[Bug c/31263] New: Misoptimization of constant function expressions

[o dot mangold at gmx dot de]

I'm not sure, if this can be called a bug, but it is at least a really bad case
of poor optimization.

The following program calls the function 'Square' several times, either with
x=1000 or x=i*2-i-i+1000 (which is also 1000). The second version is executed
much FASTER. I see no reason, why this should be so. I tested it with gcc 4.1.1
and 4.1.2. The timings are more or less equal.

> gcc -O2 f_demo.c f_demo2.c -o f_demo
> time f_demo

real    0m1.537s
user    0m1.183s
sys     0m0.345s

> gcc -D VARIABLE_PAR -O2 f_demo.c f_demo2.c -o f_demo
> time f_demo

real    0m0.700s
user    0m0.368s
sys     0m0.329s

--- f_demo.c -----------------------------------------------------------
#include <stdlib.h>

double Square(double x);

#ifdef VARIABLE_PAR
#define PAR i*2-i-i+1000
#else
#define PAR 1000
#endif

int main()
{
  const int iSize=50000000;
  int i;
  double *pdA=malloc(iSize*sizeof(double));
  for(i=0;i<iSize;i++) {
    pdA[i]=Square(PAR);
  }
}

--- f_demo2.c ----------------------------------------------------
double Square(double x)
{
  return x*x;
}


-- 
           Summary: Misoptimization of constant function expressions
           Product: gcc
           Version: 4.1.2
            Status: UNCONFIRMED
          Severity: minor
          Priority: P3
         Component: c
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: o dot mangold at gmx dot de
 GCC build triplet: i386-redhat-linux
  GCC host triplet: i386-pc-linux-gnu
GCC target triplet: i386-redhat-linux


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=31263