[demangler] Ensure proper alignment in BumpPointerAllocator

[sepavloff]

Summary

This change slightly modifies algorithm of BumpPointerAllocator so that it
provides alignment of allocated memory blocks in runtime without specific
compiler support. It allows getting rid of the specifier alignas(16) in the
declaration of BumpPointerAllocator::InitialBuffer. Object of the class
BumpPointerAllocator is created as a part of another object allocated in heap,
so this specifier actually cannot guarantee proper alignment. Microsoft
compiler emits a warning in this case.

It fixes https://bugs.llvm.org/show_bug.cgi?id=37944.

[erik.pilkington]

Hi, thanks for working on this!

We don't need 16-byte alignment on MSVC, we only need 8 because alignof(max_align_t) is 8 on that platform, and malloc is required to provide memory that's aligned to that boundary. We can't portable use max_align_t in LLVM (see the commit message in r336162), but I think we can get away with using alignof(long double). Have you considered this option instead of fixing up the alignment dynamically?

[sepavloff]

We don't need 16-byte alignment on MSVC, we only need 8 because alignof(max_align_t) is 8 on that platform, and malloc is required to provide memory that's aligned to that boundary.

If allignment provided by malloc is sufficient, there is no need in runtime alignment. and alignas(alignof(long double)) should be OK.

[sepavloff]

Updated patch

[erik.pilkington]

LGTM, thanks again! Please also commit this to the version of this file in libcxxabi/src/cxa_demangle.cpp.

lib/Demangle/ItaniumDemangle.cpp

1953 ↗

(On Diff #154108)

This can be spelt alignas(long double).

[sepavloff]

lib/Demangle/ItaniumDemangle.cpp

1953 ↗

(On Diff #154108)

Thanks!

llvm/trunk/lib/Demangle/ItaniumDemangle.cpp

class BumpPointerAllocator {
   struct BlockMeta {
     BlockMeta* Next;
     size_t Current;
   };
 
   static constexpr size_t AllocSize = 4096;
   static constexpr size_t UsableAllocSize = AllocSize - sizeof(BlockMeta);
 
-  alignas(16) char InitialBuffer[AllocSize];
+  alignas(long double) char InitialBuffer[AllocSize];
   BlockMeta* BlockList = nullptr;
 
   void grow() {
     char* NewMeta = new char[AllocSize];
     BlockList = new (NewMeta) BlockMeta{BlockList, 0};
   }
 
   void* allocateMassive(size_t NBytes) {