[PATCH] D81865: [clang] Use string tables for static diagnostic descriptions

[Nathan Froyd via Phabricator via cfe-commits]

froydnj added a comment.

In D81865#2176589 <https://reviews.llvm.org/D81865#2176589>, @dblaikie wrote:

> I believe this falls under the (using cppreference ( https://en.cppreference.com/w/cpp/language/union ) , since it's a bit easier to read) UB clause: " It's undefined behavior to read from the member of the union that wasn't most recently written. Many compilers implement, as a non-standard language extension, the ability to read inactive members of a union."
>
> Last member written to was the "StringTable" member, but then it's read from the "String" member, so that'd be UB. Commonly supported, but UB - not sure if we have a general statement that we depend on this behavior in LLVM, even though it's non-standard (but it's possible that we do make such an assumption about the compiler that's building LLVM). It'd be nice to avoid that, though - and not too difficult, I think - I /believe/ it's valid to take a pointer to an object, cast it to char*, compute a pointer to some specific member and then cast it back to the right type and access. But I could be wrong there. @rsmith would be the person to give an authoritative answer.

Thanks for the explanation.  Is the language of "writing" applicable here, given that this is a constant blob of storage?  (I suppose the compiler is permitted to designate a particular member as having been "written"?)

Would it be more palatable to write:

  struct StaticDiagInfoDescriptionStringTable {
    // members as char[] for each diagnostic
  };
  
  const StaticDiagInfoDescriptionStringTable StaticDiagInfoDescriptions = {
    // define all the members
  };
  
  ...
  
  struct StaticDiagInfoRec {
    ...
    StringRef getDescription() const {
      size_t MyIndex = this - &StaticDiagInfo[0];
      uint32_t StringOffset = StaticDiagInfoDescriptionOffsets[MyIndex];
      // Defined as a pointer to the first member, and (presumably) there is no internal padding.
      const char *StringTable = reinterpret_cast<const char*>(&StaticDiagInfoDescriptions);
      return StringRef(&StringTable[StringOffset], DescriptionLen);
  };

and then we don't have to care about how the host compiler interprets access to different members of unions?


Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D81865/new/

https://reviews.llvm.org/D81865