[sanitizer][NFCI] Document interaction of inlining with disabling instrumentation (#177672)

For ASan, MSan, TSan, UBSan

clang/docs/AddressSanitizer.rst

diff --git a/clang/docs/AddressSanitizer.rst b/clang/docs/AddressSanitizer.rst
index dc0ba79..6c1cb28 100644
--- a/clang/docs/AddressSanitizer.rst
+++ b/clang/docs/AddressSanitizer.rst
@@ -248,12 +248,27 @@
 The same attribute used on a global variable prevents AddressSanitizer
 from adding redzones around it and detecting out of bounds accesses.
 
-
 AddressSanitizer also supports
 ``__attribute__((disable_sanitizer_instrumentation))``. This attribute
 works similarly to ``__attribute__((no_sanitize("address")))``, but it also
 prevents instrumentation performed by other sanitizers.
 
+Interaction of Inlining with Disabling Sanitizer Instrumentation
+-----------------------------------------------------------------
+
+* A `no_sanitize` function will not be inlined heuristically by the compiler into a sanitized function.
+* An `always_inline` function will adopt the instrumentation status of the function it is inlined into.
+* Forcibly combining `no_sanitize` and ``__attribute__((always_inline))`` is not supported, and will often lead to unexpected results. To avoid mixing these attributes, use:
+
+.. code-block:: c
+
+    // Note, __has_feature test for sanitizers is deprecated, and Clang will support __SANITIZE_<sanitizer>__ similar to GCC.
+    #if __has_feature(address_sanitizer) || defined(__SANITIZE_ADDRESS__) || ... <other sanitizers>
+    #define ALWAYS_INLINE_IF_UNINSTRUMENTED
+    #else
+    #define ALWAYS_INLINE_IF_UNINSTRUMENTED __attribute__((always_inline))
+    #endif
+
 Conditional Sanitizer Checks with ``__builtin_allow_sanitize_check``
 --------------------------------------------------------------------
 

clang/docs/MemorySanitizer.rst

diff --git a/clang/docs/MemorySanitizer.rst b/clang/docs/MemorySanitizer.rst
index 54f88d0..0ebfed7 100644
--- a/clang/docs/MemorySanitizer.rst
+++ b/clang/docs/MemorySanitizer.rst
@@ -101,6 +101,22 @@
 required; for example for certain code that cannot tolerate any instrumentation
 and resulting side-effects. This attribute overrides ``no_sanitize("memory")``.
 
+Interaction of Inlining with Disabling Sanitizer Instrumentation
+-----------------------------------------------------------------
+
+* A `no_sanitize` function will not be inlined heuristically by the compiler into a sanitized function.
+* An `always_inline` function will adopt the instrumentation status of the function it is inlined into.
+* Forcibly combining `no_sanitize` and ``__attribute__((always_inline))`` is not supported, and will often lead to unexpected results. To avoid mixing these attributes, use:
+
+. code-block:: c
+
+    // Note, __has_feature test for sanitizers is deprecated, and Clang will support __SANITIZE_<sanitizer>__ similar to GCC.
+    #if __has_feature(memory_sanitizer) || defined(__SANITIZE_MEMORY__) || ... <other sanitizers>
+    #define ALWAYS_INLINE_IF_UNINSTRUMENTED
+    #else
+    #define ALWAYS_INLINE_IF_UNINSTRUMENTED __attribute__((always_inline))
+    #endif
+
 Conditional Sanitizer Checks with ``__builtin_allow_sanitize_check``
 --------------------------------------------------------------------
 

clang/docs/ThreadSanitizer.rst

diff --git a/clang/docs/ThreadSanitizer.rst b/clang/docs/ThreadSanitizer.rst
index c90d668..06c0cdb 100644
--- a/clang/docs/ThreadSanitizer.rst
+++ b/clang/docs/ThreadSanitizer.rst
@@ -110,6 +110,22 @@
 tolerate any instrumentation and resulting side-effects. This attribute
 overrides ``no_sanitize("thread")``.
 
+Interaction of Inlining with Disabling Sanitizer Instrumentation
+-----------------------------------------------------------------
+
+* A `no_sanitize` function will not be inlined heuristically by the compiler into a sanitized function.
+* An `always_inline` function will adopt the instrumentation status of the function it is inlined into.
+* Forcibly combining `no_sanitize` and ``__attribute__((always_inline))`` is not supported, and will often lead to unexpected results. To avoid mixing these attributes, use:
+
+. code-block:: c
+
+    // Note, __has_feature test for sanitizers is deprecated, and Clang will support __SANITIZE_<sanitizer>__ similar to GCC.
+    #if __has_feature(thread_sanitizer) || defined(__SANITIZE_THREAD__) || ... <other sanitizers>
+    #define ALWAYS_INLINE_IF_UNINSTRUMENTED
+    #else
+    #define ALWAYS_INLINE_IF_UNINSTRUMENTED __attribute__((always_inline))
+    #endif
+
 Conditional Sanitizer Checks with ``__builtin_allow_sanitize_check``
 --------------------------------------------------------------------
 

clang/docs/UndefinedBehaviorSanitizer.rst

diff --git a/clang/docs/UndefinedBehaviorSanitizer.rst b/clang/docs/UndefinedBehaviorSanitizer.rst
index 0a2d833..60c619d 100644
--- a/clang/docs/UndefinedBehaviorSanitizer.rst
+++ b/clang/docs/UndefinedBehaviorSanitizer.rst
@@ -380,6 +380,20 @@
 supported by other compilers, so consider using it together with
 ``#if defined(__clang__)``.
 
+Interaction of Inlining with Disabling Sanitizer Instrumentation
+-----------------------------------------------------------------
+
+Unlike many of the other sanitizers (e.g., ASan/MSan, TSan), UBSan is largely
+compatible with inlining, both the compiler's heuristic inlining as well as
+``__attribute__((always_inline))``.
+
+There are (at least) two exceptions:
+
+* inlining may change the layout of variables, which can affect whether
+  the `alignment` check detects an under-aligned variable
+* combining `__attribute((no_sanitize("local-bounds")))` with
+  `__attribute((always_inline))` is not supported
+
 Suppressing Errors in Recompiled Code (Ignorelist)
 --------------------------------------------------