[Bug target/70008] New: [ARM] Reverse subtract with carry can be generated in thumb2 mode

[michael.collison at linaro dot org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70008

            Bug ID: 70008
           Summary: [ARM] Reverse subtract with carry can be generated in
                    thumb2 mode
           Product: gcc
           Version: 6.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: michael.collison at linaro dot org
  Target Milestone: ---

The following pattern in arm.md allows the 'rsc' instruction to be generated in
thumb2 mode.

(define_insn "*subsi3_carryin"
  [(set (match_operand:SI 0 "s_register_operand" "=r,r")
        (minus:SI (minus:SI (match_operand:SI 1 "reg_or_int_operand" "r,I")
                            (match_operand:SI 2 "s_register_operand" "r,r"))
                  (ltu:SI (reg:CC_C CC_REGNUM) (const_int 0))))]
  "TARGET_32BIT"
  "@
   sbc%?\\t%0, %1, %2
   rsc%?\\t%0, %2, %1"

TARGET_32BIT includes the thumb2 architecture which does does support the
reverse subtract with carry (rsc) instruction.

I will post a patch upstream after testing.