Problem-statement
-----------------

        Extend spim1.md  file by  writing machine descriptions  for left
        shift and subtract.

Procedure
---------

        1) Study addsi3 RTL pattern. The pattern of shift instruction is
           similar to addsi3.
        2) Write  a new  RTL  pattern using  define_insn  with the  name
           ashlsi3 that follows the structure of the pattern for addsi3.
           Use the RTL  operator `ashift' in place of  `plus' and modify
           the  predicates  appropriately  (if required).  The  assembly
           instruction format for left shift is given below.
           
           SLL: Shift left logical (Shifts a  register value left by the
                shift amount  listed in  the instruction and  places the
                result in a third register. Zeroes are shifted in.)

                Syntax: sll reg1, reg2, const 
                Semantics: reg1 = reg2 << const

           SLLV: Shift left  logical variable  (Shifts a  register value
                 left by the  value in a second register  and places the
                 result in a third register. Zeroes are shifted in.)
           
                 Syntax: sllv reg1, reg2, reg3
                 Semantics: reg1 = reg2 << reg3 
           

        3) Write  a new  RTL  pattern using  define_insn  with the  name
           subsi3 that follows the structure  of the pattern for addsi3.
           Use the  RTL operator `minus'  in place of `plus'  and modify
           the  predicates  appropriately  (if required).  The  assembly
           format for subtract is given below.

           SUB:  Subtracts two  registers  and stores  the  result in  a
           register

                Syntax: sub reg1, reg2, reg3 
                Semantics: reg1 = reg2 - reg3
                  

        4) Build cc1 and compile the program given below. 

                int main()
                {
                        int a;
                        int b = a<<2;
                        a = b-a;
                        return 0;
                }       
                
           The expected assembly output is as follows
         
                .text
                .align 2
                .globl main
            main:
                sw      $ra,  0($sp)
                sw      $sp, -4($sp)
                sw      $fp, -8($sp)
                move    $fp,  $sp
                addi    $sp,  $fp, -36
                lw      $v0, -12($fp)
                sll     $v0,  $v0, 2
                sw      $v0, -8($fp)
                lw      $a0, -8($fp)
                lw      $v0, -12($fp)
                sub     $v0,  $a0, $v0
                sw      $v0, -12($fp)
                move    $v0,  $zero
                move    $sp,  $fp
                lw      $fp, -8($sp)
                lw      $ra,  0($sp)
                jr      $ra