llvm dev - Aug 2015 - [cfe-dev] [LLVMdev] Clang devirtualization proposal

----- Original Message -----
> From: "Chandler Carruth" <chandlerc at google.com>
> To: "Hal Finkel" <hfinkel at anl.gov>
> Cc: "Sanjoy Das" <sanjoy at playingwithpointers.com>,
"llvm-dev"
> <llvm-dev at lists.llvm.org>, cfe-dev at lists.llvm.org
> Sent: Friday, August 7, 2015 7:35:57 PM
> Subject: Re: [cfe-dev] [LLVMdev] Clang devirtualization proposal
> On Fri, Aug 7, 2015 at 5:21 PM Hal Finkel < hfinkel at anl.gov >
wrote:
> > ----- Original Message -----
> 
> > > From: "Chandler Carruth" < chandlerc at google.com
>
> 
> > > To: "Hal Finkel" < hfinkel at anl.gov >,
"Sanjoy Das" <
> > > sanjoy at playingwithpointers.com >
> 
> > > Cc: " cfe-dev at cs.uiuc.edu Developers" < cfe-dev
at cs.uiuc.edu >,
> > > "LLVM Developers Mailing List" < llvmdev at
cs.uiuc.edu >
> 
> > > Sent: Friday, August 7, 2015 5:52:04 PM
> 
> > > Subject: Re: [cfe-dev] [LLVMdev] Clang devirtualization proposal
> 
> > >
> 
> > > On Sat, Aug 1, 2015 at 6:39 AM Hal Finkel < hfinkel at anl.gov
>
> > > wrote:
> 
> > >
> 
> > > ----- Original Message -----
> 
> > > > From: "Sanjoy Das" < sanjoy at
playingwithpointers.com >
> 
> > > > To: "Reid Kleckner" < rnk at google.com >
> 
> > > > Cc: "Piotr Padlewski" < prazek at google.com
>, "
> > > > cfe-dev at cs.uiuc.edu
> 
> > > > Developers" < cfe-dev at cs.uiuc.edu >,
"LLVM Developers
> 
> > > > Mailing List" < llvmdev at cs.uiuc.edu >
> 
> > > > Sent: Saturday, August 1, 2015 1:22:50 AM
> 
> > > > Subject: Re: [LLVMdev] [cfe-dev] Clang devirtualization
> > > > proposal
> 
> > > >
> 
> > > > On Fri, Jul 31, 2015 at 6:18 PM, Reid Kleckner < rnk at
google.com
> > > > >
> 
> > > > wrote:
> 
> > > > > Consider this pseudo-IR and some possible transforms
that I
> > > > > would
> 
> > > > > expect to
> 
> > > > > be semantics preserving:
> 
> > > > >
> 
> > > > > void f(i32* readonly %a, i32* %b) {
> 
> > > > > llvm.assume(%a == %b)
> 
> > > > > store i32 42, i32* %b
> 
> > > > > }
> 
> > > > > ...
> 
> > > > > %p = alloca i32
> 
> > > > > store i32 13, i32* %p
> 
> > > > > call f(i32* readonly %p, i32* %p)
> 
> > > > > %r = load i32, i32* %p
> 
> > > > >
> 
> > > > > ; Propagate llvm.assume info
> 
> > > > > void f(i32* readonly %a, i32* %b) {
> 
> > > > > store i32 42, i32* %a
> 
> > > > > }
> 
> > > > > ...
> 
> > > > > %p = alloca i32
> 
> > > > > store i32 13, i32* %p
> 
> > > > > call f(i32* readonly %p, i32* %p)
> 
> > > > > %r = load i32, i32* %p
> 
> > > >
> 
> > > > I'd say this first transformation is incorrect.
`readonly` is
> 
> > > > effectively part of `%a`'s "type" as it
constrains and affects
> > > > the
> 
> > > > operations you can do on `%a`. Even if `%b` is bitwise
> > > > equivalent
> 
> > > > to
> 
> > > > `%a` at runtime, it is "type incompatible" to
replace `%a` with
> 
> > > > `%b`.
> 
> > > >
> 
> > > > This is similar to how you cannot replace `store i32 42, i32
> 
> > > > addrspace(1)* %a` with `store i32 42, i32 addrspace(2)* %b`,
> > > > even
> 
> > > > if
> 
> > > > you can prove `ptrtoint %a` == `ptrtoint %b` -- the nature
of
> 
> > > > `store`
> 
> > > > is dependent on the type of the pointer you store through.
> 
> > > >
> 
> > > > The glitch in LLVM IR right now is that the `readonly`ness
of
> > > > `%a`
> 
> > > > is
> 
> > > > not modeled in the type system, when I think it should be.
An
> > > > `i32
> 
> > > > readonly*` should be a different type from `i32*`. In
practice
> > > > this
> 
> > > > may be non-trivial to get right (for instance `phi`s and
> > > > `selects`
> 
> > > > will either have to do a type merge, or we'd have to
have
> > > > explicit
> 
> > > > type operators at the IR level).
> 
