https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64003
--- Comment #2 from dmalcolm at gcc dot gnu.org ---
FWIW, line 6236 of i386.md is here:
6229 (define_insn "*subv<mode>4_1"
6230 [(set (reg:CCO FLAGS_REG)
6231 (eq:CCO (minus:<DWI>
6232 (sign_extend:<DWI>
6233 (match_operand:SWI 1 "nonimmediate_operand" "0"))
6234 (match_operand:<DWI> 3 "const_int_operand" "i"))
6235 (sign_extend:<DWI>
>>6236 (minus:SWI (match_dup 1)
6237 (match_operand:SWI 2
"x86_64_immediate_operand"
6238 "<i>")))))
6239 (set (match_operand:SWI 0 "nonimmediate_operand" "=<r>m")
6240 (minus:SWI (match_dup 1) (match_dup 2)))]
6241 "ix86_binary_operator_ok (MINUS, <MODE>mode, operands)
6242 && CONST_INT_P (operands[2])
6243 && INTVAL (operands[2]) == INTVAL (operands[3])"
6244 "sub{<imodesuffix>}\t{%2, %0|%0, %2}"
6245 [(set_attr "type" "alu")
6246 (set_attr "mode" "<MODE>")
6247 (set (attr "length_immediate")
6248 (cond [(match_test "IN_RANGE (INTVAL (operands[2]), -128,
127)")
6249 (const_string "1")
6250 (match_test "<MODE_SIZE> == 8")
6251 (const_string "4")]
6252 (const_string "<MODE_SIZE>")))])
