[abainbridge]

I think the exception code _is_ loaded. It is only a theoretical possibility that loading it could be avoided.

I just built the following code with g++ v7.4 (from MSYS64 on Windows):

    #include <math.h>
    #include <stdexcept>

    void exitWithMessageException() {
        if (random() == 4321)
            throw std::runtime_error("Halt! Who goes there?");
    }

    int main() {
        exitWithMessageException();
        return 1234;
    }

The generated code mixed the exception handlers with the hot-path code. Here are the address ranges of relevant chunks:

    100401080 - Hot path of exitWithMessageException
    100401099

    10040109a - Cold path of exitWithMessageException
    10040113f

    100401140 - Start of main

[gpderetta]

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

[abainbridge]

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?

[gpderetta]

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