Thursday, March 31, 2022

Clang 14 faster at building LibreOffice

 So I've updated my Clang to the newly released version 14, and while doing the LibreOffice rebuild afterwards I noticed that it seemed to build faster. I didn't measure it, but the build finished sooner than I expected. So of course I've measured it.

As a simple reference I used my year-old post about Clang 11 building faster with PCH, where Calc'c Library_sc built in 4 minutes and 39 seconds. And indeed now it's faster:

 1105.84user 88.35system 2:55.07elapsed 682%CPU (0avgtext+0avgdata 1666576maxresident)k
 180904inputs+2272520outputs (41390major+23529938minor)pagefaults 0swaps

Just to make sure it's not something else, I went back to Clang 13 to test it:

 1581.76user 93.84system 4:14.75elapsed 657%CPU (0avgtext+0avgdata 1916380maxresident)k
 153912inputs+2316696outputs (41891major+27245569minor)pagefaults 0swaps

So yes, it's real, and it's the compiler. It also suggests that there was an improvement also between Clang 11 and Clang 13, although not as noticeable as this.

I have no idea why that is, it seems too big of a difference to be just something random, but I see nothing relevant in the release notes (and it's not DWARF5, I tested that one). I also have no idea if it's a code change or if it's the compiler being faster because it generates faster code and is self-built. But hey, it's nice. I still vaguely remember the times when I was trying to avoid full Calc rebuilds like a plague, but that seems like a long time ago.

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