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Comment by ketanmaheshwari | Hacker News Reader

I am personally interested in the code amalgamation technique that SQLite uses[0]. It seems like a free 5-10% performance improvement as is claimed by SQLite folks. Be nice if he addresses it some in one of the sessions.

[0] https://sqlite.org/amalgamation.html

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This is a pretty standard topic, and not really a compiler optimization. It's usually called a unity build.

[0] https://en.wikipedia.org/wiki/Unity_build

Unity builds have been largely supplanted by LTO. They still have uses for build time improvements in one-off builds, as LTO on a non-incremental build is usually slower than the equivalent unity build.

At my company, we have not seen any performance benefits from LTO on a GCC cross-compiled Qt application.

GCC version: 11.3 target: Cortex-A9 Qt version: 5.15

I think we tested single core and quad core, also possibly a newer GCC version, but I'm not sure. Just wanted to add my two cents.

I would expect a little benefit from devirt (but maybe in-TU optimizations are getting that already?), but if a program is pessimized enough, LTO's improvements won't be measurable.

And programs full of pointer-chasing are quite pessimized; highly-OO code is a common example, which includes almost all GUIs, even in C++.

Do you link against a version of the Qt library that provides IR objects?

In any case even with whole program optimization, O would expect that effectively devirtualizing an heavily object oriented application to be very hard.

For those of you playing at home, LTO is link-time optimization.

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