llvm dev - Jun 2015 - [LLVMdev] Constant folding inttoptr i32 0 to null pointer?

Thanks David,
It turns out, that the address space I was using was not 0, and yet the pointer
was constant folded to null.

Here is the sequence:

Unoptimized code:

define i32 @foo() #0 {
entry:
  %address.addr.i = alloca i32, align 4
  %value.i = alloca i32, align 4
  store i32 0, i32* %address.addr.i, align 4
  %0 = load i32* %address.addr.i, align 4
  %1 = inttoptr i32 %0 to i32 addrspace(1)*
  %std_ld.i = load volatile i32 addrspace(1)* %1
  store i32 %std_ld.i, i32* %value.i, align 4
  %2 = load i32* %value.i, align 4
  ret i32 %2
}

After optimization (early CSE):

define i32 @foo() #0 {
entry:
  %std_ld.i = load volatile i32 addrspace(1)* null, align 536870912
  ret i32 %std_ld.i
}

The contant folder doesn’t seem to check for address space, it simply checks if
the integer in question is zero, and folds the inttoptr to null:

Constant *llvm::ConstantFoldCastInstruction(unsigned opc, Constant *V,
                                            Type *DestTy) {

...

  if (V->isNullValue() && !DestTy->isX86_MMXTy())
    return Constant::getNullValue(DestTy);
...


Is this a bug?

Thanks
     Guy



From: David Majnemer [mailto:david.majnemer at gmail.com]
Sent: Tuesday, June 09, 2015 18:45
To: Benyei, Guy
Cc: llvmdev at cs.uiuc.edu
Subject: Re: [LLVMdev] Constant folding inttoptr i32 0 to null pointer?



On Tue, Jun 9, 2015 at 5:57 AM, Benyei, Guy <guy.benyei at
intel.com<mailto:guy.benyei at intel.com>> wrote:
Hello,
It seems that ConstantFoldCastInstruction in ConstantFold.cpp folds inttoptr
instruction with 0 as operand to a null pointer. It makes sense, when talking
about a C-style frontend, as the C99 spec (6.3.2.3) states:

“An integer constant expression with the value 0, or such an expression cast to
type void *, is called a null pointer constant.”

On the other hand, some architectures use 0 as a valid memory location, and this
constant folding seems to be possibly harmful when the code actually tries to
access the memory location at address 0.
Is this behavior intentional? Do I miss something? Will a load from address null
try to access address 0, or may it become an undef value?

LLVM assumes that the null pointer in address space zero can never be
successfully dereferenced.  You must utilize some other address space to
dereference a null pointer.


Thanks
       Guy


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'load volatile i32 addrspace(1)* null' seems fine to me.  However, it
looks
like instcombine will turn:
define i32 @foo() {
entry:
  %std_ld.i = load volatile i32, i32 addrspace(1)* null
  ret i32 %std_ld.i
}

into:
define i32 @foo() {
entry:
  %std_ld.i = load volatile i32, i32 addrspace(1)* null, align 536870912
  ret i32 %std_ld.i
}

which is not ok.

