Possible CSE quirk involving SUBREG on the i386

[Jeffrey A Law]

  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
  > 
  > The use of %edx seems wasteful.
Yes.  Wasteful.  Compiling this with the current sources yields:

func:
        pushl %ebp      # 32    movsi-2
        movl %esp,%ebp  # 34    movsi+2/1
        movzwl 8(%ebp),%eax     # 4     zero_extendhisi2+2/2
        movw %ax,c      # 16    movhi+1/1
        leave   # 37    leave
        ret     # 38    return_internal

Which is even better than what you expected ;-)

jeff