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

[glisse at gcc dot gnu.org]

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

--- Comment #1 from Marc Glisse <glisse at gcc dot gnu.org> ---
We could start with the simpler:

void f(unsigned*__restrict__ r,unsigned*__restrict__ s,unsigned a,unsigned
b,unsigned c, unsigned d){
  *r=a+b;
  *s=c+d+(*r<a);
}

After combine, we have:

(insn 34 12 20 2 (set (reg:SI 93 [ _15+4 ])
        (ltu:SI (reg:CCC 17 flags)
            (const_int 0 [0]))) 608 {*setcc_si_1_movzbl}
     (expr_list:REG_DEAD (reg:CCC 17 flags)
        (nil)))

(insn 21 20 22 2 (parallel [
            (set (reg:SI 102)
                (plus:SI (reg:SI 37 r8 [ c ])
                    (reg:SI 38 r9 [ d ])))
            (clobber (reg:CC 17 flags))
        ]) "a.c":3 213 {*addsi_1}
     (expr_list:REG_DEAD (reg:SI 38 r9 [ d ])
        (expr_list:REG_UNUSED (reg:CC 17 flags)
            (expr_list:REG_DEAD (reg:SI 37 r8 [ c ])
                (nil)))))

(insn 25 24 26 2 (parallel [
            (set (reg:SI 105)
                (plus:SI (reg:SI 102)
                    (reg:SI 93 [ _15+4 ])))
            (clobber (reg:CC 17 flags))
        ]) "a.c":3 213 {*addsi_1}
     (expr_list:REG_DEAD (reg:SI 93 [ _15+4 ])
        (expr_list:REG_UNUSED (reg:CC 17 flags)
            (expr_list:REG_DEAD (reg:SI 102)
                (nil)))))

The combine dump says "Trying 21, 34 -> 25:" but the next line is blank and it
moves on to trying something else.

If I use parentheses *s=c+(d+(*r<a)); and disable reassoc, I get:

Trying 23, 24 -> 25:
Successfully matched this instruction:
(parallel [
        (set (reg:SI 105)
            (plus:SI (plus:SI (ltu:SI (reg:CCC 17 flags)
                        (const_int 0 [0]))
                    (reg:SI 37 r8 [ c ]))
                (reg:SI 38 r9 [ d ])))
        (clobber (reg:CC 17 flags))
    ])
Instruction not appropriate for target.

I don't know where that target restriction is coming from, but at least we seem
to be getting somewhere.

If I remove c and keep *s=d+(*r<a), I get

Failed to match this instruction:
(parallel [
        (set (reg:SI 103)
            (plus:SI (ltu:SI (reg:CCC 17 flags)
                    (const_int 0 [0]))
                (reg:SI 38 r9 [ d ])))
        (clobber (reg:CC 17 flags))
    ])
Failed to match this instruction:
(set (reg:SI 103)
    (plus:SI (ltu:SI (reg:CCC 17 flags)
            (const_int 0 [0]))
        (reg:SI 38 r9 [ d ])))

we would probably need a special pattern for this case, virtually adding 0.