> > >
> 
> > > We could do this, but then we'd need to promote these things
to
> 
> > > first-class parts of the type system (and I'd need to put
further
> 
> > > thought about how this interacts with dynamically-true properties
> > > at
> 
> > > callsites and inlining).
> 
> > >
> 
> > > The alternative way of looking at it, which is true today, is
> > > that
> 
> > > @llvm.assume is not removed even when its information is
'used'.
> > > It
> 
> > > appears, given this example, that this is actually required for
> 
> > > correctness, and that dead-argument elimination needs to
> 
> > > specifically not ignore effectively-ephemeral values/arguments.
> 
> > >
> 
> > > What follows is an off-the-cuff response. I'm still thinking
> > > through
> 
> > > it, but wanted to let others do so as well.
> 
> > >
> 
> > >
> 
> > > There is yet another interpretation that we might use: the final
> 
> > > transformation Reid proposed is actually correct and allowed
> 
> > > according to the IR.
> 
> > >
> 
> > >
> 
> > > Specifically, we could make 'readonly' a property of the
memory
> > > much
> 
> > > like aliasing is. That would mean that the function promises not
> > > to
> 
> > > modify the memory pointed to by %a in this example. The optimizer
> 
> > > gets to ignore any such modifications while remaining correct.
> 
> > We could certainly do this, but it will obviously make inference
> > harder. That might not be a good thing.
> 
> The other approach that seems reasonable is to push this to the
> caller to make inference in the callee easier. In that scenario, you
> would say that the readonly tells the caller that the memory
> location represented by the argument is not written *through that
> argument* but might be written through some other argument. Since
> the caller passes two pointers which alias, it cannot forward the
> store.
Isn't that what happens today? 
> The problem I see is that if the transformation in the body of the
> callee does CSE of any form, it allows dead argument elimination to
> remove this non-readonly potentially-aliasing argument.
Right, that seems to be the problem. 
> So if we want to go this route, I think we need to at least block
> dead argument elimination from removing a potentially writable
> aliasing argument even if it is unused in the function body, because
> it might be modeling a writable way for a particular memory location
> to enter the function.
> All in all, I would prefer the stronger guarantee of the readonly
> attribute (that the memory location is unchanged, regardless of
> through which pointer it is accessed).
Perhaps this is indeed the only strategy that is completely self-consistent. I
don't object to pursuring this.
> > As I said earlier, the original problem highlighted by Reid's
> > example
> > cannot currently occur: that could only happen if you remove the
> > @llvm.assume call (thus making the other argument unused). We
> > already don't do this (because the assumes could be useful if
later
> > inlined), and now we have a second reason. Regardless, because we
> > don't actively remove @llvm.assume, I'm not convinced the
current
> > state of things is currently broken.
> 
> As others have said, *anything* which triggers CSE seems problematic
> here.
Fair enough. To record the example you provided to me on IRC here: 

chandlerc: we start with 'if (a == b) { *b = 42; } else { x(); }' 
chandlerc: then CSE to 'if (a == b) { *a = 42; } else { x(); }' 
chandlerc: then inline to 'if (a == b) { *a = 42; } else { unreachable;
}'
chandlerc: then fold to 'if (true) { *a = 42; }' 
chandlerc: b is now dead 
chandlerc: x was marked as 'readnone' 
chandlerc: and contained unreachable 

