C "optimization" (2 of 8)

Dan Klein dvk at mi-cec.UUCP
Wed Feb 15 05:20:32 AEST 1984

This is a continuation of my diatribe on "C doesn't optimize, it neatens".
In this and other articles, I compare a true optimizing compiler (Bliss-32
running under VMS) to a code neatener (C running under BSD 4.1c).  Any and
all counterexamples are welcome.  However, this is NOT a comparison of the
languages.  Both C and Bliss have their good and bad points.  This is simply
a comparison of the code they generate.  As in all examples, the source
code and uncensored assembly code is presented.  In all examples, the C source
and Bliss source are as nearly identical as language differences permit.  I
have not taken advantage of any "tricks" to get either language to perform
better or worse than the other.  The optimizer was enabled for both languages.

		-Dan Klein, Mellon Institute, Pittsburgh	(412)578-3382

In this example I show what the compilers do when given two blocks of code
that are nearly identical (i.e. they are "tail identical").  Only the head
is different (namely whether the first argument pushed to "alpha" is a 0
or a 1.  Both compilers behave well in that they share the tail identical code
between the two conditions.  The only complaint I have against C is that
it uses 4 PUSHL instructions to pass the remaining arguments, rather than a
more speed/space effifient two MOVQ instructions.  If you were sure you were
not going to run on an 11/730, you could further reduce this to a single MOVO
instruction, but that is an unreasonable request.
external routine alpha;			|	extern  int	alpha();
routine test(p,q,r,s,t) : NoValue =	|	test(p,q,r,s,t)
	begin				|	{
					|	    if (t!=0)
	if .t neq 0 then		|		alpha(p,q,r,s,0);
		alpha(.p,.q,.r,.s,0)	|	    else
	else				|		alpha(p,q,r,s,1);
		alpha(.p,.q,.r,.s,1);	|	}
	end;				|
	.TITLE  FOO			|		.data
					|		.text
	.EXTRN  ALPHA			|	LL0:	.align  1
					|		.globl  _test
	.PSECT  $CODE$,NOWRT,2		|		.set	L13,0x0
					|		.data
TEST:	.WORD	^M<>			|		.text
	TSTL	20(AP)			|	_test:  .word	L13
	BEQL	1$			|		tstl	20(ap)
	CLRL	-(SP)			|		jeql	L17
	BRB	2$			|		pushl	$0
1$:	PUSHL	#1			|		jbr	L200004
2$:	MOVQ	12(AP), -(SP)		|	L17:	pushl	$1
	MOVQ	4(AP), -(SP)		|	L200004:pushl	16(ap)
	CALLS	#5, W^ALPHA		|		pushl	12(ap)
	RET				|		pushl	8(ap)
					|		pushl	4(ap)
					|		calls	$5,_alpha
					|		ret

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