[PATCH] D124524: [demangler] Avoid nullptr UB

[Nathan Sidwell via Phabricator via llvm-commits]

urnathan added a comment.

In D124524#3477537 <https://reviews.llvm.org/D124524#3477537>, @dblaikie wrote:

> Hmm - why does this need an allocation, though? A `StringPiece(nullptr, nullptr)` sounds valid to me? Is something using a distinction between StringPiece that's zero-length but non-null and StringPiece that's null?

The UB is that we end up at `memcpy (dst, SV.begin(), SV.size())`, even though that's copying zero bytes, we have a null SV.begin().  C++20's string_view cannot be over a null pointer, and ISTM that the demangler's StringView (should) have the same restriction.  StringView is a pair of pointers, along with a FIXME to move to std::string_view when C++17 is the implementation language.  Hm, perhaps the assert belongs inside StringView?

This seemed the least-worst way of addressing it.  Alternatively I guess we could unconditionally allocate in OutputBuffer's ctor, but that seemed less pleasant.


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