-Hal 
> > -Hal
> 
> > >
> 
> > > This would, in turn, mean that the store in Reid's
"@f" function
> > > is
> 
> > > an unobservable in the case that %a == %b through some dynamic
> 
> > > property, whatever that may be. And as a consequence, the
> 
> > > store-forwarding is a correct transformation.
> 
> > >
> 
> > >
> 
> > > -Chandler
> 
> > >
> 
> > >
> 
> > >
> 
> > >
> 
> > >
> 
> > > -Hal
> 
> > >
> 
> > > >
> 
> > > > -- Sanjoy
> 
> > > > _______________________________________________
> 
> > > > LLVM Developers mailing list
> 
> > > > LLVMdev at cs.uiuc.edu http://llvm.cs.uiuc.edu
> 
> > > > http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
> 
> > > >
> 
> > >
> 
> > > --
> 
> > > Hal Finkel
> 
> > > Assistant Computational Scientist
> 
> > > Leadership Computing Facility
> 
> > > Argonne National Laboratory
> 
> > > _______________________________________________
> 
> > > cfe-dev mailing list
> 
> > > cfe-dev at cs.uiuc.edu
> 
> > > http://lists.cs.uiuc.edu/mailman/listinfo/cfe-dev
> 
> > >
> 
> > --
> 
> > Hal Finkel
> 
> > Assistant Computational Scientist
> 
> > Leadership Computing Facility
> 
> > Argonne National Laboratory
> 
-- 

Hal Finkel 
Assistant Computational Scientist 
Leadership Computing Facility 
Argonne National Laboratory 
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Summing up, it seems there are three choices here that are all reasonably
defensible:

1) We can change how CSE works such that equality is not sufficient for
substitutability.
2) We can change the requirements of 'readonly' (and related) parameter
attribute to be that the *memory location* is not modified by the function.
3) We can change argument elimination to not eliminate dead arguments which
might change the set of memory locations which can be read or written by
the function.

The only one of these I see as completely infeasible is #1.

For #3, this is at odds with how I understand dereferencable works, and it
seems like these should work the same way and dereferencable had
particularly good reasons to need to work the way it does.

For #2, it really does make inference significantly more tricky. Here is an
example of the new challenges: if the caller has already escaped a pointer
to the memory location (which we must assume it has) then any function
which might write to memory must be assumed to write to the memory aliased
by the passed in pointer.

Also for #2, we should probably finish converging with dereferencable and
have a size attached. This will become especially important with type-less
pointers.

I suspect #2 is the right design, mostly because I suspect most of the
interesting and important inference cases are going to be cases where we
can easily infer the stronger guarantee, and once inferred we will have
much more freedom to optimize based on this stronger guarantee... But I
can't say I'm completely confident in this model yet.

