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

Michael Kruse via llvm-dev llvm-dev at lists.llvm.org
Fri Oct 15 07:16:21 PDT 2021

Thanks for working on this.

I also noticed is that the paths printed by clang -v or -### are
escaping the backslashes and put them into quotes, i.e.
"C:\\path\\to\\clang.exe" -cc1 "..\\special'^`character .c"
Interestingly, it still works copy&pasting it to the Windows command
line [2], but cmd.exe's escape character is ^ and PowerShell's is the
backtick `. What would the correct output be?

[1] https://stackoverflow.com/questions/33027024/documented-behavior-for-multiple-backslashes-in-windows-paths


Am Do., 14. Okt. 2021 um 07:22 Uhr schrieb Martin Storsjö via llvm-dev
<llvm-dev at lists.llvm.org>:
> 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
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