The default precision which is used for converting floating point numbers to strings leads to many confusing results. If we take a Float32 1.00000000 value and 1.00000012 of the same type, these two, obviously are not equal. However, if we log them, we are displayed the same value. So a much more helpful display using 9 decimal digits is thus: [1.00000000 != 1.00000012] showing that the two values are in fact different.
(example taken from: http://www.boost.org/doc/libs/1_59_0/libs/test/doc/html/boost_test/test_output/log_floating_points.html)
I'm by no means a floating point number expert, however having investigated this issue I found numerous sources saying that "magic" numbers 9 and 17 for 32 and 64 bit values respectively are the correct format. Numbers 9 and 17 represent the maximum number of decimal digits that round trips. This means that number 0.100000000000000005 and 0.1000000000000000 are the same as their floating-point representations are concerned.
The C++ ABI for static locals is a bit heavy compared to dispatch_once; doing this saves more than 1KB in runtime code size. Dispatch_once/call_once is also more likely to be hot because it's also used by Swift and ObjC code.
Alas, llvm::get_execution_seed() from llvm/ADT/Hashing.h still inflicts one static local initialization on us we can't override (without forking Hashing.h, anyway).
This allows removal of the DebugDescription protocol which is invalid
because no classes actually conform to it. The problem is that we need
to send a debugDescription message to an NSObject without loading
Foundation. This is exactly what shims are for. A very simple shim
solves the problem.
Move the ObjC internal declarations to a public runtime header so they can be shared, and rename _swift_deallocClassInstance to the more descriptive name swift_rootObjCDealloc (and make it only available with ObjC interop).
Set up a separate libSwiftStubs.a archive for C++ stub functionality that's needed by the standard library but not part of the core runtime interface. Seed it with the Stubs.cpp and LibcShims.cpp files, which consist only of stubs, though a few stubs are still strewn across the runtime code base.
