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Issue 386 - switch mingw32 to dwarf-2

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Petr Bokoč 2020-04-14 16:44:08 +02:00
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@ -11,3 +11,20 @@ A new lab called Comp-NeuroFedora has been added to the suite. This change provi
For more information about Comp-NeuroFedora, see the link:https://neuro.fedoraproject.org[upstream documentation].
For a list of Fedora labs, see the link:https://labs.fedoraproject.org/[Functional bundles for Fedora] page.
== Switch mingw32 toolchain to dwarf-2 exceptions
The `mingw32` toolchain in Fedora 32 uses the `dwarf-2` exception model instead of the previously used `SJLJ`.
Notable differences include:
* `SJLJ` (`setjmp`/`longjmp`):
** Not "zero-cost": even if an exception isn't thrown, it incurs a minor performance penalty (~15% in exception heavy code).
** Allows exceptions to traverse through e.g. windows callbacks.
* `DWARF` (`DW2`, `dwarf-2`)
** No permanent runtime overhead.
** needs whole call stack to be `dwarf`-enabled, which means exceptions cannot be thrown over e.g. Windows system DLLs (i.e. throwing an exception in a system DLL callback and attempting to catch it won't work)
** DW2 potentially generates bigger libraries. The overhead however is not big (< 10%) for typical applications.
The main reason for switching to `dwarf-2` is that Rust can only be compiled to a link:https://github.com/rust-lang/rust/issues/12859#issuecomment-185081071[MinGW toolchain targeting dwarf exceptions on 32-bit], and Rust usage is starting to appear in some packages (i.e. `librsvg2`).
Switching to `dwarf-2` on `mingw32` would hence allow to keep the same consistent package offering between `mingw32` and `mingw64`, whereas otherwise one would need to either freeze the `mingw32` variants at older versions, or remove them altogether.