[llvm-dev] Bug 50482 - optimizer malloc

[Victor Yodaiken via llvm-dev]

What exactly is being optimized here? Does program performance improve?


On Tue, Nov 16, 2021 at 5:19 PM Johannes Doerfert via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> 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
> >
> >
> > _______________________________________________
> > LLVM Developers mailing list
> > llvm-dev at lists.llvm.org
> > https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
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