Maybe a optimization bug.

[Vortex]

Hi Pelle,

This code below outputs the correct result. Tested with Pelles C v14 :

Code Select Expand

#include <stdio.h>

int main(void)
{
    int i;
    int s=0;
    for (i=1;i<11;i++)
    {
        s=s+i;
    }
    printf("Sum = %d\n",s);
    return 0;
}

Code Select Expand

Sum = 55
Optimizations : Maximize speed

[John Z]

Are you sure because adding the print of i to check shows maybe not-
If i becomes corrupt but it is not looked at - is it really corrupt 
Schrödinger's cat according to quantum theory, i is simultaneously good and bad until viewed
 

Code Select Expand

#include <stdio.h>

int main(void)
{
    int i;
    int s=0;
    for (i=1;i<11;i++)
    {
        s=s+i;
    }
    printf("i = %d\n",i);
    printf("Sum = %d\n",s);
    return 0;
}
This also appears to be OK as long as not looking at the value of i -

Code Select Expand

#include <stdio.h>

int main(void)
{
    int i = 1;
    int s = 0;
   
    do {
        s = s + i;
        i++;
    } while (i <= 10);
    printf("i = %d\n", i);
    printf("Sum = %d\n", s);
    return 0;
}

using volatile int i has 'fixed' it every time...

John Z

[John Z]

I my experience, enabling the optimizer on poorly written C code is more of a problem than the optimizer itself.

I agree with this also. In general I am seeing less and less issues as I program more...

John Z

[Vortex]

I was too fast, same issue :

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_text   SEGMENT PARA 'CODE'

main    PROC
        sub     rsp, 40
        lea     rcx, [@154]
        call    printf    
        lea     rcx, [@156]
        mov     edx, 55   
        call    printf    
        xor     eax, eax  
        add     rsp, 40   
        ret               
main    ENDP

_text   ENDS

.rdata  SEGMENT PARA 'CONST'                          

@156    label byte
        db 53H, 75H, 6DH, 20H, 3DH, 20H, 25H, 64H       ; 0000 _ Sum = %d
        db 0AH, 00H                                  

@154    label byte
        db 69H, 20H, 3DH, 20H, 25H, 64H, 0AH, 00H       ; 000A _ i = %d..

.rdata  ENDS

Again, this line is fixing the issue :

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    for (i=1;i<11;i=i-(-1))

[John Z]

Many variations can complicate the use of i enough that the optimizer does not affect i.

Both of these variations within the loop also 'work' to inhibit the optimization -
s = s + (i*i)/i;
and
s = s + ((i << 1) >> 1);

We can probably find more too, but I'm fairly sure this is not helping -

Code Select Expand

#include <stdio.h>

int main(void)
{
    int i = 1;
    int s = 0;

    do {
        //s = s + (i*i)/i;         // works
          s = s + ((i << 1) >> 1); // works
        i++;   
    } while (i <= 10);
    printf("i = %d\n", i);
    printf("Sum = %d\n", s);
    return 0;
}
Another 'working' version

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#include <stdio.h>

int main(void)
{
    int i = 0;
    int s = 0;

    do {
++i;
s = s + i;
       } while (i <= 9);
    printf("i = %d\n", i);
    printf("Sum = %d\n", s);
    return 0;
}

John Z

[TimoVJL]

Hi John Z, have a fun with Clang with limits with optimizations.
Optimizations for static code isn't so important and you know reasons for that.

[Vortex]

The code below compiled as 32-bit application works fine :

Code Select Expand

#include <stdio.h>

int main(void)
{
    int i;
    int s=0;
    for (i=1;i<11;)
    {
s=s+i;
__asm inc i;
    }
    printf("i=%d , sum = %d\n",i,s);
    return 0;
}


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i=11 , sum = 55