-Chandler

On Fri, Aug 7, 2015 at 6:03 PM Hal Finkel <hfinkel at anl.gov> wrote:
> *From: *"Chandler Carruth" <chandlerc at google.com>
> *To: *"Hal Finkel" <hfinkel at anl.gov>
>
> *Cc: *"Sanjoy Das" <sanjoy at playingwithpointers.com>,
"llvm-dev" <
> llvm-dev at lists.llvm.org>, cfe-dev at lists.llvm.org
> *Sent: *Friday, August 7, 2015 7:35:57 PM
>
>
> *Subject: *Re: [cfe-dev] [LLVMdev] Clang devirtualization proposal
>
> On Fri, Aug 7, 2015 at 5:21 PM Hal Finkel <hfinkel at anl.gov> wrote:
>
>
>> > From: "Chandler Carruth" <chandlerc at google.com>
>> > To: "Hal Finkel" <hfinkel at anl.gov>,
"Sanjoy Das" <
>> sanjoy at playingwithpointers.com>
>> > Cc: "cfe-dev at cs.uiuc.edu Developers" <cfe-dev at
cs.uiuc.edu>, "LLVM
>> Developers Mailing List" <llvmdev at cs.uiuc.edu>
>> > Sent: Friday, August 7, 2015 5:52:04 PM
>> > Subject: Re: [cfe-dev] [LLVMdev] Clang devirtualization proposal
>> >
>> > On Sat, Aug 1, 2015 at 6:39 AM Hal Finkel < hfinkel at anl.gov
> wrote:
>> >
>> > ----- Original Message -----
>> > > From: "Sanjoy Das" < sanjoy at
playingwithpointers.com >
>> > > To: "Reid Kleckner" < rnk at google.com >
>> > > Cc: "Piotr Padlewski" < prazek at google.com
>, " cfe-dev at cs.uiuc.edu
>> > > Developers" < cfe-dev at cs.uiuc.edu >, "LLVM
Developers
>> > > Mailing List" < llvmdev at cs.uiuc.edu >
>> > > Sent: Saturday, August 1, 2015 1:22:50 AM
>> > > Subject: Re: [LLVMdev] [cfe-dev] Clang devirtualization
proposal
>> > >
>> > > On Fri, Jul 31, 2015 at 6:18 PM, Reid Kleckner < rnk at
google.com >
>> > > wrote:
>> > > > Consider this pseudo-IR and some possible transforms
that I would
>> > > > expect to
>> > > > be semantics preserving:
>> > > >
>> > > > void f(i32* readonly %a, i32* %b) {
>> > > > llvm.assume(%a == %b)
>> > > > store i32 42, i32* %b
>> > > > }
>> > > > ...
>> > > > %p = alloca i32
>> > > > store i32 13, i32* %p
>> > > > call f(i32* readonly %p, i32* %p)
>> > > > %r = load i32, i32* %p
>> > > >
>> > > > ; Propagate llvm.assume info
>> > > > void f(i32* readonly %a, i32* %b) {
>> > > > store i32 42, i32* %a
>> > > > }
>> > > > ...
>> > > > %p = alloca i32
>> > > > store i32 13, i32* %p
>> > > > call f(i32* readonly %p, i32* %p)
>> > > > %r = load i32, i32* %p
>> > >
>> > > I'd say this first transformation is incorrect.
`readonly` is
>> > > effectively part of `%a`'s "type" as it
constrains and affects the
>> > > operations you can do on `%a`. Even if `%b` is bitwise
equivalent
>> > > to
>> > > `%a` at runtime, it is "type incompatible" to
replace `%a` with
>> > > `%b`.
>> > >
>> > > This is similar to how you cannot replace `store i32 42, i32
>> > > addrspace(1)* %a` with `store i32 42, i32 addrspace(2)* %b`,
even
>> > > if
>> > > you can prove `ptrtoint %a` == `ptrtoint %b` -- the nature of
>> > > `store`
>> > > is dependent on the type of the pointer you store through.
>> > >
>> > > The glitch in LLVM IR right now is that the `readonly`ness of
`%a`
>> > > is
>> > > not modeled in the type system, when I think it should be. An
`i32
>> > > readonly*` should be a different type from `i32*`. In
practice this
>> > > may be non-trivial to get right (for instance `phi`s and
`selects`
>> > > will either have to do a type merge, or we'd have to have
explicit
>> > > type operators at the IR level).
>> >
>> > We could do this, but then we'd need to promote these things
to
>> > first-class parts of the type system (and I'd need to put
further
>> > thought about how this interacts with dynamically-true properties
at
>> > callsites and inlining).
>> >
>> > The alternative way of looking at it, which is true today, is that
>> > @llvm.assume is not removed even when its information is
'used'. It
>> > appears, given this example, that this is actually required for
>> > correctness, and that dead-argument elimination needs to
>> > specifically not ignore effectively-ephemeral values/arguments.
>> >
>> > What follows is an off-the-cuff response. I'm still thinking
through
>> > it, but wanted to let others do so as well.
>> >
>> >
>> > There is yet another interpretation that we might use: the final
>> > transformation Reid proposed is actually correct and allowed
>> > according to the IR.
>> >
>> >
>> > Specifically, we could make 'readonly' a property of the
