Path: utzoo!utgpu!water!watmath!clyde!att!osu-cis!tut.cis.ohio-state.edu!mailrus!ames!amdahl!pyramid!prls!mips!earl
From: earl@mips.COM (Earl Killian)
Newsgroups: comp.arch
Subject: Re: CISC instructions
Message-ID: <2912@wright.mips.COM>
Date: 28 Aug 88 06:52:05 GMT
References: <13254@mimsy.UUCP>
Lines: 70

In article <13254@mimsy.UUCP>, chris@mimsy.UUCP (Chris Torek) writes:
[much analysis of VAX CALLS instruction -- a classic CISC mistake]
...
> If I am allowed to avoid the standard stack frame format, I can cut the
> time to 7.1 seconds:
> 
> 	/* modified open coded call */
> 		.globl	_null
> 	_null:	movl	sp,fp		# build frame
> 		rsb
> 
> 		.globl	_main
> 	_main:	.word	0
> 		movl	$1000000,r11
> 	0:	movq	ap,-(sp)	# save ap, fp
> 		moval	4(sp),ap	# new ap
> 		jsb	_null		# call
> 		movq	(sp)+,ap	# restore ap, fp
> 		sobgtr	r11,0b
> 		ret
> 
> This is somewhat less realistic as no registers are saved, and none
> restored; if `null' were to use some, it would have to read:
> 
> 	_null:	pushr	$mask		# save local registers
> 		movl	sp,fp		# build frame
> 		/* body */
> 		movl	fp,sp		# set up for return
> 		popr	$mask		# restore registers
> 		rsb
> 
> which adds three instructions, two of them relatively slow (pushr and
> popr), changing the time (for mask=0) to 8.7 seconds.

You can do better than this.  Here's the output from a compiler that's
been around for 4 years or so, using its -fast_call option:

/* pastel 2.3 compiled test.p on 27 August 88 22:16 PST by earl
   4 statements, 8 instructions in 29 bytes, 0 static bytes */
.globl _a
.globl _pascal_runtime__main_end
.globl _pascal_runtime__main_start

_a:
 #   1	procedure a();
	rsb

 #   5	program test;
.globl _main
_main:
	jsb _pascal_runtime__main_start/* ?, main_start */
 #   9	    for i := 1000000 downto 1 do begin
	movl $1000000,r2		/* ?, i */
L107:
 #  10	      a();
	bsbb _a
	decl r2				/* i */
	bneq L107
	jsb _pascal_runtime__main_end/* ?, main_end */
	rsb

On a VAX 780 this is 3.3 seconds, whereas yours is 9.3 seconds, or
2.8x slower.  You mistakenly assumed you need a frame pointer, 
and used an argument pointer, both of which the compiler-generated
code avoided.  No registers saved is actually quite realistic, when
the call protocol uses a mix of both callee and caller-saved
registers.
-- 
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