Merge bitcoin#17708: prevector: avoid misaligned member accesses

5f26855 test: Remove ubsan alignment suppressions (Wladimir J. van der Laan)
9d933ef prevector: avoid misaligned member accesses (Anthony Towns)

Pull request description:

  Ensure prevector data is appropriately aligned. Earlier discussion in bitcoin#17530.

  **Edit laanwj**: In contrast to bitcoin#17530, it does this without increase in size of any of the coin cache data structures (x86_64, clang)

  | Struct        | (size,align) before           | (size,align) after  |
  | ------------- | ------------- | ------- |
  | Coin | 48, 8        |     48, 8   |
  | CCoinsCacheEntry | 56, 8    |   56, 8  |
  | CScript | 32, 1       |      32, 8  |

ACKs for top commit:
  laanwj:
    ACK 5f26855
  practicalswift:
    ACK 5f26855
  jonatack:
    ACK 5f26855

Tree-SHA512: 98d112d6856f683d5b212410b73f3071d2994f1efb046a2418a35890aa1cf1aa7c96a960fc2e963fa15241e861093c1ea41951cf5b4b5431f88345eb1dd0a98a

src/prevector.h

@@ -17,7 +17,6 @@
 
 #include <compat.h>
 
-#pragma pack(push, 1)
 /** Implements a drop-in replacement for std::vector<T> which stores up to N
  *  elements directly (without heap allocation). The types Size and Diff are
  *  used to store element counts, and can be any unsigned + signed type.
@@ -149,14 +148,20 @@ class prevector {
     };
 
 private:
-    size_type _size = 0;
+#pragma pack(push, 1)
     union direct_or_indirect {
         char direct[sizeof(T) * N];
         struct {
-            size_type capacity;
             char* indirect;
+            size_type capacity;
         };
-    } _union = {};
+    };
+#pragma pack(pop)
+    alignas(char*) direct_or_indirect _union = {};
+    size_type _size = 0;
+
+    static_assert(alignof(char*) % alignof(size_type) == 0 && sizeof(char*) % alignof(size_type) == 0, "size_type cannot have more restrictive alignment requirement than pointer");
+    static_assert(alignof(char*) % alignof(T) == 0, "value_type T cannot have more restrictive alignment requirement than pointer");
 
     T* direct_ptr(difference_type pos) { return reinterpret_cast<T*>(_union.direct) + pos; }
     const T* direct_ptr(difference_type pos) const { return reinterpret_cast<const T*>(_union.direct) + pos; }
@@ -567,6 +572,4 @@ class prevector {
     }
 };
 
-#pragma pack(pop)
-
 #endif // BITCOIN_PREVECTOR_H

test/sanitizer_suppressions/ubsan

@@ -1,5 +1,5 @@
-alignment:move.h
-alignment:prevector.h
+# -fsanitize=undefined suppressions
+# =================================
 bool:wallet/wallet.cpp
 float-divide-by-zero:policy/fees.cpp
 float-divide-by-zero:validation.cpp