addc/subc optimizations

[chfast]

Use compiler builtins to implement addc and subc primitives. This mostly benefits GCC (clang is able to match respective patterns without builtins).

[codecov-commenter]

Codecov Report

Merging #253 (c348241) into master (b05e8a1) will not change coverage. The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master      #253   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           10        10           
  Lines         1911      1958   +47     
=========================================
+ Hits          1911      1958   +47     

Flag	Coverage Δ
32bit	100.00% <100.00%> (ø)
gcc	99.48% <92.42%> (-0.52%)	:arrow_down:

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
test/unittests/test_builtins.cpp	100.00% <ø> (ø)
test/unittests/test_uint256.cpp	100.00% <ø> (ø)
include/intx/intx.hpp	100.00% <100.00%> (ø)
test/experimental/addmod.hpp	100.00% <100.00%> (ø)

[yperbasis]

According to Wiki, there are 2 different conventions for subtraction: carry & borrow. x86 uses borrow & ARM uses carry. I'm not sure which convention __builtin_subcll uses. Is there good documentation for it anywhere?

[chfast]

According to Wiki, there are 2 different conventions for subtraction: carry & borrow. x86 uses borrow & ARM uses carry. I'm not sure which convention __builtin_subcll uses. Is there good documentation for it anywhere?

Oh, interesting. This is "sub with borrow" then although the name may suggest otherwise. And looks this is not very well optimized on ARM maybe because of the reversed carry bit.

https://godbolt.org/z/sPsahE4f9