[llvm-dev] Dereferenceable load semantics & LICM

Artur Pilipenko via llvm-dev llvm-dev at lists.llvm.org
Fri Apr 7 10:51:34 PDT 2017

On 7 Apr 2017, at 00:54, Sanjoy Das <sanjoy at playingwithpointers.com<mailto:sanjoy at playingwithpointers.com>> wrote:

+CC Artur

On April 6, 2017 at 2:51:17 PM, Keno Fischer (keno at juliacomputing.com<mailto:keno at juliacomputing.com>) wrote:
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.

That is very close to what we (Azul) do internally (I think Artur
covered some of this on his EuroLLVM talk).
The slides and videos from the conference have not yet been published, but I posted my slides on slideshare:

In a few words, we introduced the notion of Java types which are expressed in the IR using attributes and metadata, we derive various facts from the Java types including dereferenceability.


If we have:

if (x instanceof java.lang.String) {
  if (arbitraryCondition) {
    char[]v = x.value;

we know we can hoist the load of x.value outside the
arbitraryCondition check (modulo aliasing).

-- Sanjoy

On Thu, Apr 6, 2017 at 5:47 PM, Sanjoy Das via llvm-dev
Hi Piotr,

On April 6, 2017 at 9:28:58 AM, Piotr Padlewski
(piotr.padlewski at gmail.com<mailto: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

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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llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>

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