On Tue, Jun 9, 2015 at 11:34 AM, Benyei, Guy <guy.benyei at intel.com>
wrote:
>  Thanks David,
>
> It turns out, that the address space I was using was not 0, and yet the
> pointer was constant folded to null.
>
>
>
> Here is the sequence:
>
>
>
> Unoptimized code:
>
>
>
> define i32 @foo() #0 {
>
> entry:
>
>   %address.addr.i = alloca i32, align 4
>
>   %value.i = alloca i32, align 4
>
>   store i32 0, i32* %address.addr.i, align 4
>
>   %0 = load i32* %address.addr.i, align 4
>
>   %1 = inttoptr i32 %0 to i32 addrspace(1)*
>
>   %std_ld.i = load volatile i32 addrspace(1)* %1
>
>   store i32 %std_ld.i, i32* %value.i, align 4
>
>   %2 = load i32* %value.i, align 4
>
>   ret i32 %2
>
> }
>
>
>
> After optimization (early CSE):
>
>
>
> define i32 @foo() #0 {
>
> entry:
>
>   %std_ld.i = load volatile i32 addrspace(1)* null, align 536870912
>
>   ret i32 %std_ld.i
>
> }
>
>
>
> The contant folder doesn’t seem to check for address space, it simply
> checks if the integer in question is zero, and folds the inttoptr to null:
>
>
>
> Constant *llvm::ConstantFoldCastInstruction(unsigned opc, Constant *V,
>
>                                             Type *DestTy) {
>
>
>
> ...
>
>
>
>   if (V->isNullValue() && !DestTy->isX86_MMXTy())
>
>     return Constant::getNullValue(DestTy);
>
> ...
>
>
>
>
>
> Is this a bug?
>
>
>
> Thanks
>
>      Guy
>
>
>
>
>
>
>
> *From:* David Majnemer [mailto:david.majnemer at gmail.com]
> *Sent:* Tuesday, June 09, 2015 18:45
> *To:* Benyei, Guy
> *Cc:* llvmdev at cs.uiuc.edu
> *Subject:* Re: [LLVMdev] Constant folding inttoptr i32 0 to null pointer?
>
>
>
>
>
>
>
> On Tue, Jun 9, 2015 at 5:57 AM, Benyei, Guy <guy.benyei at intel.com>
wrote:
>
>  Hello,
>
> It seems that ConstantFoldCastInstruction in ConstantFold.cpp folds
> inttoptr instruction with 0 as operand to a null pointer. It makes sense,
> when talking about a C-style frontend, as the C99 spec (6.3.2.3) states:
>
>
>
> “An integer constant expression with the value 0, or such an expression
> cast to type void *, is called a null pointer constant.”
>
>
>
> On the other hand, some architectures use 0 as a valid memory location,
> and this constant folding seems to be possibly harmful when the code
> actually tries to access the memory location at address 0.
>
> Is this behavior intentional? Do I miss something? Will a load from
> address null try to access address 0, or may it become an undef value?
>
>
>
> LLVM assumes that the null pointer in address space zero can never be
> successfully dereferenced.  You must utilize some other address space to
> dereference a null pointer.
>
>
>
>
>
> Thanks
>
>        Guy
>
>
>
> ---------------------------------------------------------------------
> Intel Israel (74) Limited
>
> This e-mail and any attachments may contain confidential material for
> the sole use of the intended recipient(s). Any review or distribution
> by others is strictly prohibited. If you are not the intended
> recipient, please contact the sender and delete all copies.
>
>
> _______________________________________________
> LLVM Developers mailing list
> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>
>
>
> ---------------------------------------------------------------------
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>
> This e-mail and any attachments may contain confidential material for
> the sole use of the intended recipient(s). Any review or distribution
> by others is strictly prohibited. If you are not the intended
> recipient, please contact the sender and delete all copies.
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On Tue, Jun 9, 2015 at 12:32 PM, David Majnemer <david.majnemer at
gmail.com>
wrote:
> 'load volatile i32 addrspace(1)* null' seems fine to me.  However,
it
> looks like instcombine will turn:
> define i32 @foo() {
> entry:
>   %std_ld.i = load volatile i32, i32 addrspace(1)* null
>   ret i32 %std_ld.i
> }
>
> into:
> define i32 @foo() {
> entry:
>   %std_ld.i = load volatile i32, i32 addrspace(1)* null, align 536870912
>   ret i32 %std_ld.i
> }
>
> which is not ok.
>
On second thought, I think that high alignment is benign.  It simply
indicates that your backend is free to load the value as-if it were aligned
to an arbitrary boundary (because it is 1 byte, 2 byte, 4 byte, etc.
aligned).