memory much
>> > like aliasing is. That would mean that the function promises not
to
>> > modify the memory pointed to by %a in this example. The optimizer
>> > gets to ignore any such modifications while remaining correct.
>>
>> We could certainly do this, but it will obviously make inference
harder.
>> That might not be a good thing.
>>
>
> The other approach that seems reasonable is to push this to the caller to
> make inference in the callee easier. In that scenario, you would say that
> the readonly tells the caller that the memory location represented by the
> argument is not written *through that argument* but might be written
> through some other argument. Since the caller passes two pointers which
> alias, it cannot forward the store.
>
>
> Isn't that what happens today?
>
>
>
> The problem I see is that if the transformation in the body of the callee
> does CSE of any form, it allows dead argument elimination to remove this
> non-readonly potentially-aliasing argument.
>
>
> Right, that seems to be the problem.
>
>
>
> So if we want to go this route, I think we need to at least block dead
> argument elimination from removing a potentially writable aliasing argument
> even if it is unused in the function body, because it might be modeling a
> writable way for a particular memory location to enter the function.
>
> All in all, I would prefer the stronger guarantee of the readonly
> attribute (that the memory location is unchanged, regardless of through
> which pointer it is accessed).
>
>
> Perhaps this is indeed the only strategy that is completely
> self-consistent. I don't object to pursuring this.
>
>
>
>
>> As I said earlier, the original problem highlighted by Reid's
example
>> cannot currently occur: that could only happen if you remove the
>> @llvm.assume call (thus making the other argument unused). We already
don't
>> do this (because the assumes could be useful if later inlined), and now
we
>> have a second reason. Regardless, because we don't actively remove
>> @llvm.assume, I'm not convinced the current state of things is
currently
>> broken.
>>
>
> As others have said, *anything* which triggers CSE seems problematic here.
>
>
> Fair enough. To record the example you provided to me on IRC here:
>
> chandlerc: we start with 'if (a == b) { *b = 42; } else { x(); }'
> chandlerc: then CSE to 'if (a == b) { *a = 42; } else { x(); }'
> chandlerc: then inline to 'if (a == b) { *a = 42; } else { unreachable;
}'
> chandlerc: then fold to 'if (true) { *a = 42; }'
> chandlerc: b is now dead
> chandlerc: x was marked as 'readnone'
> chandlerc: and contained unreachable
>
>
>  -Hal
>
>
>
>>
>>  -Hal
>>
>> >
>> > This would, in turn, mean that the store in Reid's
"@f" function is
>> > an unobservable in the case that %a == %b through some dynamic
>> > property, whatever that may be. And as a consequence, the
>> > store-forwarding is a correct transformation.
>> >
>> >
>> > -Chandler
>> >
>> >
>> >
>> >
>> >
>> > -Hal
>> >
>> > >
>> > > -- Sanjoy
>> > > _______________________________________________
>> > > LLVM Developers mailing list
>> > > LLVMdev at cs.uiuc.edu http://llvm.cs.uiuc.edu
>> > > http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>> > >
>> >
>> > --
>> > Hal Finkel
>> > Assistant Computational Scientist
>> > Leadership Computing Facility
>> > Argonne National Laboratory
>> > _______________________________________________
>> > cfe-dev mailing list
>> > cfe-dev at cs.uiuc.edu
>> > http://lists.cs.uiuc.edu/mailman/listinfo/cfe-dev
>> >
>>
>> --
>> Hal Finkel
>> Assistant Computational Scientist
>> Leadership Computing Facility
>> Argonne National Laboratory
>>
>
>
>
> --
> Hal Finkel
> Assistant Computational Scientist
> Leadership Computing Facility
> Argonne National Laboratory
>
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> I suspect #2 is the right design, mostly because I suspect most of the
interesting and important inference cases are going to be cases where we can
easily infer the stronger guarantee, and once inferred we will have much more
freedom to optimize based on this stronger guarantee...
Can't the stronger guarantee be represented in the existing system by
either:

