optimization/5739: "const" and "pure" function attributes pessimize code

[Dan Nicolaescu <dann at godzilla dot ics dot uci dot edu>]

>Number:         5739
>Category:       optimization
>Synopsis:       "const" and "pure" function attributes pessimize code
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    unassigned
>State:          open
>Class:          pessimizes-code
>Submitter-Id:   net
>Arrival-Date:   Wed Feb 20 14:16:00 PST 2002
>Closed-Date:
>Last-Modified:
>Originator:     Dan Nicolaescu <dann@godzilla.ics.uci.edu>
>Release:        CVS
>Organization:
>Environment:
sparc-sun-solaris2.8
>Description:
"const" and "pure" function attributes pessimize code

The following code: 

#ifdef MY_PURE
extern  double mycos (double) __attribute__ ((pure));
extern  double mysin (double) __attribute__ ((pure));
#elif defined MY_CONST
extern  double mycos (double) __attribute__ ((const));
extern  double mysin (double) __attribute__ ((const));
#else
extern  double mycos (double);
extern  double mysin (double);
#endif

struct my_complex
{
  double re;
  double im;
};


int main(void) {

  struct my_complex u[2048];
  int i;
  for (i = 0; i < 2048; ++i)
    {
      u[i].re = 1.0;
      u[i].im = 0.0;
    }
               
  for (i = 0; i < 2048; ++i) {
    struct my_complex *p = u;
    unsigned int j;
    for (j = 0; j < 2048; ++j) {
      double ur = p->re;
      double ui = p->im;
      double u2 = ur * ur + ui * ui;             
      double cosv = mycos (u2);
      double sinv = mysin (u2);
      p->re = p->re * cosv - p->im * sinv;
      p->im = p->im * sinv +  p->re * cosv;
      ++p;
    } 
  }
}

generates worse code when compiled with -O2 -DMY_PURE or
-O2 -DMY_CONST than when compiled with -O2 on SPARC with
GCC from CVS on sparc

Attached is an sdiff of the assembly files resulted when compiling with -O2 (left) and -O2 -DMY_CONST (right)

>How-To-Repeat:
Compile the above code with -O2 and -O2 -DMY_CONST and 
look at the assembly files.
>Fix:

>Release-Note:
>Audit-Trail:
>Unformatted:
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