Path: utzoo!mnetor!uunet!husc6!hao!oddjob!gargoyle!ihnp4!occrsh!uokmax!rmtodd
From: rmtodd@uokmax.UUCP (Richard Michael Todd)
Newsgroups: comp.lang.c
Subject: Re: noalias; parens honored
Message-ID: <978@uokmax.UUCP>
Date: 19 Dec 87 04:17:59 GMT
References: <6829@brl-smoke.ARPA> <2845@zeus.TEK.COM>
Reply-To: rmtodd@uokmax.UUCP ()
Organization: University of Oklahoma, Norman, OK
Lines: 118
Keywords: ANSI C standard
Summary: Some compilers are capable of doing such optimizations

In article <2845@zeus.TEK.COM> dant@tekla.UUCP (Dan Tilque) writes:
>It also seems pretty late in the process to be making major changes like
>adding keywords.  Is there any current compiler (i.e. prior art) which
>uses 'noalias'?  If so, how much better is the code generated by it?
  I don't know of any compiler that uses the 'noalias' keyword, but both Aztec
and Turbo C for the PC have a command line option to specify this type
of 'noalias' optimization for an entire file. For Aztec it's '+a', for Turbo
it's '-Z'.  
  As for how well it works, I didn't know offhand, but I just did a little
expirementing with Aztec C, compiling some C files both with and without
the +a optimization, and running diff on the assembly code output.  The files
I tried came from MicroEmacs 3.9 (I guess that counts as "real world code" :-)
I tested three files in all; the first two showed no difference whatsoever
with the optimization on and off.  The third file I tried, buffer.c, showed
some optimization at various places.  For the overly-curious, part of the 
diff listing between the files is shown below.  The part marked with '<' is
the no-optimization version, the part marked with '>' is the version with
the no-alias optimization turned on.  Mostly the optimizations look to be
simply the result of the compiler keeping better track of stuff left in
registers by previous expressions' code.
1148,1149c1140,1141
< 	push	 word ptr blistp_+2
< 	push	 word ptr blistp_
---
> 	push	es
> 	push	bx
1805,1806c1797,1798
< ^	mov	dx,word ptr -2[bp]
< 	mov	ax,word ptr -4[bp]
---
> ^	mov	dx,es
> 	mov	ax,bx
1821,1827c1813,1817
< 	mov	ax,es:word ptr 4[bx]
< 	mov	dx,es:word ptr 6[bx]
< 	mov	cx,es:word ptr 16[bx]
< 	mov	bx,es:word ptr 18[bx]
< 	cmp	dx,bx
< 	jne	$20067
< 	cmp	ax,cx
---
> 	mov	cx,es:word ptr 4[bx]
> 	mov	bx,es:word ptr 6[bx]
> 	cmp	bx,dx
> 	jne	$20067
> 	cmp	cx,ax
2073,2074c2063,2064
< ^	mov	dx,word ptr -2[bp]
< 	mov	ax,word ptr -4[bp]
---
> ^	mov	dx,es
> 	mov	ax,bx
2125,2126c2115,2116
< ^	mov	dx,word ptr -2[bp]
< 	mov	ax,word ptr -4[bp]
---
> ^	mov	dx,es
> 	mov	ax,bx
2140d2129
< 	les	bx,dword ptr -4[bp]
2144,2155c2133,2140
< ^	les	bx,dword ptr -4[bp]
< 	mov	es:word ptr 8[bx],00H
< ;                bp->b_markp = NULL;
< ^	les	bx,dword ptr -4[bp]
< 	mov	es:word ptr 12[bx],00H
< 	mov es:word ptr 10[bx],00H
< ;                bp->b_marko = 0;
< ^	les	bx,dword ptr -4[bp]
< 	mov	es:word ptr 14[bx],00H
< ;                bp->b_flag  = bflag;
< ^	mov	al,byte ptr 12[bp]
< 	les	bx,dword ptr -4[bp]
---
> ^	mov	es:word ptr 8[bx],00H
> ;                bp->b_markp = NULL;
> ^	mov	es:word ptr 12[bx],00H
> 	mov es:word ptr 10[bx],00H
> ;                bp->b_marko = 0;
> ^	mov	es:word ptr 14[bx],00H
> ;                bp->b_flag  = bflag;
> ^	mov	al,byte ptr 12[bp]
2159,2163c2144,2146
< 	les	bx,dword ptr -4[bp]
< 	mov	es:word ptr 23[bx],ax
< ;                bp->b_nwnd  = 0;
< ^	les	bx,dword ptr -4[bp]
< 	mov	es:byte ptr 21[bx],00H
---
> 	mov	es:word ptr 23[bx],ax
> ;                bp->b_nwnd  = 0;
> ^	mov	es:byte ptr 21[bx],00H
2167d2149
< 	les	bx,dword ptr -4[bp]
2174,2176c2156,2158
< 	mov	dx,word ptr -2[bp]
< 	mov	ax,word ptr -4[bp]
< 	add	ax,25
---
> 	add	bx,25
> 	mov	dx,es
> 	mov	ax,bx
2199c2181,2182
< 	les	bx,dword ptr -12[bp]
---
> 	mov	es,dx
> 	mov	bx,ax
2203,2205c2186,2187
< ^	mov	dx,word ptr -10[bp]
< 	mov	ax,word ptr -12[bp]
< 	les	bx,dword ptr -12[bp]
---
> ^	mov	dx,es
> 	mov	ax,bx
___________________________________________________________________________
Richard Todd
USSnail:820 Annie Court,Norman OK 73069
UUCP: {allegra!cbosgd|ihnp4}!occrsh!uokmax!rmtodd