[llvm-dev] [GSoC 2016] Better Alias Analysis By Default - Mid Term Summary

[Jia Chen via llvm-dev]

Dear Joerg,

I also want to add that except for the interprocedural analysis part, 
whatever changes I made to CFL-AA I always tried avoid being 
semantically more aggressive than BasicAA. So if certain UB gets 
exploited in CFL-AA now, BasicAA would probably do the same as well.

On 06/21/2016 06:07 PM, George Burgess IV wrote:
> +Hal
>
> The use-case we dropped support for, specifically, was:
>
> ```
> void foo() {
>    int a;
>    int *ap = &a;
>    int *ap2 = (int*)((char*)NULL + (uintptr_t)&a);
>    // now ap and ap2 may alias
> }
> ```
>
> Our justification is that LLVM (specifically, BasicAA) already
> explicitly doesn't support this pattern, so we shouldn't have to support
> it in CFLAA.
>
> On Tue, Jun 21, 2016 at 3:50 PM, Joerg Sonnenberger via llvm-dev
> <llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>> wrote:
>
>     On Tue, Jun 21, 2016 at 05:29:13PM -0500, Jia Chen via llvm-dev wrote:
>     > - [r271322] Remove aliasing relations between GEP pointers and GEP indices.
>     > Before this patch, CFL-AA will claim that a and b may-alias each other in
>     > the following codes:
>     >
>     > int a[10], b[10];
>     > a[N] = ...;
>     > b[N] = ...;
>     >
>     > This seemingly insane behavior was actually there to safeguard certain cases
>     > where people do crazy stuffs with pointer arithmetics. Later we figured out
>     > that those crazy behaviors were in fact UB and therefore we could drop
>     > support for it.
>
>     What exactly is the semantic justification here? I'm asking because
>     there are a number of crucial use cases for aliasing global arrayish
>     variables behind the compiler like linker sets. We had quite some fun in
>     NetBSD with newer GCC introducing breakage by exploiting UB in fun ways.
>     It would be nice to avoid breaking valid applications in system/embedded
>     land.
>
>     Joerg
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>
>

-- 
Best Regards,

--
Jia Chen