* Adding 'readonly' to all the aliasing pointer arguments.
* Adding 'noalias' to the pointer argument (if there aren't any
aliasing pointer arguments).

The proposal sounds interesting but it seems like it would prevent
languages with strong versions of 'const' (or similar) from marking
pointers as 'readonly', which might be a useful parameter attribute
for APIs (i.e. function declarations where inference isn't possible).

I may not have followed the discussion completely, but would it be
possible to simply strip the 'readonly' attributes when dead arguments
are eliminated?

On Sat, Aug 8, 2015 at 2:27 AM, Chandler Carruth <chandlerc at google.com>
wrote:
> Summing up, it seems there are three choices here that are all reasonably
> defensible:
>
> 1) We can change how CSE works such that equality is not sufficient for
> substitutability.
> 2) We can change the requirements of 'readonly' (and related)
parameter
> attribute to be that the *memory location* is not modified by the function.
> 3) We can change argument elimination to not eliminate dead arguments which
> might change the set of memory locations which can be read or written by
the
> function.
>
> The only one of these I see as completely infeasible is #1.
>
> For #3, this is at odds with how I understand dereferencable works, and it
> seems like these should work the same way and dereferencable had
> particularly good reasons to need to work the way it does.
>
> For #2, it really does make inference significantly more tricky. Here is an
> example of the new challenges: if the caller has already escaped a pointer
> to the memory location (which we must assume it has) then any function
which
> might write to memory must be assumed to write to the memory aliased by the
> passed in pointer.
>
> Also for #2, we should probably finish converging with dereferencable and
> have a size attached. This will become especially important with type-less
> pointers.
>
> I suspect #2 is the right design, mostly because I suspect most of the
> interesting and important inference cases are going to be cases where we
can
> easily infer the stronger guarantee, and once inferred we will have much
> more freedom to optimize based on this stronger guarantee... But I
can't say
> I'm completely confident in this model yet.
>
> -Chandler
>
> On Fri, Aug 7, 2015 at 6:03 PM Hal Finkel <hfinkel at anl.gov> wrote:
>>
>> From: "Chandler Carruth" <chandlerc at google.com>
>> To: "Hal Finkel" <hfinkel at anl.gov>
>>
>> Cc: "Sanjoy Das" <sanjoy at playingwithpointers.com>,
"llvm-dev"
>> <llvm-dev at lists.llvm.org>, cfe-dev at lists.llvm.org
>> Sent: Friday, August 7, 2015 7:35:57 PM
>>
>>
>> Subject: Re: [cfe-dev] [LLVMdev] Clang devirtualization proposal
>>
>> On Fri, Aug 7, 2015 at 5:21 PM Hal Finkel <hfinkel at anl.gov>
wrote:
>>>
>>>
>>> > From: "Chandler Carruth" <chandlerc at
google.com>
>>> > To: "Hal Finkel" <hfinkel at anl.gov>,
"Sanjoy Das"
>>> > <sanjoy at playingwithpointers.com>
>>> > Cc: "cfe-dev at cs.uiuc.edu Developers" <cfe-dev
at cs.uiuc.edu>, "LLVM
>>> > Developers Mailing List" <llvmdev at cs.uiuc.edu>
>>> > Sent: Friday, August 7, 2015 5:52:04 PM
>>> > Subject: Re: [cfe-dev] [LLVMdev] Clang devirtualization
proposal
>>> >
>>> > On Sat, Aug 1, 2015 at 6:39 AM Hal Finkel < hfinkel at
anl.gov > wrote:
>>> >
>>> > ----- Original Message -----
>>> > > From: "Sanjoy Das" < sanjoy at
playingwithpointers.com >
>>> > > To: "Reid Kleckner" < rnk at google.com >
>>> > > Cc: "Piotr Padlewski" < prazek at google.com
>, " cfe-dev at cs.uiuc.edu
