[DOC][DTLTO] Update DTLTO documentation for the LLVM 22 release (#177368)

bd1976bris · c-rhodes · commit 5bebd32e87ec · 2026-02-02T09:04:13.000Z
This change updates the documentation to reflect work completed during the LLVM 22 timeframe, including support for the ThinLTO cache and static libraries/archives. It also clarifies that the goal of DTLTO is to support distribution of ThinLTO backend compilations for any in-process ThinLTO invocation. SIE Internal Tracker: TOOLCHAIN-21016 (cherry picked from commit 88478ab)
diff --git a/llvm/docs/DTLTO.rst b/llvm/docs/DTLTO.rst
@@ -24,7 +24,11 @@ here:
 Using the *separate thin-link* approach requires a build system capable of
 handling the dynamic dependencies specified in the individual summary index
 files, such as Bazel. DTLTO removes this requirement, allowing it to be used
-with any build process that supports in-process ThinLTO.
+with any build process that supports in-process ThinLTO. To facilitate this,
+DTLTO supports common in-process ThinLTO features, such as caching. Bitcode
+objects in static libraries/archives (e.g. libc.a) are also handled
+transparently by temporarily extracting referenced objects for distribution;
+when thin archives are used, no extraction is required.
 
 The following commands show the steps used for the *separate thin-link*
 approach for a basic example:
@@ -66,7 +70,7 @@ such as the following can be used:
 
 .. code-block:: console
 
-   clang -flto=thin -fuse-ld=lld -O2 t1.o t2.o -fthinlto-distributor=$(which python3) \
+   clang -flto=thin -fuse-ld=lld -O2 t1.o t2.o libt3.a -fthinlto-distributor=$(which python3) \
      -Xthinlto-distributor=$LLVMSRC/llvm/utils/dtlto/local.py
 
 Distributors
@@ -165,22 +169,22 @@ following backend compilation commands with maximum parallelism:
     /usr/bin/clang -O2 -c -fprofile-sample-use=my.prof t2.o  -fthinlto-index=t2.o.thinlto.bc -o t2.native.o \
       -fproc-stat-report=t2.stats.txt
 
-TODOs
------
+Limitations
+-----------
 
-The following features are planned for DTLTO but not yet implemented:
+The following features are not implemented at this time:
 
-- Support for the ThinLTO in-process cache.
 - Support for platforms other than ELF and COFF.
-- Support for archives with bitcode members.
-- Support for more LTO configurations; only a very limited set of LTO
-  configurations is supported currently, e.g., support for basic block sections
-  is not currently available.
+- Support for more CodeGen configurations in the LTO backend; only a very
+  limited set is supported currently, e.g. support for basic block sections is
+  not currently available.
+- Support for sharing ThinLTO cache entries with in-process ThinLTO entries.
+  This is not currently supported because identical code generation is not yet
+  guaranteed.
 
 Constraints
 -----------
 
 - Matching versions of Clang and LLD should be used.
 - The distributor used must support the JSON schema generated by the version of
   LLD in use.
-
