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Path: utzoo!utcsri!greg
From: greg@utcsri.UUCP (Gregory Smith)
Newsgroups: net.lang
Subject: Re: Re: Multiple assignments
Message-ID: <2883@utcsri.UUCP>
Date: Sun, 1-Jun-86 12:55:39 EDT
Article-I.D.: utcsri.2883
Posted: Sun Jun  1 12:55:39 1986
Date-Received: Sun, 1-Jun-86 13:32:31 EDT
References: <1455@mmintl.UUCP> <825@bentley.UUCP> <1482@mmintl.UUCP> <201@cdx39.UUCP> <2846@utcsri.UUCP> <1516@mmintl.UUCP>
Reply-To: greg@utcsri.UUCP (Gregory Smith)
Organization: CSRI, University of Toronto
Lines: 48
Summary: 

In article <1516@mmintl.UUCP> franka@mmintl.UUCP (Frank Adams) writes:
>In article <2846@utcsri.UUCP> greg@utcsri.UUCP (Gregory Smith) writes:
>>It becomes the same as
>>	x = 0;
>>	y = x;	/* actually y=(double)x */
>>
>>Are you suggesting that given (x,y)=0; it will be easier for a compiler
>>to detect the incompatibility and generate
>>	x = 0;
>>	y = 0.0;	????
>>I don't really think so.
>
>Yes, because "(x,y)=0;" is equivalent to "x=0; y=0;", unlike "x=y=0", which
>as you note is equivalent to "y=0; x=y;".

'Equivalent' in both cases meaning 'yielding the same result' and not nec.
'implemented as'. ( I'm sure you meant that too ).

>
>On the flip side, a naive compiler will generate less efficient code for
>something like:
>
>   int i, j;
>   float a;
>   (i,j)=a;
>
>than it would when the last line is "i=j=a"; it will do the float to int
>conversion twice.

It would screw up  because it would compile i=a;j=a;. But what about
(i,j) =array[ func(foo-bar)+i]*17? If a compiler treated *that* as two
statements, it would be more than just naive.
So the code generator would have to take a look at the RHS to see if
splitting the assignment in two is a good idea or not.  My point is
that the same decision can be made with the C format of i=j=...
So the problem is not inherent in 'i=j=' - the problem is that few
( if any ) compilers go to this trouble.
I will admit that the semantics of i=j= can get funny when i and j are of
different types -especially when j is fewer bits than j.
>
>Frank Adams                           ihnp4!philabs!pwa-b!mmintl!franka



-- 
"We demand rigidly defined areas of doubt and uncertainty!" - Vroomfondel
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Greg Smith     University of Toronto      UUCP: ..utzoo!utcsri!greg