================
@@ -652,6 +652,141 @@ 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
+---------------------------------
+
+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
feedbacks
+
+Before Clang 19, a change in BMI of any (transitive) dependency would case 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;
+there are no transitive changes.
+
+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.
+
+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.
----------------
mathstuf wrote:
Ah, ok.
FWIW, I was thinking that even `a.pcm` would get reported in `-MF` for your
example and elided if it didn't contribute. As for the `restat = 1` with a
swap, yes, that is build-system side. I think some docs saying something like:
```md
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.
```
https://github.com/llvm/llvm-project/pull/96453
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