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Re: Use of Temporary( ) vs an Optimised Compiler [message #28197 is a reply to message #28130]

Mon, 26 November 2001 21:42

Craig Markwardt
Messages: 1869
Registered: November 1996

Senior Member

"Martin Downing" <martin.downing@ntlworld.com> writes:

> Here's a thought for the day:
> we have all had to get used to using the temporary function to enable memory
> efficient code. Some of us less effectively than others!
> ie: instead of
> a = 2*a + b/a
> write
> a = 2*a + b/TEMPORARY(a)
>
> Personally although good practice I find it makes code hard to read. Who
> agrees that this could and should be dealt with at the compilation stage,
> obviously if A is being reassigned then the previous contents are lost so
> the compiler could reuse A when processing the last copy of A on the right
> hand side. Would that be so hard for RSI to implement?

I agree, Martin. A compiler writer would know exactly when a variable
on the right hand side is being reassigned. However, your example
points out at least one of the problems.

> a = 2*a + b/TEMPORARY(a)

Since A appears twice on the right hand side, the compiler would need
to be smart enough to not overwrite A after its first appearance. In
fact, I am not sure that IDL makes any guarantees about order of
evaluation and side effects. Isn't it possible that the TEMPORARY()
gets called before the first A is evaluated? [ Not sure on this, but
that's why I avoid the situation. ]

Second of all, if the compiler automatically TEMPORARY()'ed every
variable that was reassigned, it makes debugging harder. What if your
expression was:

> a = 2*a + F(B)

If F(B) crashed after 2*A was evaluated, then there may be no way to
recover the original value of A. So, there would probably need to be
a "debug" vs. "performance" compilation flag.

Craig

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Craig B. Markwardt, Ph.D. EMAIL: craigmnet@cow.physics.wisc.edu
Astrophysics, IDL, Finance, Derivatives | Remove "net" for better response
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[Message index]

		Use of Temporary( ) vs an Optimised Compiler By: Martin Downing on Mon, 26 November 2001 07:38
		Re: Use of Temporary( ) vs an Optimised Compiler By: Pavel A. Romashkin on Tue, 27 November 2001 09:24
		Re: Use of Temporary( ) vs an Optimised Compiler By: Paul van Delst on Tue, 27 November 2001 10:46
		Re: Use of Temporary( ) vs an Optimised Compiler By: Martin Downing on Tue, 27 November 2001 02:42
		Re: Use of Temporary( ) vs an Optimised Compiler By: Craig Markwardt on Mon, 26 November 2001 21:42
		Re: Use of Temporary( ) vs an Optimised Compiler By: Liam E. Gumley on Tue, 27 November 2001 11:13

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