>>> > > Developers" < cfe-dev at cs.uiuc.edu >,
"LLVM Developers
>>> > > Mailing List" < llvmdev at cs.uiuc.edu >
>>> > > Sent: Saturday, August 1, 2015 1:22:50 AM
>>> > > Subject: Re: [LLVMdev] [cfe-dev] Clang devirtualization
proposal
>>> > >
>>> > > On Fri, Jul 31, 2015 at 6:18 PM, Reid Kleckner < rnk
at google.com >
>>> > > wrote:
>>> > > > Consider this pseudo-IR and some possible transforms
that I would
>>> > > > expect to
>>> > > > be semantics preserving:
>>> > > >
>>> > > > void f(i32* readonly %a, i32* %b) {
>>> > > > llvm.assume(%a == %b)
>>> > > > store i32 42, i32* %b
>>> > > > }
>>> > > > ...
>>> > > > %p = alloca i32
>>> > > > store i32 13, i32* %p
>>> > > > call f(i32* readonly %p, i32* %p)
>>> > > > %r = load i32, i32* %p
>>> > > >
>>> > > > ; Propagate llvm.assume info
>>> > > > void f(i32* readonly %a, i32* %b) {
>>> > > > store i32 42, i32* %a
>>> > > > }
>>> > > > ...
>>> > > > %p = alloca i32
>>> > > > store i32 13, i32* %p
>>> > > > call f(i32* readonly %p, i32* %p)
>>> > > > %r = load i32, i32* %p
>>> > >
>>> > > I'd say this first transformation is incorrect.
`readonly` is
>>> > > effectively part of `%a`'s "type" as it
constrains and affects the
>>> > > operations you can do on `%a`. Even if `%b` is bitwise
equivalent
>>> > > to
>>> > > `%a` at runtime, it is "type incompatible" to
replace `%a` with
>>> > > `%b`.
>>> > >
>>> > > This is similar to how you cannot replace `store i32 42,
i32
>>> > > addrspace(1)* %a` with `store i32 42, i32 addrspace(2)*
%b`, even
>>> > > if
>>> > > you can prove `ptrtoint %a` == `ptrtoint %b` -- the
nature of
>>> > > `store`
>>> > > is dependent on the type of the pointer you store
through.
>>> > >
>>> > > The glitch in LLVM IR right now is that the
`readonly`ness of `%a`
>>> > > is
>>> > > not modeled in the type system, when I think it should
be. An `i32
>>> > > readonly*` should be a different type from `i32*`. In
practice this
>>> > > may be non-trivial to get right (for instance `phi`s and
`selects`
>>> > > will either have to do a type merge, or we'd have to
have explicit
>>> > > type operators at the IR level).
>>> >
>>> > We could do this, but then we'd need to promote these
things to
>>> > first-class parts of the type system (and I'd need to put
further
>>> > thought about how this interacts with dynamically-true
properties at
>>> > callsites and inlining).
>>> >
>>> > The alternative way of looking at it, which is true today, is
that
>>> > @llvm.assume is not removed even when its information is
'used'. It
>>> > appears, given this example, that this is actually required
for
>>> > correctness, and that dead-argument elimination needs to
>>> > specifically not ignore effectively-ephemeral
values/arguments.
>>> >
>>> > What follows is an off-the-cuff response. I'm still
thinking through
>>> > it, but wanted to let others do so as well.
>>> >
>>> >
>>> > There is yet another interpretation that we might use: the
final
>>> > transformation Reid proposed is actually correct and allowed
>>> > according to the IR.
>>> >
>>> >
>>> > Specifically, we could make 'readonly' a property of
the memory much
>>> > like aliasing is. That would mean that the function promises
not to
>>> > modify the memory pointed to by %a in this example. The
optimizer
>>> > gets to ignore any such modifications while remaining correct.
>>>
>>> We could certainly do this, but it will obviously make inference
harder.
>>> That might not be a good thing.
>>
>>
>> The other approach that seems reasonable is to push this to the caller