>
> On Tue, Jun 9, 2015 at 11:34 AM, Benyei, Guy <guy.benyei at
intel.com> wrote:
>
>>  Thanks David,
>>
>> It turns out, that the address space I was using was not 0, and yet the
>> pointer was constant folded to null.
>>
>>
>>
>> Here is the sequence:
>>
>>
>>
>> Unoptimized code:
>>
>>
>>
>> define i32 @foo() #0 {
>>
>> entry:
>>
>>   %address.addr.i = alloca i32, align 4
>>
>>   %value.i = alloca i32, align 4
>>
>>   store i32 0, i32* %address.addr.i, align 4
>>
>>   %0 = load i32* %address.addr.i, align 4
>>
>>   %1 = inttoptr i32 %0 to i32 addrspace(1)*
>>
>>   %std_ld.i = load volatile i32 addrspace(1)* %1
>>
>>   store i32 %std_ld.i, i32* %value.i, align 4
>>
>>   %2 = load i32* %value.i, align 4
>>
>>   ret i32 %2
>>
>> }
>>
>>
>>
>> After optimization (early CSE):
>>
>>
>>
>> define i32 @foo() #0 {
>>
>> entry:
>>
>>   %std_ld.i = load volatile i32 addrspace(1)* null, align 536870912
>>
>>   ret i32 %std_ld.i
>>
>> }
>>
>>
>>
>> The contant folder doesn’t seem to check for address space, it simply
>> checks if the integer in question is zero, and folds the inttoptr to
null:
>>
>>
>>
>> Constant *llvm::ConstantFoldCastInstruction(unsigned opc, Constant *V,
>>
>>                                             Type *DestTy) {
>>
>>
>>
>> ...
>>
>>
>>
>>   if (V->isNullValue() && !DestTy->isX86_MMXTy())
>>
>>     return Constant::getNullValue(DestTy);
>>
>> ...
>>
>>
>>
>>
>>
>> Is this a bug?
>>
>>
>>
>> Thanks
>>
>>      Guy
>>
>>
>>
>>
>>
>>
>>
>> *From:* David Majnemer [mailto:david.majnemer at gmail.com]
>> *Sent:* Tuesday, June 09, 2015 18:45
>> *To:* Benyei, Guy
>> *Cc:* llvmdev at cs.uiuc.edu
>> *Subject:* Re: [LLVMdev] Constant folding inttoptr i32 0 to null
pointer?
>>
>>
>>
>>
>>
>>
>>
>> On Tue, Jun 9, 2015 at 5:57 AM, Benyei, Guy <guy.benyei at
intel.com> wrote:
>>
>>  Hello,
>>
>> It seems that ConstantFoldCastInstruction in ConstantFold.cpp folds
>> inttoptr instruction with 0 as operand to a null pointer. It makes
sense,
>> when talking about a C-style frontend, as the C99 spec (6.3.2.3)
states:
>>
>>
>>
>> “An integer constant expression with the value 0, or such an expression
>> cast to type void *, is called a null pointer constant.”
>>
>>
>>
>> On the other hand, some architectures use 0 as a valid memory location,
>> and this constant folding seems to be possibly harmful when the code
>> actually tries to access the memory location at address 0.
>>
>> Is this behavior intentional? Do I miss something? Will a load from
>> address null try to access address 0, or may it become an undef value?
>>
>>
>>
>> LLVM assumes that the null pointer in address space zero can never be
>> successfully dereferenced.  You must utilize some other address space
to
>> dereference a null pointer.
>>
>>
>>
>>
>>
>> Thanks
>>
>>        Guy
>>
>>
>>
>> ---------------------------------------------------------------------
>> Intel Israel (74) Limited
>>
>> This e-mail and any attachments may contain confidential material for
>> the sole use of the intended recipient(s). Any review or distribution
>> by others is strictly prohibited. If you are not the intended
>> recipient, please contact the sender and delete all copies.
>>
>>
>> _______________________________________________
>> LLVM Developers mailing list
>> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
>> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>>
>>
>>
>> ---------------------------------------------------------------------
>> Intel Israel (74) Limited
>>
>> This e-mail and any attachments may contain confidential material for
>> the sole use of the intended recipient(s). Any review or distribution
>> by others is strictly prohibited. If you are not the intended
>> recipient, please contact the sender and delete all copies.
>>
>
>
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On Tue, Jun 9, 2015 at 11:34 AM, Benyei, Guy <guy.benyei at intel.com>
wrote:
> Thanks David,
>
> It turns out, that the address space I was using was not 0, and yet the
> pointer was constant folded to null.
>
The lang ref is not entirely clear on whether 0 is always equal to the
null pointer. It (at least to me) implies it is:
"
Any memory access must be done through a pointer value associated with
an address range of the memory access, otherwise the behavior is
undefined. Pointer values are associated with address ranges according
to the following rules:
...

