MemoryDependenceAnalysis reports dependencies between NoAlias pointers

Hi all,

I have a custom alias analysis pass that enforces that pointers from different address spaces do not alias, and I'm using MemoryDependenceAnalysis to, well, figure out dependence analysis.

The AA pass is extremely simple, it only checks the address space of pointers, returns NoAlias if they're different, and delegates otherwise. It is the last alias analysis pass added to my PassManager (basically, after BasicAA only). As far as I can check with breakpoints and logging, it appears to work as intended. MemoryDependenceAnalysis often appears in the call stack when my alias method is called, and it does return NoAlias appropriately.

However, in another custom module pass that I have, I'm observing that MemDep reports dependences between loads and stores of pointers to different address spaces, despite my AA pass. For instance, an AA request against these parameters returns NoAlias:

  %7 = inttoptr i64 %4 to i64 addrspace(1)*
  %9 = getelementptr inbounds %struct.mystruct, %struct.mystruct* %0, i64 0, i32 7, i32 0

But then, memdep suggests that there is a clobber dependency between these two instructions:

  %10 = load i64, i64* %9, align 8
  store i64 %6, i64 addrspace(1)* %7, align 8

I would expect that since the locations cannot alias, the load cannot depend on the store. When used from my module pass, MemoryDependenceAnalysis never ends up calling my AA pass, but I would imagine that this is because GVN or some other pass already did all the queries that I do and that they are cached.

Am I missing something?

Félix

First, you should step through memdep and see exactly what is going on.
You can invalidate the memdep cache if you need to for these pointers,
which should cause your AA to be called.

Second, a bit more info is needed.

On which of these pointers and what memdep functions are you calling?

(and can you attach the full .ll file?)

Note that memorydependence's main client is GVN, and they have a
pretty incestuous relationship.

So the API may not give you what you really want in some cases.
For example, iit will return load-load clobbers in noalias cases
sometimes so that gvn can use them for load widening, etc.

But it's not at all clear that you are hitting one of these cases.

Thanks Daniel,

I'll do a debug build of LLVM (I guess that'll teach me) and step through it as soon as I'll find a power outlet.

I'm calling getDependency on the load instruction, and it returns the store instruction. Suspiciously enough, calling invalidateCachedPointerInfo on load->getPointerOperand() does not cause my AA pass to be called again when I use getDependency (or is that not what I needed to do?).

I joined the .ll that shows the relevant function as it gets to my pass (memdep is a function pass, right?). Ultimately, my goal is to find out when loads from address space 0 depend on calls. Given my usage pattern, I can overlook non-local dependencies.

Félix

memdepprob.ll (3.54 KB)

Thanks Daniel,

I'll do a debug build of LLVM (I guess that'll teach me) and step through it as soon as I'll find a power outlet.

I'm calling getDependency on the load instruction, and it returns the store instruction. Suspiciously enough, calling invalidateCachedPointerInfo on load->getPointerOperand() does not cause my AA pass to be called again when I use getDependency (or is that not what I needed to do?).

No, that should do it.
I would step through and see what is happening.

I joined the .ll that shows the relevant function as it gets to my pass (memdep is a function pass, right?). Ultimately, my goal is to find out when loads from address space 0 depend on calls.
Given my usage pattern, I can overlook non-local dependencies.

Interesting. FWIW, memory ssa will make this easier (and i'm just
getting back to adding a ton of tests for it after having my time
eaten for a while. It should go in in a few weeks).
http://reviews.llvm.org/D7864

You would just look at loads, and the immediate definition of the
MemoryUse on the load will tell you whether it is dependent on a call
or not
if it's a call, it's dependent on that call. If it's something else,
A. It is not dependent on any calls in between that thing and your load
B. It's killed by that something else, regardless of whether any call
before it would also kill it :slight_smile:

Interesting. FWIW, memory ssa will make this easier (and i'm just
getting back to adding a ton of tests for it after having my time
eaten for a while. It should go in in a few weeks).
⚙ D7864 This patch introduces MemorySSA, a virtual SSA form for memory. Details on what it looks like are in MemorySSA.h

You would just look at loads, and the immediate definition of the
MemoryUse on the load will tell you whether it is dependent on a call
or not
if it's a call, it's dependent on that call. If it's something else,
A. It is not dependent on any calls in between that thing and your load
B. It's killed by that something else, regardless of whether any call
before it would also kill it :slight_smile:

This is extremely relevant to what I'm doing. I was using MemDep because that's what came closest to memory SSA. I think I'll grab the patch right away.

I'm still pretty new to LLVM and just starting to figure things out, and this is just a hobby thing (no corporate resources to back my project), but if you would like feedback, I can probably give some.

Félix

The latest version is part of
git@github.com:dberlin/llvm-gvn-rewrite.git on the newgvn branch

Grab it and ignore the NewGVN stuff that comes with it (ie don't use -newgvn)

This is the branch i split memoryssa out of, so it has all the latest
bug fixes.

A lesser version of this (IE with the updating/etc) is what is going
to get committed as the first version of MemorySSA in a few weeks.