[lld] 13fc178 - Force GHashCell to be 8-byte-aligned.

[Eli Friedman via llvm-commits]

Author: Eli Friedman
Date: 2022-04-18T08:46:03-07:00
New Revision: 13fc1781735a327699d9522e8e44899acf92a61a

URL: https://github.com/llvm/llvm-project/commit/13fc1781735a327699d9522e8e44899acf92a61a
DIFF: https://github.com/llvm/llvm-project/commit/13fc1781735a327699d9522e8e44899acf92a61a.diff

LOG: Force GHashCell to be 8-byte-aligned.

Otherwise, with recent versions of libstdc++, clang can't tell that the
atomic operations are properly aligned, and generates calls to
libatomic.  (Actually, because of the use of reinterpret_cast, it wasn't
guaranteed to be aligned, but I think it ended up being aligned in
practice.)

Fixes https://github.com/llvm/llvm-project/issues/54790 , the part where
LLVM failed to build.

Differential Revision: https://reviews.llvm.org/D123872

Added: 
    

Modified: 
    lld/COFF/DebugTypes.cpp

Removed: 
    


################################################################################
diff  --git a/lld/COFF/DebugTypes.cpp b/lld/COFF/DebugTypes.cpp
index aa2ce3bd9bbbf..0d25de464f9f6 100644
--- a/lld/COFF/DebugTypes.cpp
+++ b/lld/COFF/DebugTypes.cpp
@@ -902,7 +902,11 @@ struct GHashTable {
 
 /// 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;