[Axel-Naumann]

Instead of looking for libraries resolving each unresolved symbol,
just load each lib resolving symbols: this will allow subsequent
missing symbols to be resolved from the loaded library quickly,
without touching disk.

When loading an existing ACLiC library, and if we expect it
to contain the dependencies (explicit linking), just load itt,
instead of trying to (re-)determine its dependencies from its
undefined symbols: the outcome should be just the library
dependencies we expect the library to know already.

[Axel-Naumann]

I'll clean up git history once the tests pass - there's root-project/roottest#699 and the very different projectroot.core.metacling.test.gtest_core_metacling_test_TClingTest

[Axel-Naumann]

Loading a library that is needed to resolve symbols (as done by this PR) means that libraries linked against that library are loaded as a side-effect, without TCling / TSystem seeing it. TClingTest's gInterpreter->GetSharedLibDeps("libTreePlayer") thus doesn't report all actual library dependencies but only a subset. This isn't an issue for explicit linking, though (neither for implicit linking, which will load all libraries separately and thus enumerate them), but it's a potential correctness concern and an inconsistency for explicit vs implicit linking / loading.

[pcanal]

This call is still needed for the not-explicitly-linked case. It has the side effect of load the dependencies from the rootmap files so that the subsequent LoadLibrary can use that information. See commit 8fdd5c3

[pcanal]

Related we should probably deprecate the option to disable explicit linking in ACLiC (it is already removed from the main build). Eitherway removing this line probably require to add (or check we already have) a test for the problem solved by commit 8fdd5c3

[pcanal]

Approved but need one more correction.

[ferdymercury]

Other speedup related issues:
#14287
#7123
https://its.cern.ch/jira/browse/ROOT-8848

[github-actions]

Test Results

    17 files      17 suites   4d 3h 48m 11s ⏱️
 2 694 tests  2 694 ✅ 0 💤 0 ❌
44 129 runs  44 129 ✅ 0 💤 0 ❌

Results for commit ed10012.

♻️ This comment has been updated with latest results.

[guitargeek]

Thanks a lot @Axel-Naumann for this, and @pcanal and @vgvassilev for the reviews!

I'm closing this PR in favor of #23538, which takes over its core part: when the ACLiC library is explicitly linked (ACLiC.LinkLibs & 1, the default on Linux and macOS), skip the dependency scan with GetSharedLibDeps(lib, /*tryDyld*/ true) before loading it. The dependencies are already recorded in the library itself, since ACLiC runs the same scan at build time and relinks against what it finds. On current master, this scan alone takes about 2.9 s for a macro using RDataFrame, RooFit, Tree, Hist and Gpad, so .L macro.C+ with an up-to-date library goes from 4.9 s to 2.35 s.

The new PR leaves out the rest of this one:

• It keeps gInterpreter->GetSharedLibDeps(library) before loading an existing library, for the rootmap side effect @pcanal pointed out (8fdd5c3). It costs next to nothing anyway.
• Deprecating !(ACLiC.LinkLibs & 1) and removing the ancient macOS case can be done separately.

Axel is credited as co-author of the new commit. Thanks again!