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

[David Blaikie via Phabricator via cfe-commits]

dblaikie added a comment.

In D81865#2178542 <https://reviews.llvm.org/D81865#2178542>, @froydnj wrote:

> 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"?)

I /believe/ it is applicable, though I could be wrong.

Ah, cppreference's example supports that theory at least: `S s = {0x12345678}; // initializes the first member, s.n is now the active member` - that the initialization itself does set the active member of the union.

(oh, and a later example on cppreference, which is lifted from the C++ spec (though not sure exactly which version) says similarly: "Y y = { { 1, 2 } }; // OK, y.x is active union member (9.2)")

> 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?

I think so? I guess that's essentially the point of offsetof.

The comment about padding probably isn't needed, I think? Even if there was padding, the StringOffset comes from "offsetof" so it describes the offset including any padding involved?

Be great if @rsmith got a chance to weigh in here.


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