Re: Possible CSE quirk involving SUBREG on the i386

[Jeffrey A Law <law@cygnus.com>]

  In message < 199806300534.BAA27602@jwlab.FEITH.COM >you write:
  > Compiling:
  > 
  >   unsigned short c;
  > 
  >   int
  >   func(unsigned short a)
  >     {
  >     unsigned short b;
  >  
  >     b = a;
  >     c = b;
  > 
  >     return b;
  >     }
  > 
  > for the i386 using egcs current with -O -S yields:
  > 
  > _func:
  > 	pushl %ebp
  > 	movl %esp,%ebp
  > 	movl 8(%ebp),%eax
  > 	movl %eax,%edx
  > 	movw %ax,_c
  > 	movzwl %dx,%eax
  > 	leave
  > 	ret
  > 
  > Which is interesting since I expected:
  > 
  > _func:
  > 	pushl %ebp
  > 	movl %esp,%ebp
  > 	movl 8(%ebp),%eax
  > 	movw %ax,_c
  > 	movzwl %ax,%eax
  > 	leave
  > 	ret
I'm not sure this is really a CSE issue.  Though it might be a combine
issue.

;; Start of basic block 0, registers live: 6 [bp] 16 []
(insn 4 2 6 (set (reg:SI 22)
        (mem:SI (reg:SI 16 %argp))) 54 {movsi+2} (nil)
    (expr_list:REG_EQUIV (mem:SI (reg:SI 16 %argp))
        (nil)))
        
(insn 6 4 7 (set (reg/v:HI 21)
        (subreg:HI (reg:SI 22) 0)) 58 {movhi+1} (insn_list 4 (nil))
    (nil))

(note 7 6 8 "" NOTE_INSN_FUNCTION_BEG)

(note 8 7 10 "" NOTE_INSN_DELETED)

(note 10 8 16 0 NOTE_INSN_BLOCK_BEG)

(insn 16 10 18 (set (mem:HI (symbol_ref:SI ("c")))
        (subreg:HI (reg:SI 22) 0)) 58 {movhi+1} (nil)
    (expr_list:REG_DEAD (reg:SI 22)
        (nil)))

(insn 18 16 20 (set (reg:SI 24)
        (zero_extend:SI (reg/v:HI 21))) 86 {zero_extendhisi2} (insn_list 6 (nil))
    (expr_list:REG_DEAD (reg/v:HI 21)
        (nil)))

If we were to combine insns 6 & 18 I think we'd get the code you want.

I don't know why combine isn't doing this.

jeff