just_select in combine.c:force_to_mode

[SenthilKumar.Selvaraj]

Hi,

  When debugging PR 88253, I found that force_to_mode uses a parameter
  (just_select) to prevent the function from returning a const0_rtx even
  if none of the bits set by the rtx are needed. The comment says

   "If JUST_SELECT is nonzero, don't optimize by noticing that bits in MASK
   are all off in X.  This is used when X will be complemented, by either
   NOT, NEG, or XOR."

   and the code behaves the same way, but could someone help me
   understand why?

   I ran into this, when I found that force_to_mode converts
   
   (ior:QI (subreg:QI (ior:HI (ashift:HI (zero_extend:HI (reg/v:QI 46 [ h ]))
                (const_int 8 [0x8]))                                            
           
            (reg:HI 55 [ D.1627 ])) 0)
    (const_int 42 [0x2a]))

    into

   (set (reg:QI 44 [ D.1626 ])
      (ior:QI (subreg:QI (reg:HI 55 [ D.1627 ]) 0)
          (const_int 42 [0x2a])))

   but is unable to do the same thing for the below rtx.

   (xor:QI (subreg:QI (ior:HI (ashift:HI (zero_extend:HI (reg/v:QI 46 [ h ]))   
              
                (const_int 8 [0x8]))                                            
           
            (reg:HI 55 [ D.1627 ])) 0)                                          
           
    (reg:QI 58))  

   The only difference is the xor instead of ior at the outermost sexp,
   and force_to_mode returns

   (set (reg:QI 44 [ D.1626 ])                                                  
              
    (xor:QI (ior:QI (subreg:QI (ashift:HI (zero_extend:HI (reg/v:QI 46 [ h ]))  
           
                    (const_int 8 [0x8])) 0)                                     
           
            (subreg:QI (reg:HI 55 [ D.1627 ]) 0))                               
           
        (reg:QI 58)))

   This more complicated pattern doesn't match, and combine
   moves on to another combination of insns, eventually resulting in PR 88253.

   Isn't the simplification of 

   (subreg:QI (ior:HI (ashift:HI (zero_extend:HI (reg/v:QI 46 [ h ]))           
      
                (const_int 8 [0x8]))                                            
           
            (reg:HI 55 [ D.1627 ])) 

   to

   (subreg:QI (reg:HI 55 [ D.1627 ])

   safe to do, wheter the outer insn code is XOR or IOR?

Regards
Senthil