[llvm-dev] inttoptr and noalias returns

Fri Apr 16 09:48:08 PDT 2021

That's a very long story.. let me try to summarize why you can't do
"inttoptr(ptrtoint(x)) -> x" *blindly* (it's correct in some cases).

1.	Integers carry no provenance information and can be interchanged at
will.

This means that this transformation is always correct:

if (x == y)

  f(x);

=>

if (x == y)

  f(y);

2.	There are many pointers whose addresses are equal. For example:

char p[n];

char q[m];

char r[3];

We may have that (int)(p+n) == (int)q == (int)(r-m).

Even if we focus just on inbounds pointers (because we e.g. augmented
inttoptr to have an inbounds tag), we can still have 2 pointers with the
same address: p+n & q.

3.	Pointers have provenance. You can't use p+n to change memory of q.

p[n] = 42; // UB, to make the life of the alias analysis easier

If we put the three pieces together, we get that it's possible for the
compiler to swap a ptrtoint of a dereferenceable pointer with something else
and then if you blindly fold the ptrtoint/inttoptr chain, you get a wrong
pointer. Something like:

int x = p + n;

int y = q;

if (x == y)

  *(char*)y = 3;

=>  (GVN)

int x = p + n;

int y = q;

if (x == y)

  *(char*)x = 3;

=>  (invalid fold of inttoptr/ptrtoin chain)

int x = p + n;

int y = q;

if (x == y)

  *(p+n) = 3;

=>  (access OOB is UB)

int x = p + n;

int y = q;

if (x == y)

  UB;

I've a few slides on LLVM's AA that may help:
https://web.ist.utl.pt/nuno.lopes/pres/pointers-eurollvm18.pptx

Nuno

From: Joseph Tremoulet <jotrem at microsoft.com> 
Sent: 16 April 2021 15:48
To: Nuno Lopes <nunoplopes at sapo.pt>
Cc: llvm-dev at lists.llvm.org
Subject: RE: [EXTERNAL] RE: [llvm-dev] inttoptr and noalias returns

> otherwise relies on the incorrect transformation "inttoptr(ptrtoint(x)) ->
x"

Could you point me to an example/explanation of why that transformation is
incorrect?  It's not clear to me from the LangRef.

> A big issue with LLVM's static analysis is caching, since everything is
done lazily. If you want to add something more expensive to BasicAA, you
need to make sure that information is cached somehow to avoid recomputing it
a thousand times. Compilation time is quite sensitive to the performance of
BasicAA.

The IsCapturedCache in AAQueryInfo is pretty close to what I'm after, but I
don't really understand why the code in aliasCheck is using the weaker
isEscapeSource as opposed to !isNonEscapingLocalObject.

> escape pointers just like ptrtoint does

Yeah, so if the rule for ptrtoint is simply that the source pointer escapes,
then I'd think we could take advantage of the flip side of that and
isEscapeSource could return true for inttoptr, without needing expensive
analysis/caching.  But I know this can be a subtle area, so I'm not sure
that's the rule.  I see [1] that Ryan Taylor added discussing it to the
agenda for the February AA conference call, I'm curious what the outcome of
that was.

Thanks,

-Joseph

1 - https://lists.llvm.org/pipermail/llvm-dev/2021-February/148671.html

From: Nuno Lopes <nunoplopes at sapo.pt <mailto:nunoplopes at sapo.pt> > 
Sent: Thursday, April 15, 2021 6:37 AM
To: Joseph Tremoulet <jotrem at microsoft.com <mailto:jotrem at microsoft.com> >
Cc: llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org> 
Subject: [EXTERNAL] RE: [llvm-dev] inttoptr and noalias returns

You're right that LLVM is very conservative in handling inttoptr. And
otherwise relies on the incorrect transformation "inttoptr(ptrtoint(x)) ->
x" to get rid of inttoptr.

I agree the store should have been removed in your second example. I guess
inttoptr is not frequently used, and even less after a bunch of fixes to
prevent optimizers from creating new ones.

BasicAA is quite basic, but that's all LLVM has. The other alias analyses in
git are either not useful in practice, unfinished or buggy. (I haven't
looked into that dir in a couple of years, so things may have changed in the
meantime).

