[jordanella]

A build with nothing to compile still relinked the 440 MB executable and the renderer, a build from a second checkout of the same tree recompiled all eight hot pieces (~10-20 minutes) because their keys named the play profile by its path, and clangd had no compile database. Three small changes to nx2/build.py from the dev-loop proposal (#2636, A4 and A8); one PR because they touch the same lines, and I can split them if you prefer.

Change

nx2/build.py:

• A4, no link when nothing changed: run_link skips a link whose inputs are the last successful one's. Its key is kept beside the output (<output>.linkkey) with the output's size:

  • the command, the compiler and the work folder's system-headers stamp;
  • each object's key (its .o.sha1; its size and time without one);
  • the contents of the other files it names, inside options too (the hot symbol order, the libraries, the resources).

  A failed link, or an output of another size or gone, links again. The executable's, the renderer's and the game DLL's links go through it.

• A8, compile_commands.json: every compile as compile_one runs it, written to the build folder for clangd and clang-tidy (compile_commands.json at the checkout's top is git-ignored, for a copy there).

• The hot pieces' keys name the profile by its contents: flag_text leaves -fprofile-instr-use=<path> out of the key, since -DHOOKSHOT_PROFILE_SHA1 beside it already says what the key needs of the profile. This changes their keys once.

Measured

Windows 11, Ryzen 9 3900X, a dev work folder:

• A build with nothing to do: 16 s against 6 s, both links skipped.
• One renderer edit: only the renderer's link runs.
• A second checkout of the same tree building in the same work folder: the eight hot pieces from the cache, where main compiled them again.

In the game

Every build I ran in the game today used this build.py. The latest: Windows 11, RTX 3080 (NVIDIA 621.14), a dev-profile build, from a save, headless through the control bridge: in play in Kokiri Forest, 28-31 fps, nothing in the log about a lost device or a crash. A skipped link leaves the same binary in place.

Testing

• tests/test_link_skip.py (new): a stand-in linker through the real run_link: the same inputs aren't linked again; a changed object, library or symbol order is; so is an output of another size or gone; a failed link forgets its key; the files inside options are inputs; the compile commands are the compiles; the build's links go through it.
• tests/test_profile_path_key.py (new): two paths, one profile give one key; another profile, another key; every use of the profile's path names its contents too.
• Every check that reads the changed files (37), through run_all.py, rebased on today's main, Windows: pass, apart from four that fail the same way on main here (test_driver_cache_folder, test_fpx_sticky, test_linux_update, nx2/test_dll.py). Rebasing found that main's build.py doesn't import re (perf(build): a renderer header edit compiles the files that read it, not the whole back end #2706 adds it); this PR imports it too.
• Not tested: Linux.

See #2636 (A4, A8).

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