llvm dev - May 2007 - [LLVMdev] Is 'Implicit def' bug fixed in LLVM 2.0?

Hello, guys.
I remember that Jeffrey Poznanovic posted ths issue about 'Implicit def'
on Apr. 29 and Chris Lattner mentioned it seemed to be a bug.

I am using ver 1.9 and also found 'Implicit def' while using codegen for
the code follows:

-------------------------------------------------------------
#define f c_f
#define fs c_fs
 
int f(int x1) {
    return x1;
}

int fs(){
    int i, sum;
    for (i=0; i<10; i++)
      sum +=  f(i);
    return sum;
}
#ifdef LINUX
#include <stdio.h>
#include <stdlib.h>

int c_f(int);
int c_fs();

int main (int argc, char* argv[]) {
    int j;
    j = c_fs();

    printf ("%i\n",j);
    printf ("You got it\n");

}
#endif
-------------------------------------------------------------
The code above was converted to LLVM assembly as follows:

-------------------------------------------------------------
implementation   ; Functions:

int %c_f(int %x1) {
entry:
        ret int %x1
}

int %c_fs() {
entry:
        ret int undef
}

-------------------------------------------------------------

You can see 'int undef' at the end of this assembly which is shown as
'Implicit def' in target specific assembly code.

I just wonder if this is fixed in the newer version.
Have a good weekend.

Thanks,
Seung J. Lee

On Fri, 25 May 2007, Seung Jae Lee wrote:
> I remember that Jeffrey Poznanovic posted ths issue about 'Implicit
def'
> on Apr. 29 and Chris Lattner mentioned it seemed to be a bug.
Ok, I don't recall the bug but:
> I am using ver 1.9 and also found 'Implicit def' while using
codegen for
> the code follows:
The implicit def instruction often comes from use of the 'undef' value. 
In your case, that comes from this function
> int %c_fs() {
> entry:
>        ret int undef
> }
>
So your real question seems to be why the llvm optimizer reduces c_fs to 
undef.   The answer is because your code is not computing a defined value:
> int c_fs(){
>    int i, sum;
>    for (i=0; i<10; i++)
>      sum +=  f(i);
>    return sum;
> }
Try initializing 'sum' to zero.

-Chris
> #ifdef LINUX
> #include <stdio.h>
> #include <stdlib.h>
>
> int c_f(int);
> int c_fs();
>
> int main (int argc, char* argv[]) {
>    int j;
>    j = c_fs();
>
>    printf ("%i\n",j);
>    printf ("You got it\n");
>
> }
> #endif
> -------------------------------------------------------------
> The code above was converted to LLVM assembly as follows:
>
> -------------------------------------------------------------
> implementation   ; Functions:
>
> int %c_f(int %x1) {
> entry:
>        ret int %x1
> }
>
> -------------------------------------------------------------
>
> You can see 'int undef' at the end of this assembly which is shown
as 'Implicit def' in target specific assembly code.
>
> I just wonder if this is fixed in the newer version.
> Have a good weekend.
>
> Thanks,
> Seung J. Lee
> _______________________________________________
> LLVM Developers mailing list
> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>
-Chris

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