llvm dev - Nov 2021 - Bug 50482 - optimizer malloc

Hi,
    This is my first post and I am looking at the bug 50482 . It is related
to malloc optimization. I have a doubt related to malloc optimization.
Here is the code:-

#include <stdlib.h>
int test() {
  char *x = malloc(-1);
  char *y = malloc(2);
  int ret = (x != NULL) && (y != NULL);
  free(x); free(y);
  return ret;
}

Above program returns 1.

During optimization (-O1) when llvm IR goes through instruction
combining pass then program behaves incorrect.

This link follows Instruction combining pass.

https://github.com/llvm/llvm-project/blob/main/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp#L2639

Where it is mentioned that

If we have a malloc call which is only used in any amount of
comparisons to null and free calls, delete the calls and replace the
comparisons with true or false as appropriate.

but is it feasible solution to replace every malloc call with true or
false when comparison to null and free calls?

Thank you,

Bhumitram Kumar
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Hi Bhumitram Kumar,

First, great you are interested in working on LLVM.

I think you might have stumbled on a bug report that is not great for 
beginners.

I don't think there is a bug here, optimizing this to return 1 seems 
fine. That
said, the people reporting the bug obviously disagree which will at 
least cause
some discussion before any decision on how to deal with this is made. 
Long story
short, you might want to look at another bug.

All the best,
   Johannes


On 11/16/21 12:33, bhumitram kumar via llvm-dev wrote:
> Hi,
>      This is my first post and I am looking at the bug 50482 . It is
related
> to malloc optimization. I have a doubt related to malloc optimization.
> Here is the code:-
>
> #include <stdlib.h>
> int test() {
>    char *x = malloc(-1);
>    char *y = malloc(2);
>    int ret = (x != NULL) && (y != NULL);
>    free(x); free(y);
>    return ret;
> }
>
> Above program returns 1.
>
> During optimization (-O1) when llvm IR goes through instruction
> combining pass then program behaves incorrect.
>
> This link follows Instruction combining pass.
>
>
https://github.com/llvm/llvm-project/blob/main/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp#L2639
>
> Where it is mentioned that
>
> If we have a malloc call which is only used in any amount of
> comparisons to null and free calls, delete the calls and replace the
> comparisons with true or false as appropriate.
>
> but is it feasible solution to replace every malloc call with true or
> false when comparison to null and free calls?
>
> Thank you,
>
> Bhumitram Kumar
>
>
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