llvm dev - Sep 2016 - RFC: ConstantData should not have use-lists

r261464 added a type called ConstantData to the Value hierarchy.  This
is a parent type for constants with no operands, such as i32 0 and null.

Since then, I've removed most instances of iterating through the
use-lists of an instance of ConstantData.  I'd like to make this
illegal.  Since the users of ConstantData are spread across an
LLVMContext, most code that looks at the users is wrong.  Adding an
assertion should catch a lot of bugs (see r263853 and r263875) and
avoid some expensive walks through uninteresting code.

(The same is not true of Constant, generally.  A GlobalValue's use-list
will local to the GlobalValue's Module.  Any ConstantVector,
ConstantArray, or ConstantStruct that points at a GlobalValue will also
be local to the same Module.  In these cases, we also need RAUW
support.)

Besides catching bugs, removing use-lists from ConstantData will
guarantee that the compiler output *does not* depend on the use-list
order of something like i32 0.

Finally, this should dramatically reduce the overhead of serializing
use-list order in bitcode.  We will no longer track the arbitrary
order of references to things like i32 0 and null. 

What's left?
===========
I just filed PR30513 to track remaining work.

 1. Avoid the remaining uses of ConstantData use-lists.  There are only
    a couple of cases left, highlighted in the WIP HACK patches attached
    below (0001 and 0002).
 
 2. Remove the use-lists!  Replace them with ref-counts to keep most of
    the use-list API functional (and minimize the size of the change).
    See the WIP patch below (0003).

 3. (Optional) Remove use-lists from other non-GlobalValue Constants
    that do not reference any GlobalValues.  This would require some
    sort of magic in, e.g., ConstantVector to conditionally have a
    use-list.  Call sites of API like Value::use_begin would have to
    check for Value::hasUseList.

 4. (Optional) Remove the ref-count from ConstantData (and, potentially,
    other use-list-free Constants).  This would eliminate ref-count
    traffic, but would also require checking at call sites before using
    any use-list-related API.

Feedback
=======
- Does anyone disagree with this general direction?  Why?
- Any thoughts on #3?
- Any thoughts on #4?

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> On Sep 24, 2016, at 3:06 PM, Duncan P. N. Exon Smith via llvm-dev
<llvm-dev at lists.llvm.org> wrote:
> 
> r261464 added a type called ConstantData to the Value hierarchy.  This
> is a parent type for constants with no operands, such as i32 0 and null.
> 
> Since then, I've removed most instances of iterating through the
> use-lists of an instance of ConstantData.  I'd like to make this
> illegal.  Since the users of ConstantData are spread across an
> LLVMContext, most code that looks at the users is wrong.  Adding an
> assertion should catch a lot of bugs (see r263853 and r263875) and
> avoid some expensive walks through uninteresting code.
> 
> (The same is not true of Constant, generally.  A GlobalValue's use-list
> will local to the GlobalValue's Module.  Any ConstantVector,
> ConstantArray, or ConstantStruct that points at a GlobalValue will also
> be local to the same Module.  In these cases, we also need RAUW
> support.)
> 
> Besides catching bugs, removing use-lists from ConstantData will
> guarantee that the compiler output *does not* depend on the use-list
> order of something like i32 0.
> 
> Finally, this should dramatically reduce the overhead of serializing
> use-list order in bitcode.  We will no longer track the arbitrary
> order of references to things like i32 0 and null. 
> 
> What's left?
> ===========> 
> I just filed PR30513 to track remaining work.
> 
> 1. Avoid the remaining uses of ConstantData use-lists.  There are only
>    a couple of cases left, highlighted in the WIP HACK patches attached
>    below (0001 and 0002).
> 
> 2. Remove the use-lists!  Replace them with ref-counts to keep most of
>    the use-list API functional (and minimize the size of the change).
>    See the WIP patch below (0003).
> 
> 3. (Optional) Remove use-lists from other non-GlobalValue Constants
>    that do not reference any GlobalValues.  This would require some
>    sort of magic in, e.g., ConstantVector to conditionally have a
>    use-list.
I wonder if the constant class hierarchy should not be revisited in light of
this?
For instance you identified that some are local to a module while others are
“context-wide”.

