my
whole series at the moment) I'd start sending the initial patches for
review.

// Martin

[1]
https://github.com/msys2/MINGW-packages/blob/master/mingw-w64-clang/README-patches.md

[2] https://github.com/llvm/llvm-project/compare/main...mstorsjo:path-separator