Re:Thriving in a Crowded and Changing World: C++ 2006–2020 [...
by _huayra_ on Saturday July 17, 2021 @01:35AM
(#27863729)
Yes when you can compile from scratch it does not matter anymore, but as you mentioned it has mostly to do with software vendors (OS distros, proprietary apps, etc). There are things that are basically disallowed for upgrades in the standard library because certain things (e.g. class member layout, default vs. empty user-defined constructor, such as the register promotion issue when pair<> was '=default' from the empty constructor).
I think Marshall Clow gives a pretty good overview of the ABI issues at a high level [0].
When the compiler can see both the call site and the function definition (such as when the function is inline, or a template) it is free to ignore ABI.
If there is a break, it will either be wholly backward-compatible, so that new code can still call old functions, or be a whole separate build target, like the jump from x86 to amd64, with no possibility of linkage mistakes. (There must be people reading here who don't recall the move from 32- to 64-bit x86. It was wholly painless for programmers; not so much for OS distributors.) Imagine building for ARM today; you are at absolutely no risk of accidentally linking x86 code.