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From: chris@mimsy.UUCP (Chris Torek)
Newsgroups: comp.sys.ibm.pc,comp.lang.c
Subject: Re: TC2.0 bugfree ?
Message-ID: <15560@mimsy.UUCP>
Date: 20 Jan 89 21:20:38 GMT
References: <3785@druwy.ATT.COM> <16674@iuvax.cs.indiana.edu>
Organization: U of Maryland, Dept. of Computer Science, Coll. Pk., MD 20742
Lines: 60

In article <16674@iuvax.cs.indiana.edu> bobmon@iuvax.cs.indiana.edu
(RAMontante) writes:
>I am cross-posting the following to comp.lang.c, because the language
>expertise is there.  I am not convinced that you have any guarantee about
>just when the post-increments happen -- consider p.50 of K&R 1st edition,
>for example.  So it's not a TC bug, just an implementor's choice.

This is correct.  The pANS uses the notion of `sequence points' to
decide when a side effect (such as post-increment) must happen; there
is no sequence point within a simple assignment expression like the
one quoted below.

Incidentally, the quoted assembly shows that TC2.0 (Turbo C, I suppose)
misses one possible optimisation.

>>	i = (*cp++ << 8) + *cp++;

cp is in the SI register at this point; the goal is to compute AX=[SI]<<8:

>>	mov	al,byte ptr [si]
>>	mov	ah,0
>>	mov	cl,8
>>	shl	ax,cl

A much better sequence is

	mov	al,0
	mov	ah,byte ptr [si]

A simple peephole optimiser should be able to catch this.  More complex
analysis might even figure out that

	i = (cp[0] << 8) + cp[1];
	cp += 2;

could be computed as

	mov	ah,byte ptr [si]
	inc	si
	mov	al,byte ptr [si]
	inc	si
	; i is now in ax

while

	i = cp[0] + (cp[1] << 8);

is even more simple:

	mov	ax,word ptr [si]

but I would not expect most compilers to manage either of these.  (If
you rearranged the source to read

	i = *(int *)cp;

I would expect the latter sequence.)
-- 
In-Real-Life: Chris Torek, Univ of MD Comp Sci Dept (+1 301 454 7163)
Domain:	chris@mimsy.umd.edu	Path:	uunet!mimsy!chris