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From: tim@nucleus.amd.com (Tim Olson)
Newsgroups: comp.lang.c
Subject: Re: Compilers and programming style (was Re: A question of style)
Message-ID: <28716@amdcad.AMD.COM>
Date: 9 Jan 90 15:19:03 GMT
References: <1989Dec22.100135.2903@gdt.bath.ac.uk> <4367@rtech.rtech.com> <1989Dec31.153241.16479@gdt.bath.ac.uk> <649@codonics.COM> <1990Jan5.100042.26760@gdt.bath.ac.uk> <2283@dataio.Data-IO.COM> <534@bbxsda.UUCP>
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Reply-To: tim@amd.com (Tim Olson)
Organization: Advanced Micro Devices, Inc., Austin, Texas
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In article <534@bbxsda.UUCP> scott@bbxsda.UUCP (Scott Amspoker) writes:
| In article <2283@dataio.Data-IO.COM> bright@dataio.Data-IO.COM (Walter Bright) writes:
| >volatile int *p;
| >*p;		/* has side effects and must not be optimized away	*/
| 
| Just curious - what kind of code should be generated for the above
| C fragment?

The integer pointed to by p should be accessed from memory and thrown
away (this may be useful in talking to certain peripheral devices).
For example:

w "t.c",L3/C3:  q: Expression has no side-effects.

;f(int *q, volatile int *p)
;{
;       *q;
;       *p;
        load    0,0,gr96,lr3
;}      jmpi    lr0
        nop


The expression "*q;" generated no code and produced a compiler warning
that it had no side-effects.  However, the expression "*p;", since p
was declared as a pointer to a volatile object, generates an access to
that object which is then subsequently thrown away.


	-- Tim Olson
	Advanced Micro Devices
	(tim@amd.com)