Force GHashCell to be 8-byte-aligned.

[efriedma]

Summary

Otherwise, with recent versions of libstdc++, clang can't tell that the atomic operations are properly aligned, and generates calls to libatomic.

Fixes https://github.com/llvm/llvm-project/issues/54790 , at least for that specific case.

[jyknight]

Sidenote: Oh, yikes! I didn't even notice before that this code is doing reinterpret_cast from a GHashCell* to a std::atomic<GHashCell>*. That's...kinda not great...

[MaskRay]

Seems worth mentioning auto *cellPtr = reinterpret_cast<std::atomic<GHashCell> *>(&table[idx]);

[MaskRay]

I know nearly nothing about COFF debugging. I assume there is better way avoiding the reinterpret_cast but this patch as-is is safe for 14.x backport.

[rnk]

lgtm

In D123872#3454636, @jyknight wrote:

Sidenote: Oh, yikes! I didn't even notice before that this code is doing reinterpret_cast from a GHashCell* to a std::atomic<GHashCell>*. That's...kinda not great...

Yeah, I think I was excited with the performance results, and shipped some prototype code. The idea is that we only need to do atomic operations during hash table insertion. Afterwards, the table is used as readonly data. I wouldn't be surprised if there are issues lurking here for ISAs with weak memory models.

lld/COFF/DebugTypes.cpp

struct GHashTable {
   /// position in the table. It is safe for the caller to store the insertion
   /// position because the table cannot be resized.
   uint32_t insert(COFFLinkerContext &ctx, GloballyHashedType ghash,
                   GHashCell newCell);
 };
 
 /// A ghash table cell for deduplicating types from TpiSources.
 class GHashCell {
-  uint64_t data = 0;
+  // Force "data" to be 64-bit aligned; otherwise, some versions of clang
+  // will generate calls to libatomic when using some versions of libstdc++
+  // on 32-bit targets.  (Also, in theory, there could be a target where
+  // new[] doesn't always return an 8-byte-aligned allocation.)
+  alignas(sizeof(uint64_t)) uint64_t data = 0;
 
 public:
   GHashCell() = default;
 
   // Construct data most to least significant so that sorting works well:
   // - isItem
   // - tpiSrcIdx
   // - ghashIdx