[libcxx-commits] [PATCH] D77924: ld128 demangle: allow space for 'L' suffix.

[Eli Friedman via Phabricator via libcxx-commits]

efriedma added a comment.

> My understanding is that floating point literals are encoded using the bytes of their internal representation (https://itanium-cxx-abi.github.io/cxx-abi/abi.html#mangling.literal), and we don't know what the encoding is from the mangled name, all we can know is the builtin C++ type and the size of the encoding in bytes. We could potentially try to use that as a heuristic (e.g. 10 bytes probably means x87), but I'm not sure if that would always give the right answer (?)

We could probably decode 4, 8, and 16 bytes literals as IEEE floats, and 10 bytes as x86 long double, and be right 99% of the time.  The only remotely realistic failure mode is that there's an alternative 16-byte float type on certain PowerPC variants.

Alternatively, we could skip decoding and dump the raw bytes; it's rare for a mangled name to contain a float literal in the first place, so most people probably won't notice.

> and it would mean that we'd have to write our own floating-point printers for other representations in libcxxabi, which sounds like a pain. WDYT?

LLVM's APFloat has the necessary code.  I guess you can't use that directly from libcxxabi, though?

> (Regardless though, I think we should land this crash-fix.)

Yes.  And maybe also change the code to use snprintf, so we never end up with a buffer overflow.


Repository:
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  https://reviews.llvm.org/D77924/new/

https://reviews.llvm.org/D77924