[llvm-dev] Dereferenceable load semantics & LICM

[Keno Fischer via llvm-dev]

I left a comment on https://reviews.llvm.org/D18738 earlier which I
feel is related to this discussion, so I'm reproducing it here:

I wonder though if what we want to express isn't some sort of
"type-based dereferencability annotations". For example the semantics
I care about are essentially, "if you know you have a defereferencable
pointer, you can go and dereference any other valid (managed) pointers
the pointed to data references (recursively)". This has to be true for
me, because the GC walks all pointers recursively that way. Of course
the problem with this is that the compiler doesn't know which part of
the referenced data are valid pointers for this purpose (and it can't
just be based on the struct type, because we do store pointers to
unmanaged data). So if we had a way to express to the compiler "here
are the pointers in this struct that you may assume dereferencable",
that would work very well for me.

On Thu, Apr 6, 2017 at 5:47 PM, Sanjoy Das via llvm-dev
<llvm-dev at lists.llvm.org> wrote:
> Hi Piotr,
>
> On April 6, 2017 at 9:28:58 AM, Piotr Padlewski
> (piotr.padlewski at gmail.com) wrote:
>> Hi Sanjoy,
>> My point is that this it is exactly the same way as normal !dereferenceable
>> introduces UB.
>>
>> ptr = load i8*, i8** %ptrptr, !dereferenceable !{i64 8}
>> if (false) {
>> int val = *ptr;
>> }
>
> These are two different things.
>
> In the above example, your original program executes a load that was
> supposed to produce a dereferenceable value, but it did not.  This
> means the original program is undefined, so we can optimize it to do
> whatever we want.
>
> OTOH, look at this program:
>
> void main() {
>   if (false) {
>     // ptrptr does not contain a dereferenceable pointer, but is
> itself dereferenceable
>     ptr = load i8*, i8** %ptrptr, !dereferenceable !{i64 8}, !global
>     int val = *ptr;
>   }
> }
>
> What is the behavior of the above program?  It better be well defined,
> since it does nothing!
>
> However, if we follow your rules, we can first xform it to
>
> void main() {
>   ptr = load i8*, i8** %ptrptr, !dereferenceable !{i64 8}, !global
>   if (false) {
>     int val = *ptr;
>   }
> }
>
> and then to
>
> void main() {
>   ptr = load i8*, i8** %ptrptr, !dereferenceable !{i64 8}, !global
>   int val = *ptr;
>   if (false) {
>   }
> }
>
> which introduces UB into a program that was well defined to begin
> with.
>
>> If frontend says that something is dereferenceable, which is not actually
>> dereferenceable, then it is UB and everything can happen - like the
>> execution of dead instruction.
>> This is exactly the same with the global properties - we are giving a
>> guarantee that pointer it is dereferenceable even if we would hoist or sink
>> it, and if it is not true then it is UB.
>
> But then you're saying dead code (code that would not actually execute
> in the original program) can affect program behavior, and: "if (false)
> X;" is not a no-op for some values of X.  It is in this respect that
> your proposal is similar to the speculatable proposal.
>
> -- Sanjoy
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