llvm dev - Oct 2021 - RFC: Support for preferring paths with forward slashes on Windows

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

Michael

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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On Fri, 15 Oct 2021, Michael Kruse wrote:
> 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?
I wasn't aware that cmd.exe had an escape char per se (other than ^ for 
line continuations?).

The fact that such slashes are printed double is, iirc, an intentional 
quirk, so that the command lines are copypasteable in a variety of 
contexts: cmd.exe don't need them doubled (but tolerates them), bash 
unescapes them so it also can execute them.

This is actually one downside of using forward slashes, as cmd.exe 
wouldn't be able to execute such a command straight out (only the slash 
direction of the command executable itself matters though).

// Martin