Use optimisation level 2

[tfpf]

Visibility - GCC Wiki

Closes #247.

[tfpf]

Hiding symbols is not required because that is already the default when using Meson. Kudos again to the Meson developers for setting sane (genius) defaults!

[tfpf]

There is insufficient evidence that optimisation level 2 gives better performance than level 3. The following numbers were obtained using a machine different from the one I usually run benchmarks on.

Membership Check

Optimisation Level 2

	0.00 in d	0.33 in d	0.67 in d	1.00 in d
100000	466.196116	557.165672	450.109114	445.309144
1000000	541.194241	718.284451	603.605544	490.465720
10000000	648.149269	805.398502	748.876120	589.742698

Optimisation Level 3

	0.00 in d	0.33 in d	0.67 in d	1.00 in d
100000	490.693192	565.387975	457.075284	456.654925
1000000	553.142049	719.515105	607.979110	502.855061
10000000	592.341685	810.753989	726.519562	597.176215

Insertion and Deletion

Optimisation Level 2

	set_del(d, keys_33)	set_del(d, keys_67)	set_del(d, keys_100)
100000	58.211082	131.633938	209.740966
1000000	74.561862	168.646467	302.007536
10000000	89.416885	262.711641	428.468641

Optimisation Level 3

	set_del(d, keys_33)	set_del(d, keys_67)	set_del(d, keys_100)
100000	56.350561	120.354840	187.630554
1000000	71.076539	169.000136	298.549570
10000000	88.913401	258.136030	453.049115

Iteration

Optimisation Level 2

	for _ in d: pass	for _ in reversed(d): pass
100000	0.020122	0.023647
1000000	0.240947	0.300003
10000000	3.068003	3.952579

Optimisation Level 3

	for _ in d: pass	for _ in reversed(d): pass
100000	0.018108	0.023875
1000000	0.233428	0.301671
10000000	3.065221	3.997450

[tfpf]

I see optimisation level 3 giving better performance than level 2.

See the note on Optimize Options (Using the GNU Compiler Collection (GCC)):

If you do not specify an optimization level option -O at link time, then GCC uses the highest optimization level used when compiling the object files.

which means the Meson not supplying only the LTO flag but not the CTO flag to the linker is not a problem.