I haven’t given too much thoughts about this, but I'm curious if you did?

— 
Mehdi

>  Call sites of API like Value::use_begin would have to
>    check for Value::hasUseList.
> 
> 4. (Optional) Remove the ref-count from ConstantData (and, potentially,
>    other use-list-free Constants).  This would eliminate ref-count
>    traffic, but would also require checking at call sites before using
>    any use-list-related API.
> 
> Feedback
> =======> 
> - Does anyone disagree with this general direction?  Why?
> - Any thoughts on #3?
> - Any thoughts on #4?
> 
> <0001-WIP-HACK-SimplifyLibCalls-Disable-optimizeSinCosPi-o.patch>
> <0002-WIP-HACK-LICM-Ignore-stores-to-UndefValue-and-Consta.patch>
>
<0003-WIP-IR-Remove-use-lists-from-ConstantData.patch>_______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev

> On 2016-Sep-24, at 15:16, Mehdi Amini <mehdi.amini at apple.com>
wrote:
> 
>> 
>> On Sep 24, 2016, at 3:06 PM, Duncan P. N. Exon Smith via llvm-dev
<llvm-dev at lists.llvm.org> wrote:
>> 
>> r261464 added a type called ConstantData to the Value hierarchy.  This
>> is a parent type for constants with no operands, such as i32 0 and
null.
>> 
>> Since then, I've removed most instances of iterating through the
>> use-lists of an instance of ConstantData.  I'd like to make this
>> illegal.  Since the users of ConstantData are spread across an
>> LLVMContext, most code that looks at the users is wrong.  Adding an
>> assertion should catch a lot of bugs (see r263853 and r263875) and
>> avoid some expensive walks through uninteresting code.
>> 
>> (The same is not true of Constant, generally.  A GlobalValue's
use-list
>> will local to the GlobalValue's Module.  Any ConstantVector,
>> ConstantArray, or ConstantStruct that points at a GlobalValue will also
>> be local to the same Module.  In these cases, we also need RAUW
>> support.)
>> 
>> Besides catching bugs, removing use-lists from ConstantData will
>> guarantee that the compiler output *does not* depend on the use-list
>> order of something like i32 0.
>> 
>> Finally, this should dramatically reduce the overhead of serializing
>> use-list order in bitcode.  We will no longer track the arbitrary
>> order of references to things like i32 0 and null. 
>> 
>> What's left?
>> ===========>> 
>> I just filed PR30513 to track remaining work.
>> 
>> 1. Avoid the remaining uses of ConstantData use-lists.  There are only
>>   a couple of cases left, highlighted in the WIP HACK patches attached
>>   below (0001 and 0002).
>> 
>> 2. Remove the use-lists!  Replace them with ref-counts to keep most of
>>   the use-list API functional (and minimize the size of the change).
>>   See the WIP patch below (0003).
>> 
>> 3. (Optional) Remove use-lists from other non-GlobalValue Constants
>>   that do not reference any GlobalValues.  This would require some
>>   sort of magic in, e.g., ConstantVector to conditionally have a
>>   use-list.
> 
> I wonder if the constant class hierarchy should not be revisited in light
of this?
> For instance you identified that some are local to a module while others
are “context-wide”.
> 
> I haven’t given too much thoughts about this, but I'm curious if you
did?
You mean something like PureConstantVector (which cannot transitively reference
GlobalValue) vs ConstantVectorWithGlobalRef (which can/must transitively
reference GlobalValue), right?  (And also for ConstantStruct, ConstantArray, and
ConstantExpr, etc.)

