[PATCH] D95734: Use alias analysis to remove redundant instrumentation for Asan

[Chijin via Phabricator via llvm-commits]

ChijinZ added a comment.

In D95734#2551939 <https://reviews.llvm.org/D95734#2551939>, @eugenis wrote:

> This is what Vitaly is talking about:
>
>   __attribute__((noinline)) void deallocate(void *p) {
>     free(p);
>   }
>   
>   int main(int argc, char **argv) {
>     int *p = (int *)malloc(sizeof(int));
>     *p = 42;
>     deallocate(p);
>     return *p;
>   }
>
> Your optimization is missing the bug. Any call to a function without the "nofree" attribute needs to forget about the earlier checks.
>
> In general, optimizations like this are dangerous because there is no good test set for memory bugs. We could be missing bugs left and right and will never notice...

It seems that my optimization doesn't miss the bug. If you pass a pointer as an argument to a function, then the following use of this pointer will *MayAlias* the previous use. Thus, the corresponding check will not be removed. In my experiment, all the asan checks of this example are not removed. Did I miss something?

Besides, I don't think it is dangerous because the implementation of llvm's alias analysis is always conservative, and my optimization is also conservative (only remove one when "MustAlias").


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