Copybara import of the project:

--
3439d8a5cd745dfd2776593916eead65cb456afc by Kevin Zakka <kevinarmandzakka@gmail.com>:

Respect an explicit CMAKE_INTERPROCEDURAL_OPTIMIZATION=OFF

The default-LTO guard checked the value rather than whether it was DEFINED, so an
explicit -DCMAKE_INTERPROCEDURAL_OPTIMIZATION=OFF (used in CI to cut build time)
was silently flipped back ON. Check DEFINED instead; default-on for Release is
preserved when the caller makes no choice.

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bd15d1b264e5ab9594669b58fa1e0388d0a16f6b by Kevin Zakka <kevinarmandzakka@gmail.com>:

Speed up CI

~10x faster on the POSIX jobs (~45m -> ~4-5m) and ~2x overall wall-clock.

- Build Studio/Filament and WASM only in their dedicated canary jobs, not in
  every matrix job (WASM uses emcc, which ignores the host compiler).
- Compiler matrix -> build_matrix.json with core/extended tiers: PRs build the
  core set, pushes to main run the full sweep.
- ccache across build jobs (studio cache keyed on the Filament pin); fix the
  Python-bindings build so ccache hits (CCACHE_BASEDIR).
- Disable IPO/LTO on POSIX (it was silently on); keep it on Windows where /GL-off
  exposes a latent test bug and build time is not the bottleneck.
- Run ctest in parallel on POSIX (~2x); uv for Python installs (~29s -> ~8s).
- Move MJX (compiler-independent) to a single dedicated job.
- Restrict GITHUB_TOKEN to contents: read; bump actions off deprecated Node20.

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c0618ab7aef524b238e0a30f2f23c50c4ce0c9b1 by Kevin Zakka <kevinarmandzakka@gmail.com>:

Build the gcc jobs with LTO off too

Restores LTO-off for gcc (recovering the build-time win). That surfaces known
gcc-12 -Wrestrict false positives in libstdc++ <char_traits> at -O3; two clean,
behavior-preserving rewrites in the XML writer avoid them. Confirmed gcc-12 and
gcc-14 build clean with LTO off (PR #3325).

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2c38da0f48f77635e58e4b7931b86d60f3c253c7 by Kevin Zakka <kevinarmandzakka@gmail.com>:

Respect explicit IPO=OFF in the Simulate and Sample options too

Apply the same DEFINED check as cmake/MujocoOptions.cmake to the simulate/ and
sample/ subprojects (which carry identical copies of the guard) to keep the three
in sync, as the internal import requires. Since the guard now honors =OFF, stop
forcing IPO=OFF on the tiny samples/simulate CI builds so they keep their default
LTO and don't expose the gcc -Werror false positives that -O3-without-LTO triggers.

PiperOrigin-RevId: 930008698
Change-Id: Ibe9c73f688ecafa470dc786cf8e1777b44a58f6d

Author: kevinzakka

cmake/MpcOptions.cmake

@@ -107,7 +107,12 @@ if(CMAKE_CXX_COMPILER_ID STREQUAL "GNU" OR (CMAKE_CXX_COMPILER_ID MATCHES "Clang
   endif()
 endif()
 
-if(NOT CMAKE_INTERPROCEDURAL_OPTIMIZATION AND (CMAKE_BUILD_TYPE AND NOT CMAKE_BUILD_TYPE STREQUAL "Debug"))
+# Enable interprocedural optimization (LTO) by default for non-Debug builds, but
+# only when the caller has not made an explicit choice. Checking the value (rather
+# than whether it is DEFINED) meant an explicit `-DCMAKE_INTERPROCEDURAL_OPTIMIZATION=OFF`
+# was silently overridden back to ON, e.g. in CI where LTO is disabled to cut
+# build time.
+if(NOT DEFINED CMAKE_INTERPROCEDURAL_OPTIMIZATION AND (CMAKE_BUILD_TYPE AND NOT CMAKE_BUILD_TYPE STREQUAL "Debug"))
   set(CMAKE_INTERPROCEDURAL_OPTIMIZATION ON)
 endif()