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Bee O'Problem

@aeva @mdiluz but where does the call to unreachable come from? Your logic only explains the loop being optimized out unless I'm missing something.

9 Feb 2023 at 3:14 | Wall-to-wall | Open on mastodon.gamedev.place
6 comments
RAOF

@beeoproblem @aeva @mdiluz The reasoning I've seen is:
You generate code for main() and unreachable(). You use the UB to dead-code eliminate main(). Now the code for unreachable() is where the code for main() was; ie: it has become the entrypoint.

9 Feb 2023 at 3:32 | Open on toot.cat
aeva

@RAOF @beeoproblem @mdiluz if you're missing a main function other functions don't get promoted to main. I tested it out in godbolt earlier, what happens is the ret instruction in main gets optimized out, thus the program counter just advances to the next instruction in memory which is the other function. The entry points for main and the other function still exist.

9 Feb 2023 at 4:14 | Open on mastodon.gamedev.place
aeva

@RAOF @beeoproblem @mdiluz gcc and msvc both do what you'd expect. Clang and zig's c++ compiler both optimize away the loop and the return.

9 Feb 2023 at 4:15 | Open on mastodon.gamedev.place
Marqin

@beeoproblem @aeva @mdiluz the return call from main() got optimised away and so the CPU doesn’t jump away, and keeps taking further instructions from the assembly, which are the body of unreachable()

9 Feb 2023 at 3:45 | Open on mastodon.gamedev.place
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