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From: "segher at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug target/97437] builtins subcarry and addcarry still not generate
 the right code. Not get optimized to immediate value
Date: Thu, 15 Oct 2020 13:57:41 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D97437

--- Comment #5 from Segher Boessenkool <segher at gcc dot gnu.org> ---
Trying 7 -> 9:
    7: r97:SI=3D0x2a
    9: {flags:CCC=3Dcmp(r97:SI+r98:SI,r97:SI);r99:SI=3Dr97:SI+r98:SI;}
      REG_DEAD r98:SI
      REG_DEAD r97:SI
Failed to match this instruction:
(parallel [
        (set (reg:CC 17 flags)
            (compare:CC (reg:SI 98 [ *b_12(D) ])
                (const_int -42 [0xffffffffffffffd6])))
        (set (reg:SI 99)
            (plus:SI (reg:SI 98 [ *b_12(D) ])
                (const_int 42 [0x2a])))
    ])

On rs6000 we have four special variants for the immediate add-with-carry
insn patterns: imm 0, imm -1, imm pos, imm neg.  All of these have
different canonical RTL.=