Reduce smart pointer copies in destructors

[mcm001]

#1441 introduced a new recursive destructor to avoid stack overflows. We can improve runtime a bit by avoiding copying shared pointers around when we don't need to. I tested this using a robot stationary at 0,0,0 looking at an apriltag 2.5 meters in front of me a total of 50,000 observations, updating ISAM with each observation. This decreases mean runtime by 17% in this particular toy problem (see below).

With this optimization applied and for this -particular- toy problem, I'm still seeing ~half my runtime spent in the BayesTree destructor (see attached perf data from a release build):

perf.zip

IsamBenchmark.cpp (program that generates the output below): https://github.com/mcm001/gtsam/blob/44d36c1e173fe9c9c3752eb0dab376059610272e/examples/IsamBenchmark.cpp

	Without patch	With patch
Run times, microseconds	6.01E+07	6.28E+07
	8.22E+07	7.07E+07
	8.02E+07	8.14E+07
	8.32E+07	7.25E+07
	8.24E+07	6.72E+07
	8.61E+07	5.99E+07
	8.39E+07	6.51E+07
	8.28E+07	6.29E+07
	8.44E+07	6.27E+07
	8.49E+07	6.34E+07
Mean, seconds	81.01	66.86
	p-value, 2-tailed t-test, equal variance	2.62E-04

I suspect that there's further opportunities for optimization in these destructors, but I'll leave that to people smarter than I :)

Also fixes some stray ifdef/ifs for boost builds that squeaked in in MR #1948 and #1789 (see 6697452 ). Perhaps this is an indication that we need to add a no-boost build without boost installed in CI?

-- ===============================================================	
-- ================  Configuration Options  ======================	
--  CMAKE_CXX_COMPILER_ID type                       : GNU	
--  CMAKE_CXX_COMPILER_VERSION                       : 11.4.0	
--  CMake version                                    : 3.31.4	
--  CMake generator                                  : Ninja	
--  CMake build tool                                 : /usr/bin/ninja	
-- Build flags	
--  Build Tests                                      : Enabled	
--  Build examples with 'make all'                   : Enabled	
--  Build timing scripts with 'make all'             : Disabled	
--  Build shared GTSAM libraries                     : Enabled	
--  Put build type in library name                   : Enabled	
--  Build libgtsam_unstable                          : Enabled	
--  Build GTSAM unstable Python                      : Enabled	
--  Build MATLAB Toolbox for unstable                : Disabled	
--  Build for native architecture                    : Disabled	
--  Build type                                       : Release	
--  C compilation flags                              :  -O3 -DNDEBUG	
--  C++ compilation flags                            :  -O3 -DNDEBUG	
--  Enable Boost serialization                       : OFF	
--  GTSAM_COMPILE_FEATURES_PUBLIC                    : cxx_std_17	
--  GTSAM_COMPILE_OPTIONS_PUBLIC                     :	
--  GTSAM_COMPILE_DEFINITIONS_PUBLIC                 :	
--  GTSAM_COMPILE_OPTIONS_PUBLIC_RELEASE             :	
--  GTSAM_COMPILE_DEFINITIONS_PUBLIC_RELEASE         :	
--  Use System Eigen                                 : OFF (Using version: 3.4.0)	
--  Use System Metis                                 : OFF	
--  Using Boost version                              :	
--  Use Intel TBB                                    : Yes (Version: 2021.5.0)	
--  Eigen will use MKL                               : MKL not found	
--  Eigen will use MKL and OpenMP                    : OpenMP found but GTSAM_WITH_EIGEN_MKL is disabled	
--  Default allocator                                : TBB	
--  Cheirality exceptions enabled                    : YES	
--  Build with ccache                                : Yes	
-- Packaging flags	
--  CPack Source Generator                           : TGZ	
--  CPack Generator                                  : TGZ	
-- GTSAM flags	
--  Quaternions as default Rot3                      : Disabled	
--  Runtime consistency checking                     : Disabled	
--  Build with Memory Sanitizer                      : Disabled	
--  Rot3 retract is full ExpMap                      : Enabled	
--  Pose3 retract is full ExpMap                     : Enabled	
--  Enable branch merging in DecisionTree            : Enabled	
--  Enable timing machinery                          : Disabled	
--  Allow features deprecated in GTSAM 4.3           : Enabled	
--  Metis-based Nested Dissection                    : Enabled	
--  Use tangent-space preintegration                 : Enabled	
-- MATLAB toolbox flags	
--  Install MATLAB toolbox                           : Disabled	
-- Python toolbox flags	
--  Build Python module with pybind                  : Disabled	
-- ===============================================================	

[calcmogul]

We can improve runtime a bit by avoiding copying shared pointers around when we don't need to.

Well, the underlying pointers are still getting copied around. The difference is moving instead of copying the shared pointer here avoids incrementing then immediately decrementing the refcount.

Since the std::queue is emptied before the next for loop iteration, you could try hoisting the declaration out of the for loop to hopefully reuse the underlying storage.

[dellaert]

Wow. LGTM. Note some of the #if changes where also made in the concurrent #1984.

[dellaert]

PS I agree with you on having a CI flow that catches it - I guess the issue is that we compile without boost but boots is still installed and that masks issues?