Export of internal Abseil changes

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6eac0cc7bca997ee95afd4ec485077ae8fd99333 by Abseil Team <[email protected]>:

Add comment not to use `ABSL_ATTRIBUTE_PACKED` with `std::atomic`.

PiperOrigin-RevId: 395231460
GitOrigin-RevId: 6eac0cc7bca997ee95afd4ec485077ae8fd99333
Change-Id: Ib4e83bed0f313724b309b6278e9e24a6e5fe9b2c

Author: Abseil Team

absl/base/attributes.h

@@ -548,13 +548,19 @@
 // ABSL_ATTRIBUTE_PACKED
 //
 // Instructs the compiler not to use natural alignment for a tagged data
-// structure, but instead to reduce its alignment to 1. This attribute can
-// either be applied to members of a structure or to a structure in its
-// entirety. Applying this attribute (judiciously) to a structure in its
-// entirety to optimize the memory footprint of very commonly-used structs is
-// fine. Do not apply this attribute to a structure in its entirety if the
-// purpose is to control the offsets of the members in the structure. Instead,
-// apply this attribute only to structure members that need it.
+// structure, but instead to reduce its alignment to 1.
+//
+// Therefore, DO NOT APPLY THIS ATTRIBUTE TO STRUCTS CONTAINING ATOMICS. Doing
+// so can cause atomic variables to be mis-aligned and silently violate
+// atomicity on x86.
+//
+// This attribute can either be applied to members of a structure or to a
+// structure in its entirety. Applying this attribute (judiciously) to a
+// structure in its entirety to optimize the memory footprint of very
+// commonly-used structs is fine. Do not apply this attribute to a structure in
+// its entirety if the purpose is to control the offsets of the members in the
+// structure. Instead, apply this attribute only to structure members that need
+// it.
 //
 // When applying ABSL_ATTRIBUTE_PACKED only to specific structure members the
 // natural alignment of structure members not annotated is preserved. Aligned