llvm dev - May 2015 - [LLVMdev] 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

On Thu, May 21, 2015 at 11:37 AM, Félix Cloutier <felixcca at yahoo.ca>
wrote:
> 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.
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

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> Le 2015-05-21 à 14:53:44, Daniel Berlin <dberlin at dberlin.org> a
écrit :
> 
> On Thu, May 21, 2015 at 11:37 AM, Félix Cloutier <felixcca at
yahoo.ca> wrote:
>> 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.
> 
> 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.