================
@@ -652,6 +652,134 @@ 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 No Transitive Change
+---------------------------------
+
+Starting with clang19.x, we introduced an experimental feature: the
non-transitive
+change for modules, aimed at reducing unnecessary recompilations. 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();
+ }
+
+Now let's 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
+
+then let's change the interface of ``m-partA.cppm`` to:
+
+.. code-block:: c++
+
+ // m-partA.v1.cppm
+ export module m:partA;
+ export int getA() { return 43; }
+
+Let's compile the BMI for `useBOnly` again:
+
+.. 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
+
+We observed that the contents of useBOnly.pcm remain unchanged.
+Consequently, if the build system bases recompilation decisions on directly
imported modules only,
+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.
+This concept is called as no transitive changes.
+
+When clang generates a BMI, it records the hash values of all potentially
contributory BMIs
+into the currently written BMI. This ensures that build systems are not
required to consider
+transitively imported modules when deciding on recompilations.
+
+The definition for potential contributory BMIs is implementation defined. We
don't intend to
+display detailed rules for users. The contract is:
+
+1. It is a severe bug if a BMI remains unchanged erroneously following an
observable change
+ that affects its users.
+2. It is an potential improvement opportunity if a BMI changes after an
unobservable change
+ happens.
+
+We suggest build systems to support this feature as a configurable option for
a long time.
+So that users can go back to the transitive change mode safely at any time.
----------------
ChuanqiXu9 wrote:
Done
https://github.com/llvm/llvm-project/pull/96453
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