Changes/Mingw32GccDwarf2

Switch mingw32 toolchain to dwarf-2 exceptions

Summary

Switch the mingw32 toolchain to dwarf-2 exceptions, instead of SJLJ.

Owner

• Name: Sandro Mani
• Email: manisandro@gmail.com
• Release notes owner:

Current status

• Targeted release: Fedora 32
• Last updated: 2019-09-23
• Tracker bug: #1754660
• Release notes tracker: #386

Detailed Description

The two exception modesl supported by mingw32 targets are SJLJ (currently used in Fedora, and still default upstream), and the more recent dwarf-2. According to various sources [1][2][3], the key differences between the two are:

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 MinGW toolchain targeting dwarf exceptions on 32-bit [4], 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.

• [1] https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=540782
• [2] https://sourceforge.net/p/mingw-w64/mailman/message/30532139/
• [3] https://wiki.qt.io/MinGW-64-bit#Exception_handling:_SJLJ.2C_DWARF.2C_and_SEH
• [4] https://github.com/rust-lang/rust/issues/12859#issuecomment-185081071

Benefit to Fedora

Continue being able to ship the same package set on mingw32 as well as mingw64.

Scope

• Proposal owners:

GCC for mingw32 will be built with --disable-sjlj-exceptions --with-dwarf2 and all mingw-* packages rebuilt.

• Other developers:

None

• Release engineering: Rebuild in a side-tag
• Policies and guidelines: No policies need to be changed

Upgrade/compatibility impact

No impact

Contingency Plan

• Contingency mechanism: Revert to SJLJ
• Contingency deadline: Before release
• Blocks release? Yes
• Blocks product? No

Documentation

None

Release Notes

The mingw32 toolchain in Fedora 32 uses the dwarf-2 exception model instead of SJLJ.