[Bug c/20429] New: Free Lunch?

[Bug c/20429] New: Free Lunch?

[hillcino368 at hotmail dot com]

Hi,
S1: Dell Diminsion p4 2.533ghz 2gig ram
Batch
g++ %1.c -o %1.exe -O3 -mtune=pentium4
g++ -dumpversion
 
S2: Dell xps p3 1.0ghz 256 meg ram 
g++ %1.c -o %1.exe -O3 -mtune=pentium3
g++ -dumpversion

The following code gives much faster timings for 2000000000 vs 1000000000.
Is this a free-lunch or is it a bug? Also it appears that including the
line sum+=1 causes the loop to run faster on the p4. At any rate Gcc runs 
about 30% faster than Lcc. Also the loop
 MOV EAX,2000000000
 L1:
   DEC EAX
   JNZ L1
A386 takes 1.21 sec on S1
Gcc is truly a code optimizer.

[CODE]
#include <stdio.h>
#include <time.h>
#define timer clock()/1000.0
float t1,t2;
unsigned long count (unsigned long);
unsigned long billions = 2000000000;
int main()
{
t1=timer;
printf("%u",count(2000000000)); 
//printf("%u",count(1000000000));
//printf("%u",count(billions));
t2=timer;
printf("%s%G\n"," Sec = ",t2-t1);
getchar();
return 0;
}
unsigned long count (unsigned long i)
{
unsigned long  sum;
sum=0;
for(;i>0;i--)
 {
 sum +=1;
 }
return sum;
}
[/CODE]

Test 2000000000
S1 output -> 2000000000 Sec = 0.031
S2 output -> 2000000000 Sec = 0.18

Test 1000000000
S1 output -> 1000000000 Sec = 0.484 
S2 output -> 1000000000 Sec = 2.103

Test billions = 2000000000
S1 output -> 2000000000 Sec = 1.218
S2 output -> 2000000000 Sec = 4.486

Test sum +=1; commented out for 2000000000 
S1 output ->  0 Sec = 0.984
S2 output ->  0 sec = 4.186

Test sum +=1; commented out for billions = 2000000000 
S1 output ->  0 Sec = 0.968
S2 output ->  0 sec = 4.155

-- 
           Summary: Free Lunch?
           Product: gcc
           Version: 3.4.2
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: c
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: hillcino368 at hotmail dot com
                CC: gcc-bugs at gcc dot gnu dot org


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=20429

[Bug c/20429] Free Lunch?

[pinskia at gcc dot gnu dot org]

------- Additional Comments From pinskia at gcc dot gnu dot org  2005-03-11 18:53 -------
Well it changes the loop into:
.L5:
        incl    %eax
        cmpl    %eax, %edx
        jne     .L5

so yes this is free lunch.

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|                            |INVALID


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=20429