[docs] Reword the alignment implications for atomic instructions. (#75871)

Atomic instructions (load / store/ atomicrwm / cmpxchg) are not
really undefined behavior if they lack natural alignment. They will
(with AtomicExpand pass enabled) be converted into libcalls.

Update the language reference to reflect this.

Author: JonPsson1

llvm/docs/LangRef.rst

@@ -10515,9 +10515,10 @@ Atomic loads produce :ref:`defined <memmodel>` results when they may see
 multiple atomic stores. The type of the pointee must be an integer, pointer, or
 floating-point type whose bit width is a power of two greater than or equal to
 eight and less than or equal to a target-specific size limit.  ``align`` must be
-explicitly specified on atomic loads, and the load has undefined behavior if the
-alignment is not set to a value which is at least the size in bytes of the
-pointee. ``!nontemporal`` does not have any defined semantics for atomic loads.
+explicitly specified on atomic loads. Note: if the alignment is not greater or
+equal to the size of the `<value>` type, the atomic operation is likely to
+require a lock and have poor performance. ``!nontemporal`` does not have any
+defined semantics for atomic loads.
 
 The optional constant ``align`` argument specifies the alignment of the
 operation (that is, the alignment of the memory address). It is the
@@ -10655,9 +10656,10 @@ Atomic loads produce :ref:`defined <memmodel>` results when they may see
 multiple atomic stores. The type of the pointee must be an integer, pointer, or
 floating-point type whose bit width is a power of two greater than or equal to
 eight and less than or equal to a target-specific size limit.  ``align`` must be
-explicitly specified on atomic stores, and the store has undefined behavior if
-the alignment is not set to a value which is at least the size in bytes of the
-pointee. ``!nontemporal`` does not have any defined semantics for atomic stores.
+explicitly specified on atomic stores. Note: if the alignment is not greater or
+equal to the size of the `<value>` type, the atomic operation is likely to
+require a lock and have poor performance. ``!nontemporal`` does not have any
+defined semantics for atomic stores.
 
 The optional constant ``align`` argument specifies the alignment of the
 operation (that is, the alignment of the memory address). It is the
@@ -10807,8 +10809,9 @@ must be at least ``monotonic``, the failure ordering cannot be either
 A ``cmpxchg`` instruction can also take an optional
 ":ref:`syncscope <syncscope>`" argument.
 
-The alignment must be a power of two greater or equal to the size of the
-`<value>` type.
+Note: if the alignment is not greater or equal to the size of the `<value>`
+type, the atomic operation is likely to require a lock and have poor
+performance.
 
 The alignment is only optional when parsing textual IR; for in-memory IR, it is
 always present. If unspecified, the alignment is assumed to be equal to the
@@ -10910,8 +10913,9 @@ the ``atomicrmw`` is marked as ``volatile``, then the optimizer is not
 allowed to modify the number or order of execution of this
 ``atomicrmw`` with other :ref:`volatile operations <volatile>`.
 
-The alignment must be a power of two greater or equal to the size of the
-`<value>` type.
+Note: if the alignment is not greater or equal to the size of the `<value>`
+type, the atomic operation is likely to require a lock and have poor
+performance.
 
 The alignment is only optional when parsing textual IR; for in-memory IR, it is
 always present. If unspecified, the alignment is assumed to be equal to the