A big issue with LLVM's static analysis is caching, since everything is done
lazily. If you want to add something more expensive to BasicAA, you need to
make sure that information is cached somehow to avoid recomputing it a
thousand times. Compilation time is quite sensitive to the performance of
BasicAA.

Although there's no definitive semantics for pointer comparisons yet
(soonish I hope), LLVM's behavior implies that pointer comparisons indeed
escape pointers just like ptrtoint does (except if the two pointers being
compared are inbounds and point to the same object, and therefore the
comparison is only around offsets and thus their address doesn't leak).

Nuno

From: llvm-dev <llvm-dev-bounces at lists.llvm.org
<mailto:llvm-dev-bounces at lists.llvm.org> > On Behalf Of Joseph Tremoulet via
llvm-dev
Sent: 02 April 2021 19:26
To: llvm-dev <llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org> >
Subject: Re: [llvm-dev] inttoptr and noalias returns

Stepping through this in the debugger, I see this code in BasicAliasAnalysis
doing a check similar to the sort that I would have expected to see proving
NoAlias for this case, but it's not because (ISTM) it's being pretty
conservative:

    // If one pointer is the result of a call/invoke or load and the other
is a

    // non-escaping local object within the same function, then we know the

    // object couldn't escape to a point where the call could return it.

    //

    // Note that if the pointers are in different functions, there are a

    // variety of complications. A call with a nocapture argument may still

    // temporary store the nocapture argument's value in a temporary memory

    // location if that memory location doesn't escape. Or it may pass a

    // nocapture value to other functions as long as they don't capture it.

    if (isEscapeSource(O1) &&

        isNonEscapingLocalObject(O2, &AAQI.IsCapturedCache))

      return NoAlias;

    if (isEscapeSource(O2) &&

        isNonEscapingLocalObject(O1, &AAQI.IsCapturedCache))

      return NoAlias;

  }

and

/// Returns true if the pointer is one which would have been considered an

/// escape by isNonEscapingLocalObject.

static bool isEscapeSource(const Value *V) {

  if (isa<CallBase>(V))

    return true;

  if (isa<Argument>(V))

    return true;

  // The load case works because isNonEscapingLocalObject considers all

  // stores to be escapes (it passes true for the StoreCaptures argument

  // to PointerMayBeCaptured).

  if (isa<LoadInst>(V))

    return true;

  return false;

}

Since we have to look through all the uses of O1/O2 (including certain
transitive ones) to prove isNonEscapingLocalObject, an
expensive-but-more-precise analysis could just check if O2/O1 is in that
set, IIUC.  I get why BasicAliasAnalysis isn't the right place to do that.
Is there some more expensive alias analysis that I could opt into and get
that sort of check?

Alternatively, following the logic that we can assume isEscapeSource for
loads because we treat stores as escapes, is there room to assume
isEscapeSource for inttoptrs because we treat ptrtoints, and things that let
you subtly intify pointers such as certain compares, as escapes?

Thanks,

-Joseph

From: llvm-dev <llvm-dev-bounces at lists.llvm.org
<mailto:llvm-dev-bounces at lists.llvm.org> > On Behalf Of Joseph Tremoulet via
llvm-dev
Sent: Wednesday, March 31, 2021 2:09 PM
To: llvm-dev <llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org> >
Subject: [EXTERNAL] [llvm-dev] inttoptr and noalias returns

Hi,

I'm a bit confused about the interaction between inttoptr and noalias, and
would like to better understand our model.

I realize there's a bunch of in-flight work around restrict modeling and
that ptrtoint was on the agenda for last week's AA call.  I'm interested in
understanding both the current state and the thinking/plans for the future.
And I'm happy for pointers to anywhere this is already written down, I
didn't find it from skimming the AA call minutes or the mailing list
archive, but I could easily have overlooked it, and haven't really dug into
the set of restrict patches (nor do I know where to get a list of those).

I also realize that with aliasing questions there can always be a gap
between what the model says we can infer and how aggressive analyses and
optimizations are about actually making use of those inferences.  Again I'm
interested in both answers (and happy for either).

In the LangRef section on pointer aliasing rules [1], I see

An integer constant other than zero or a pointer value returned from a
function not defined within LLVM may be associated with address ranges
allocated through mechanisms other than those provided by LLVM. Such ranges
shall not overlap with any ranges of addresses allocated by mechanisms
provided by LLVM.

