[Doc] Update documentation for no-transitive-change (#96453)

(Some backgrounds, not required to read:
https://discourse.llvm.org/t/rfc-c-20-modules-introduce-thin-bmi-and-decls-hash/74755)

This is the document part for the no-transitive-change
(#86912,
#92083,
#92085,
#92511) to provide the ability
for build system to skip some unnecessary recompilations.

See the patch for examples.

clang/docs/ReleaseNotes.rst

@@ -174,6 +174,13 @@ here. Generic improvements to Clang as a whole or to its underlying
 infrastructure are described first, followed by language-specific
 sections with improvements to Clang's support for those languages.
 
+- Implemented improvements to BMIs for C++20 Modules that can reduce
+  the number of rebuilds during incremental recompilation. We are seeking
+  feedback from Build System authors and other interested users, especially
+  when you feel Clang changes the BMI and misses an opportunity to avoid
+  recompilations or causes correctness issues. See StandardCPlusPlusModules
+  `StandardCPlusPlusModules <StandardCPlusPlusModules.html>`_ for more details.
+
 - The ``\par`` documentation comment command now supports an optional
   argument, which denotes the header of the paragraph started by
   an instance of the ``\par`` command comment. The implementation

clang/docs/StandardCPlusPlusModules.rst

@@ -652,6 +652,149 @@ in the future. The expected roadmap for Reduced BMIs as of Clang 19.x is:
    comes, the term BMI will refer to the Reduced BMI and the Full BMI will only
    be meaningful to build systems which elect to support two-phase compilation.
 
+Experimental Non-Cascading Changes
+----------------------------------
+
+This section is primarily for build system vendors. For end compiler users,
+if you don't want to read it all, this is helpful to reduce recompilations.
+We encourage build system vendors and end users try this out and bring feedback.
+
+Before Clang 19, a change in BMI of any (transitive) dependency would cause the
+outputs of the BMI to change. Starting with Clang 19, changes to non-direct
+dependencies should not directly affect the output BMI, unless they affect the
+results of the compilations. We expect that there are many more opportunities
+for this optimization than we currently have realized and would appreaciate 
+feedback about missed optimization opportunities. For example,
+
+.. code-block:: c++
+
+  // m-partA.cppm
+  export module m:partA;
+
+  // m-partB.cppm
+  export module m:partB;
+  export int getB() { return 44; }
+
+  // m.cppm
+  export module m;
+  export import :partA;
+  export import :partB;
+
+  // useBOnly.cppm
+  export module useBOnly;
+  import m;
+  export int B() {
+    return getB();
+  }
+
+  // Use.cc
+  import useBOnly;
+  int get() {
+    return B();
+  }
+
+To compile the project (for brevity, some commands are omitted.):
+
+.. code-block:: console
+
+  $ clang++ -std=c++20 m-partA.cppm --precompile -o m-partA.pcm
+  $ clang++ -std=c++20 m-partB.cppm --precompile -o m-partB.pcm
+  $ clang++ -std=c++20 m.cppm --precompile -o m.pcm -fprebuilt-module-path=.
+  $ clang++ -std=c++20 useBOnly.cppm --precompile -o useBOnly.pcm -fprebuilt-module-path=.
+  $ md5sum useBOnly.pcm
+  07656bf4a6908626795729295f9608da  useBOnly.pcm
+
+If the interface of ``m-partA.cppm`` is changed to:
+
+.. code-block:: c++
+
+  // m-partA.v1.cppm
+  export module m:partA;
+  export int getA() { return 43; }
+
+and the BMI for ``useBOnly`` is recompiled as in:
+
+.. code-block:: console
+
+  $ clang++ -std=c++20 m-partA.cppm --precompile -o m-partA.pcm
+  $ clang++ -std=c++20 m-partB.cppm --precompile -o m-partB.pcm
+  $ clang++ -std=c++20 m.cppm --precompile -o m.pcm -fprebuilt-module-path=.
+  $ clang++ -std=c++20 useBOnly.cppm --precompile -o useBOnly.pcm -fprebuilt-module-path=.
+  $ md5sum useBOnly.pcm
+  07656bf4a6908626795729295f9608da  useBOnly.pcm
+
+then the contents of ``useBOnly.pcm`` remain unchanged.
+Consequently, if the build system only bases recompilation decisions on directly imported modules,
+it becomes possible to skip the recompilation of ``Use.cc``.
+It should be fine because the altered interfaces do not affect ``Use.cc`` in any way;
+the changes do not cascade.
+
+When ``Clang`` generates a BMI, it records the hash values of all potentially contributory BMIs
+for the BMI being produced. This ensures that build systems are not required to consider
+transitively imported modules when deciding whether to recompile.
+
+What is considered to be a potential contributory BMIs is currently unspecified.
+However, it is a severe bug for a BMI to remain unchanged following an observable change
+that affects its consumers.
+
+Build systems may utilize this optimization by doing an update-if-changed operation to the BMI
+that is consumed from the BMI that is output by the compiler.
+
+We encourage build systems to add an experimental mode that
+reuses the cached BMI when **direct** dependencies did not change,
+even if **transitive** dependencies did change.
+
+Given there are potential compiler bugs, we recommend that build systems
+support this feature as a configurable option so that users
+can go back to the transitive change mode safely at any time.
+
+Interactions with Reduced BMI
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+With reduced BMI, non-cascading changes can be more powerful. For example,
+
+.. code-block:: c++
+
+  // A.cppm
+  export module A;
+  export int a() { return 44; }
+
+  // B.cppm
+  export module B;
+  import A;
+  export int b() { return a(); }
+
+.. code-block:: console
+
+  $ clang++ -std=c++20 A.cppm -c -fmodule-output=A.pcm  -fexperimental-modules-reduced-bmi -o A.o
+  $ clang++ -std=c++20 B.cppm -c -fmodule-output=B.pcm  -fexperimental-modules-reduced-bmi -o B.o -fmodule-file=A=A.pcm
+  $ md5sum B.pcm
+  6c2bd452ca32ab418bf35cd141b060b9  B.pcm
+
+And let's change the implementation for ``A.cppm`` into:
+
+.. code-block:: c++
+
+  export module A;
+  int a_impl() { return 99; }
+  export int a() { return a_impl(); }
+
+and recompile the example:
+
+.. code-block:: console
+
+  $ clang++ -std=c++20 A.cppm -c -fmodule-output=A.pcm  -fexperimental-modules-reduced-bmi -o A.o
+  $ clang++ -std=c++20 B.cppm -c -fmodule-output=B.pcm  -fexperimental-modules-reduced-bmi -o B.o -fmodule-file=A=A.pcm
+  $ md5sum B.pcm
+  6c2bd452ca32ab418bf35cd141b060b9  B.pcm
+
+We should find the contents of ``B.pcm`` remains the same. In this case, the build system is
+allowed to skip recompilations of TUs which solely and directly depend on module ``B``.
+
+This only happens with a reduced BMI. With reduced BMIs, we won't record the function body
+of ``int b()`` in the BMI for ``B`` so that the module ``A`` doesn't contribute to the BMI of ``B``
+and we have less dependencies.
+
 Performance Tips
 ----------------