"Maciej W. Rozycki" <email@example.com> writes:
> On Thu, 5 Feb 2015, Toma Tabacu wrote:
>> > 2. It considers these character pairs to be unicode escapes in the first
>> > place given that they do not follow the syntax required for such
>> > escapes, that is `\unnnn', where `n' are hex digits.
>> It doesn't actually treat them as unicode escapes, but it still warns
>> the user, in case they were meant to be unicode escapes. Here's the
>> warning message:
>> arch/mips/include/asm/asmmacro.h:197:51: warning: \u used with no following
>> hex digits; treating as '\' followed by identifier [-Wunicode]
>> .word 0x41000000 | (\rt << 16) | (\rd << 11) | (\u << 5) | (\sel)
>> I'll add it to the summary in v2.
> Thanks, that makes things clearer. It always makes sense to include the
> exact error message produced where applicable or otherwise people do not
> necessarily know what the matter is.
>> > Of course it may be reasonable for us to work this bug around as we've
>> > been doing for years with GCC, but has the issue been reported back to
>> > clang maintainers? What was their response?
>> It hasn't been reported, but I don't think they would agree with removing
>> unicode escape sequences from the assembler-with-cpp mode because it is
>> currently being used for other languages as well, not just assembly.
> First, preprocessing rules surely have to be language specific. The C
> language standard does not specify what the preprocessor is meant to do
> (if anything) for other languages. GCC or clang -- that's no different.
> The assembly language has a different syntax and `\u' has a different
> meaning in the context of assembly macro expansion than it would have in a
> name of a symbol, where such a Unicode escape sequence might indeed be
> interpreted as such and character encoded propagated to the symbol
> produced. But that's up to the assembler -- GAS for example does not
> AFAIK support Unicode escape sequences in symbol names right now, but I
> suppose such a feature could be added if desired.
> Which prompts another question of course: how does the clang C compiler
> represent Unicode characters in identifiers in its assembly output?
> I have looked into the C language standard and it appears to me like the
> translation phase to interpret universal character names at has not been
> defined. This is probably why the standard does specify the result of
> pasting preprocessor tokens together as undefined if a universal character
> name is produced this way.
That is my interpretation as well.
> Consequently I think an important question in this context is: does
> clang's preprocessor actually convert these sequences anyhow before
> passing them down to the compiler? How for example does C output from a
> trivial example that contains such Unicode escape sequences look like
>> One such language is Haskell (ghc, to be more specific), for which
>> the clang developers had to actually stop the preprocessor from
>> enforcing the C universal character name restrictions in
>> assembler-with-cpp mode, which suggests that ghc wants the
>> preprocessor to check for unicode escape sequences.
>> At the moment, we can either disable -Wunicode for asmmacro.h or
>> refrain from using '\u' as an identifier.
> To be clear: it's `u' here that is the identifier, the leading `\' is
> merely how assembly syntax has been specified for references to macro
> arguments. And TBH I find banning any macro arguments starting with `u'
> rather silly.
> I'm leaning towards considering having -Wunicode disabled for all
> assembly sources, or maybe even for the whole Linux compilation, the
> right solution. It's not like we have a need for Unicode identifiers.
It might be an idea to disable -Wunicode and have checkpatch warn about
Unicode escapes instead if people are worried about this. Personally, I
doubt there's much cause for concern here.