[clang] [analyzer][docs] Further improve the optin.core.FixedAddressDereference docs (PR #182037)

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llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-clang-static-analyzer-1

Author: Balázs Benics (steakhal)

<details>
<summary>Changes</summary>

This addresses the raised points in:
https://github.com/llvm/llvm-project/pull/181858#pullrequestreview-3820145678

---
Full diff: https://github.com/llvm/llvm-project/pull/182037.diff


1 Files Affected:

- (modified) clang/docs/analyzer/checkers.rst (+12-8) 


``````````diff
diff --git a/clang/docs/analyzer/checkers.rst b/clang/docs/analyzer/checkers.rst
index e51015655de65..29cb261d26aeb 100644
--- a/clang/docs/analyzer/checkers.rst
+++ b/clang/docs/analyzer/checkers.rst
@@ -877,26 +877,30 @@ numerical value.
    int x = (*p_function)('x', 'y'); // NO warning yet at functon pointer calls
  }
 
+Access of fixed numerical addresses can be legitimate in low-level projects (e.g. firmware) and hardware interop.
+These values are often marked as ``volatile`` (to prevent unwanted compiler optimizations),
+so this checker doesn't report situations where the pointee of the fixed address is ``volatile``.
+If this suppression is not sufficient on a low-level project, then consider disabling this ``optin`` checker.
+Note that null pointers will still be reported by :ref:`core.NullDereference <core-NullDereference>`
+regardless if the pointee is ``volatile`` or not.
+
+.. code-block:: c
+
  void volatile_pointee() {
-   *(volatile int *)0x404 = 1; // no warning: constant non-null "volatile" pointee, you must know what you are doing
+   *(volatile int *)0x404 = 1; // no warning: fixed non-null "volatile" pointee, you must know what you are doing
  }
 
  void deref_volatile_nullptr() {
    *(volatile int *)0 = 1; // core.NullDereference still warns about this
  }
 
-If your project is low-level (e.g., firmware), or deals with hardware interop with a lot of genuine constant addresses, then consider disabling this checker.
-The checker automatically suppresses issues if the type of the pointee of the address is ``volatile``.
-You probably already need this to be ``volatile`` for legitimate access, so the checker suppresses such issues to avoid false-positives.
-Note that null pointers will still be reported by :ref:`core.NullDereference <core-NullDereference>`
-regardless if the pointee is ``volatile`` or not.
-
 If the analyzer option ``suppress-dereferences-from-any-address-space`` is set
 to true (the default value), then this checker never reports dereference of
 pointers with a specified address space. If the option is set to false, then
 reports from the specific x86 address spaces 256, 257 and 258 are still
 suppressed, but fixed address dereferences from other address spaces are
-reported.
+reported. Do not use ``address_space`` attributes to suppress the reports -
+it just happens so that the checker also doesn't raise issues if the attribute is present.
 
 .. _optin-cplusplus-UninitializedObject:
 

``````````

</details>


https://github.com/llvm/llvm-project/pull/182037