[Bug target/79173] New: add-with-carry and subtract-with-borrow support (x86_64 and others)

["vincent-gcc at vinc17 dot net" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=79173

            Bug ID: 79173
           Summary: add-with-carry and subtract-with-borrow support
                    (x86_64 and others)
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: vincent-gcc at vinc17 dot net
  Target Milestone: ---

There should be a way to support full add-with-carry and subtract-with-borrow
by generating adc / sbb instructions on x86_64 (and similar instructions on
other targets).

GCC could add builtins, such as __builtin_addc* and __builtin_subc* (two
arguments, carry in, carry out, and the result), similar to Clang:
http://clang.llvm.org/docs/LanguageExtensions.html#multiprecision-arithmetic-builtins
as suggested in PR 60206 comment 3.

Detection of special constructs in standard C/... code would be useful too.
Here are some examples from
https://gcc.gnu.org/ml/gcc-help/2017-01/msg00067.html for subtraction:

typedef unsigned long T;

void sub1 (T *p, T u0, T u1, T u2, T v0, T v1, T v2)
{
  T t1;
  int b0, b1;

  p[0] = u0 - v0;
  b0 = u0 < v0;
  t1 = u1 - v1;
  b1 = u1 < v1;
  p[1] = t1 - b0;
  b1 |= p[1] > t1;
  p[2] = u2 - v2 - b1;
}

void sub2 (T *p, T u0, T u1, T u2, T v0, T v1, T v2)
{
  int b0, b1;

  p[0] = u0 - v0;
  b0 = u0 < v0;
  p[1] = u1 - v1 - b0;
  b1 = u1 < v1 || (u1 == v1 && b0 != 0);
  p[2] = u2 - v2 - b1;
}

In the second example, the b1 line could also be replaced by:

  b1 = u1 < v1 + b0 || v1 + b0 < v1;

For the subtractions, optimal code would contain 1 sub and 2 sbb's.