And I'm curious what "mechanisms provided by LLVM" for allocation means.
Alloca, presumably.  Global variables?  Certain intrinsics?  Any function
with a noalias return value?

In the LangRef description of the noalias attribute [2], I see

This indicates that memory locations accessed via pointer values based on
the argument or return value are not also accessed, during the execution of
the function, via pointer values not based on the argument or return value .
On function return values, the noalias attribute indicates that the function
acts like a system memory allocation function, returning a pointer to
allocated storage disjoint from the storage for any other object accessible
to the caller.

The phrase "the storage for any other object accessible to the caller" in
the noalias description sounds like a broader category than the phrase
"mechanisms provided by LLVM" from the pointer aliasing section, so I would
expect that if the pointer returned from a call to a function with return
attribute noalias does not escape, then loads/stores through it would not
alias loads/stores through a pointer produced by inttoptr.  Am I
interpreting that correctly?

I wrote some snippets [3] to see what the optimizer would do.  Each case has
a store of value 86 via pointer %p that I'd expect dead store elimination to
remove if we think it does not alias the subsequent load via pointer %q
(because immediately after that is another store to %p).

In each case, %q is the result of a call to a function whose return value is
annotated noalias.

When %p is a pointer parameter, I indeed see the optimizer removing the dead
store:

define i8 @test1(i8* %p) {

    %q = call i8* @allocate()

    store i8 86, i8* %p ; <-- this gets removed

    %result = load i8, i8* %q

    store i8 0, i8* %p

    ret i8 %result

}

When %p is the result of inttoptr, I do not see the store being removed, and
I'm wondering if this is because of a subtle aliasing rule or an intentional
conservativism in the optimizer or just a blind spot in the analysis:

define i8 @test2(i64 %p_as_int) {

    %p = inttoptr i64 %p_as_int to i8*

    %q = call i8* @allocate()

    store i8 86, i8* %p ; <-- this does not get removed

    %result = load i8, i8* %q

    store i8 0, i8* %p

    ret i8 %result

}

When I outline the inttoptr into a separate function, I again see the
optimizer remove the dead store, which again I'm wondering if the difference
between this and the previous case is an intentional subtle point or what.

define i8* @launder(i64 %int) noinline {

  %ptr = inttoptr i64 %int to i8*

  ret i8* %ptr

}

define i8 @test3(i64 %p_as_int) {

    %p = call i8* @launder(i64 %p_as_int)

    %q = call i8* @allocate()

    store i8 86, i8* %p ; <-- this gets removed

    %result = load i8, i8* %q

    store i8 0, i8* %p

    ret i8 %result

}

Happy for any insights you can share.

Thanks,

-Joseph

1 - https://llvm.org/docs/LangRef.html#pointeraliasing
<https://nam06.safelinks.protection.outlook.com/?url=https%3A%2F%2Fllvm.org%
2Fdocs%2FLangRef.html%23pointeraliasing&data=04%7C01%7Cjotrem%40microsoft.co
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ha%2FWcNfK%2FkAgoamYEJSM%3D&reserved=0> 

2 - https://llvm.org/docs/LangRef.html#parameter-attributes
<https://nam06.safelinks.protection.outlook.com/?url=https%3A%2F%2Fllvm.org%
2Fdocs%2FLangRef.html%23parameter-attributes&data=04%7C01%7Cjotrem%40microso
ft.com%7Ca920365acb0b45f6dab508d8fffa7870%7C72f988bf86f141af91ab2d7cd011db47
%7C1%7C0%7C637540798542822348%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLC
JQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C1000&sdata=EOiYYW2c3RKk6gRh2q
T5tteyUnr0B3xQsBF5fUkLYRA%3D&reserved=0> 

3 - https://godbolt.org/z/x8e41G33Y
<https://nam06.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgodbolt.o
rg%2Fz%2Fx8e41G33Y&data=04%7C01%7Cjotrem%40microsoft.com%7Ca920365acb0b45f6d
ab508d8fffa7870%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C637540798542832
302%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1h
aWwiLCJXVCI6Mn0%3D%7C1000&sdata=yBwkhCH%2BhdzS%2BaKWZP9riVgdVo%2F11tQ8tGugy2
1QPh0%3D&reserved=0> 

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