to
>> make inference in the callee easier. In that scenario, you would say
that
>> the readonly tells the caller that the memory location represented by
the
>> argument is not written *through that argument* but might be written
through
>> some other argument. Since the caller passes two pointers which alias,
it
>> cannot forward the store.
>>
>>
>> Isn't that what happens today?
>>
>>
>>
>> The problem I see is that if the transformation in the body of the
callee
>> does CSE of any form, it allows dead argument elimination to remove
this
>> non-readonly potentially-aliasing argument.
>>
>>
>> Right, that seems to be the problem.
>>
>>
>>
>> So if we want to go this route, I think we need to at least block dead
>> argument elimination from removing a potentially writable aliasing
argument
>> even if it is unused in the function body, because it might be modeling
a
>> writable way for a particular memory location to enter the function.
>>
>> All in all, I would prefer the stronger guarantee of the readonly
>> attribute (that the memory location is unchanged, regardless of through
>> which pointer it is accessed).
>>
>>
>> Perhaps this is indeed the only strategy that is completely
>> self-consistent. I don't object to pursuring this.
>>
>>
>>
>>>
>>> As I said earlier, the original problem highlighted by Reid's
example
>>> cannot currently occur: that could only happen if you remove the
>>> @llvm.assume call (thus making the other argument unused). We
already don't
>>> do this (because the assumes could be useful if later inlined), and
now we
>>> have a second reason. Regardless, because we don't actively
remove
>>> @llvm.assume, I'm not convinced the current state of things is
currently
>>> broken.
>>
>>
>> As others have said, *anything* which triggers CSE seems problematic
here.
>>
>>
>> Fair enough. To record the example you provided to me on IRC here:
>>
>> chandlerc: we start with 'if (a == b) { *b = 42; } else { x();
}'
>> chandlerc: then CSE to 'if (a == b) { *a = 42; } else { x(); }'
>> chandlerc: then inline to 'if (a == b) { *a = 42; } else {
unreachable; }'
>> chandlerc: then fold to 'if (true) { *a = 42; }'
>> chandlerc: b is now dead
>> chandlerc: x was marked as 'readnone'
>> chandlerc: and contained unreachable
>>
>>
>>  -Hal
>>
>>
>>>
>>>
>>>  -Hal
>>>
>>> >
>>> > This would, in turn, mean that the store in Reid's
"@f" function is
>>> > an unobservable in the case that %a == %b through some dynamic
>>> > property, whatever that may be. And as a consequence, the
>>> > store-forwarding is a correct transformation.
>>> >
>>> >
>>> > -Chandler
>>> >
>>> >
>>> >
>>> >
>>> >
>>> > -Hal
>>> >
>>> > >
>>> > > -- Sanjoy
>>> > > _______________________________________________
>>> > > LLVM Developers mailing list
>>> > > LLVMdev at cs.uiuc.edu http://llvm.cs.uiuc.edu
>>> > > http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>>> > >
>>> >
>>> > --
>>> > Hal Finkel
>>> > Assistant Computational Scientist
>>> > Leadership Computing Facility
>>> > Argonne National Laboratory
>>> > _______________________________________________
>>> > cfe-dev mailing list
>>> > cfe-dev at cs.uiuc.edu
>>> > http://lists.cs.uiuc.edu/mailman/listinfo/cfe-dev
>>> >
>>>
>>> --
>>> Hal Finkel
>>> Assistant Computational Scientist
>>> Leadership Computing Facility
>>> Argonne National Laboratory
>>
>>
>>
>>
>> --
>> Hal Finkel
>> Assistant Computational Scientist
>> Leadership Computing Facility
>> Argonne National Laboratory
>
>
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