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Interesting, GCC 7.x seems to simply puts the cold branch on a separate nop-padded cacheline.

GCC 9 [1] instead moves the exception throwing branch into a cold clone of exitWithMessageException function. The behaviour seems to have changed on starting from GCC 8.x.

[1] https://godbolt.org/z/PKKZ8m

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Ooo, fancy. There is still a long way from just that to actually getting the savings in a real program running on a real operating system. For example, if I have thousands of source files, each with a few hundred bytes of cold exception handlers, do they get coalesced into a single block for the whole program?

coalescing functions in the same section should be the linker job, yes.

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