A pointer value is associated with the addresses associated with any
value it is based on.
A null pointer in the default address-space is associated with no address.
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.
....

A pointer value formed by an inttoptr is based on all pointer values
that contribute (directly or indirectly) to the computation of the
pointer’s value.

"
(Fun lawyering: This, and the remaining clauses, do not ever define
the behavior of a null pointer constant in the non-default address
space, which means it's behavior is undefined by the first sentence. I
know this is not what is intended, so this should probably be cleaned
up)

Anyway, the above to me implies the inttoptr of the integer constant
zero is the null pointer, because if it isn't, it's a pretty striking
omission to cover everything *but* the integer constant zero ;).

As such, folding it to null should be correct, and not cause wrong
behavior for your program.

As David says, the question of whether it can be dereferenced or not
is separate, specifically to allow the null pointer in non-default
address spaces to do different things.

Do you have a case where it does something wrong?
>
> The contant folder doesn’t seem to check for address space, it simply
checks
> if the integer in question is zero, and folds the inttoptr to null:
>
>
I believe this is correct by the above.

Currently it seems to work fine, but as you said, this behavior is not exactly
well defined. I would really expect any access to null - no matter in what
address space - to be replaced with undef by some optimization pass.

"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."

Doesn't it mean, that the integer constant zero cannot be associated with
any kind of memory, including non-default address space memory? Seems that
address 0 still has to be handled somehow...

Thanks
        Guy

-----Original Message-----
From: Daniel Berlin [mailto:dberlin at dberlin.org] 
Sent: Tuesday, June 09, 2015 23:22
To: Benyei, Guy
Cc: David Majnemer; llvmdev at cs.uiuc.edu
Subject: Re: [LLVMdev] Constant folding inttoptr i32 0 to null pointer?

On Tue, Jun 9, 2015 at 11:34 AM, Benyei, Guy <guy.benyei at intel.com>
wrote:
> Thanks David,
>
> It turns out, that the address space I was using was not 0, and yet 
> the pointer was constant folded to null.
>
The lang ref is not entirely clear on whether 0 is always equal to the null
pointer. It (at least to me) implies it is:
"
Any memory access must be done through a pointer value associated with an
address range of the memory access, otherwise the behavior is undefined. Pointer
values are associated with address ranges according to the following rules:
...

A pointer value is associated with the addresses associated with any value it is
based on.
A null pointer in the default address-space is associated with no address.
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.
....

A pointer value formed by an inttoptr is based on all pointer values that
contribute (directly or indirectly) to the computation of the pointer’s value.

"
(Fun lawyering: This, and the remaining clauses, do not ever define the behavior
of a null pointer constant in the non-default address space, which means
it's behavior is undefined by the first sentence. I know this is not what is
intended, so this should probably be cleaned
up)

Anyway, the above to me implies the inttoptr of the integer constant zero is the
null pointer, because if it isn't, it's a pretty striking omission to
cover everything *but* the integer constant zero ;).

As such, folding it to null should be correct, and not cause wrong behavior for
your program.

As David says, the question of whether it can be dereferenced or not is
separate, specifically to allow the null pointer in non-default address spaces
to do different things.

Do you have a case where it does something wrong?
>
> The contant folder doesn’t seem to check for address space, it simply 
> checks if the integer in question is zero, and folds the inttoptr to null:
>
>
I believe this is correct by the above.
---------------------------------------------------------------------
Intel Israel (74) Limited

This e-mail and any attachments may contain confidential material for
the sole use of the intended recipient(s). Any review or distribution
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