"Maciej W. Rozycki" <macro@xxxxxxxxxxxxxx> 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 > then? > >> 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. Agreed. > 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. -- Måns Rullgård mans@xxxxxxxxx
