llvm dev - Nov 2021 - Bug 50482 - optimizer malloc

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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On Tue, Nov 16, 2021 at 09:18:09PM -0600, Victor Yodaiken via llvm-dev
wrote:
> What exactly is being optimized here? Does program performance improve?
Consider a program that allocates memory, computes something in it and
frees the memory. If the computation can be done at compile time and all
stores are removed by Dead Store Elimination and the address doesn't
otherwise leak, the program can be reduced to malloc + free (or
new+delete). That makes it very beneficial to be able to remove the
allocation pair as well. 

Joerg