Path: utzoo!utgpu!jarvis.csri.toronto.edu!cs.utexas.edu!usc!jarthur!uci-ics!rfg
From: rfg@ics.uci.edu (Ron Guilmette)
Newsgroups: gnu.gcc.bug
Subject: Re: Performance anomoly using constructor expressions
Message-ID: <25B7EACF.23324@paris.ics.uci.edu>
Date: 20 Jan 90 04:36:31 GMT
References: <9001121726.AA22371@dopey.cs.unc.edu>
Reply-To: rfg@ics.uci.edu (Ron Guilmette)
Distribution: gnu
Organization: UC Irvine Department of ICS
Lines: 28

In article <9001121726.AA22371@dopey.cs.unc.edu> alexande@cs.unc.edu (Geoffrey D. Alexander) writes:
>I have encountered a strange peformance anomoly using contstructor expressions.
...
>
>Now, modify the this program so that the value of MULT_COMPLEX(x,x) is saved in
>a temporary variable.
...
>
>Now, compile the programs as follows:
>
>  gcc test4a.c -O -o test4a
>  gcc test4b.c -O -o test4b
>
>Running on a Sun3-60M (w/o floating point chip) under SunOS Release 4.0.3,
>test4a takes 4.1 seconds, while test4b takes only 2.3 seconds.  Anyone care
>to explain why?

I believe that it is because of the fact that constructor expressions are
treated as "executable" by the compiler (regardless of whether or not their
"components" are compile-time constants).  Thus, each time you "execute"
the constructor expression, code must be executed to (again) copy each of
the components into some (often temporary) area.

If you "construct" only once and copy the value into your own "temporary"
structure and then use that from then on, then you only pay for
the actual "construction" operation once.

// rfg