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Re: Possible CSE quirk involving SUBREG on the i386
- To: john at feith dot com (John Wehle)
- Subject: Re: Possible CSE quirk involving SUBREG on the i386
- From: Jeffrey A Law <law at hurl dot cygnus dot com>
- Date: Tue, 09 Mar 1999 02:09:28 -0700
- cc: egcs at egcs dot cygnus dot com
- Reply-To: law at cygnus dot 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
>
> 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