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From: alexande@cs.unc.edu (Geoffrey D. Alexander)
Newsgroups: gnu.gcc.bug
Subject: Performance anomoly using constructor expressions
Message-ID: <9001121726.AA22371@dopey.cs.unc.edu>
Date: 12 Jan 90 17:26:09 GMT
Sender: news@tut.cis.ohio-state.edu
Distribution: gnu
Organization: GNUs Not Usenet
Lines: 72

I have encountered a strange peformance anomoly using contstructor expressions.
Consider the following program.

====test4a.c===================================================================
typedef struct {
  double real;
  double imaginary;
} complex;

#define ADD_COMPLEX(x, y) \
  (complex){(x).real+y.real, (x).imaginary+(y).imaginary}

#define MULT_COMPLEX(x, y) \
  (complex){((x).real*(y).real)-((x).imaginary*(y).imaginary), \
    ((x).real*(y).imaginary)+((y).real*(x).imaginary)}

main() {
  complex c;
  int     i;
  complex x;
  c=(complex){1.0,1.0};
  x=(complex){0.0,0.0};
  for (i=1;i<=10000;i++) {
    x=ADD_COMPLEX(MULT_COMPLEX(x,x),c);
  }
  exit(0);
}
===============================================================================

Now, modify the this program so that the value of MULT_COMPLEX(x,x) is saved in
a temporary variable.

====test4b.c===================================================================
typedef struct {
  double real;
  double imaginary;
} complex;

#define ADD_COMPLEX(x, y) \
  (complex){(x).real+y.real, (x).imaginary+(y).imaginary}

#define MULT_COMPLEX(x, y) \
  (complex){((x).real*(y).real)-((x).imaginary*(y).imaginary), \
    ((x).real*(y).imaginary)+((y).real*(x).imaginary)}

main() {
  complex c;
  int     i;
  complex x;
  complex y;
  c=(complex){1.0,1.0};
  x=(complex){0.0,0.0};
  for (i=1;i<=10000;i++) {
    y=MULT_COMPLEX(x,x);
    x=ADD_COMPLEX(y,c);
  }
  exit(0);
}
===============================================================================

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?

Note that I am using gcc version 1.36.

Geoff Alexander