cc @ajs124

Each of the did build on my machine and appear to be working fine. I've not done any kind of benchmarks. The builds seem to take about 1 minute longer than usually on my 24 cores which is probably fine.

I rebased this to solve the merge conflict that I caused in another PR.

Using the Speedometer 2.0 on an Intel Core 2 Duo CPU T7250 @ 2.00GHz.

Iteration 1	22.64 runs/min
Iteration 2	22.29 runs/min
Iteration 3	22.03 runs/min
Iteration 4	22.31 runs/min
Iteration 5	21.76 runs/min
Iteration 6	22.39 runs/min
Iteration 7	22.60 runs/min
Iteration 8	21.51 runs/min
Iteration 9	21.95 runs/min
Iteration 10	22.28 runs/min
Arithmetic Mean: 22.2 ± 0.26 (1.2%)

Iteration 1	25.67 runs/min
Iteration 2	24.89 runs/min
Iteration 3	24.43 runs/min
Iteration 4	24.37 runs/min
Iteration 5	24.12 runs/min
Iteration 6	24.53 runs/min
Iteration 7	24.81 runs/min
Iteration 8	22.44 runs/min
Iteration 9	23.76 runs/min
Iteration 10	23.67 runs/min
Arithmetic Mean: 24.3 ± 0.62 (2.5%)

Iteration 1	26.70 runs/min
Iteration 2	27.38 runs/min
Iteration 3	26.27 runs/min
Iteration 4	27.67 runs/min
Iteration 5	28.08 runs/min
Iteration 6	27.07 runs/min
Iteration 7	27.58 runs/min
Iteration 8	28.34 runs/min
Iteration 9	27.78 runs/min
Iteration 10	27.47 runs/min
Arithmetic Mean: 27.4 ± 0.44 (1.6%)

The benchmark seems to show a performance increase of ~9% when going from non-LTO to LTO.
Comparing non-LTO to LTO+PGO (firefox-bin) there is a substantial performance increase of ~23%.
Comparing LTO to LTO+PGO shows LTO helps us get closer but not quite to the firefox-bin numbers, which firefox-bin providing ~13% increase over plain LTO.

This is a considerably old machine so I am not sure how accurate the benchmark results are but I can give my opinion saying it feels smoother.

This is looking good. Will try this one more time.

This is looking good. Will try this one more time.

This is also looking good here. @andir, did you do some more testing?

@ofborg eval

While this is quite an improvement, it will break reproducibility, right? (assuming it ever was reproducible)
Probably not a big deal atm, but might become an issue in the future.

LTO should not have that much of an impact on that front. PGO might have an impact but we haven't added that yet.

LTO should not have that much of an impact on that front. PGO might have an impact but we haven't added that yet.

Ah sorry this is just about LTO, should be fine so far then