I hadn't considered that, and it seems worth thinking about.  I'm unsure
whether using isa<>() would really be cleaner than using
Value::hasUseList; and it would certainly be intrusive.  Do you see any concrete
benefits?

One possible long-term thing (after #4)... we could add ConstantDataUser (vs.
User), which can only reference a Constant-with-no-GlobalValue, and has operands
the size of a pointer.  Obviously nice to save on operand-size, but I'm not
convinced it would save sufficient memory to be worthwhile: IIRC, Instruction
accounts for most instances of User.
> — 
> Mehdi
> 
> 
>> Call sites of API like Value::use_begin would have to
>>   check for Value::hasUseList.
>> 
>> 4. (Optional) Remove the ref-count from ConstantData (and, potentially,
>>   other use-list-free Constants).  This would eliminate ref-count
>>   traffic, but would also require checking at call sites before using
>>   any use-list-related API.
>> 
>> Feedback
>> =======>> 
>> - Does anyone disagree with this general direction?  Why?
>> - Any thoughts on #3?
>> - Any thoughts on #4?
>> 
>> <0001-WIP-HACK-SimplifyLibCalls-Disable-optimizeSinCosPi-o.patch>
>> <0002-WIP-HACK-LICM-Ignore-stores-to-UndefValue-and-Consta.patch>
>>
<0003-WIP-IR-Remove-use-lists-from-ConstantData.patch>_______________________________________________
>> LLVM Developers mailing list
>> llvm-dev at lists.llvm.org
>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev

> On Sep 24, 2016, at 3:06 PM, Duncan P. N. Exon Smith <dexonsmith at
apple.com> wrote:
> 
> r261464 added a type called ConstantData to the Value hierarchy.  This
> is a parent type for constants with no operands, such as i32 0 and null.
> 
> Since then, I've removed most instances of iterating through the
> use-lists of an instance of ConstantData.  I'd like to make this
> illegal.  Since the users of ConstantData are spread across an
> LLVMContext, most code that looks at the users is wrong.  Adding an
> assertion should catch a lot of bugs (see r263853 and r263875) and
> avoid some expensive walks through uninteresting code.
> 
> (The same is not true of Constant, generally.  A GlobalValue's use-list
> will local to the GlobalValue's Module.  Any ConstantVector,
> ConstantArray, or ConstantStruct that points at a GlobalValue will also
> be local to the same Module.  In these cases, we also need RAUW
> support.)
> 
> Besides catching bugs, removing use-lists from ConstantData will
> guarantee that the compiler output *does not* depend on the use-list
> order of something like i32 0.
> 
> Finally, this should dramatically reduce the overhead of serializing
> use-list order in bitcode.  We will no longer track the arbitrary
> order of references to things like i32 0 and null. 
> 
> What's left?
> ===========> 
> I just filed PR30513 to track remaining work.
> 
> 1. Avoid the remaining uses of ConstantData use-lists.  There are only
>    a couple of cases left, highlighted in the WIP HACK patches attached
>    below (0001 and 0002).
> 
> 2. Remove the use-lists!  Replace them with ref-counts to keep most of
>    the use-list API functional (and minimize the size of the change).
>    See the WIP patch below (0003).
> 
> 3. (Optional) Remove use-lists from other non-GlobalValue Constants
>    that do not reference any GlobalValues.  This would require some
>    sort of magic in, e.g., ConstantVector to conditionally have a
>    use-list.  Call sites of API like Value::use_begin would have to
>    check for Value::hasUseList.
Could you explain why #3 requires checking for hasListList() or e.g
isa<ConstantVectorWithGlobalRef>() before calling use_begin()?

> 4. (Optional) Remove the ref-count from ConstantData (and, potentially,
>    other use-list-free Constants).  This would eliminate ref-count
>    traffic, but would also require checking at call sites before using
>    any use-list-related API.
> 
> Feedback
> =======> 
> - Does anyone disagree with this general direction?  Why?
> - Any thoughts on #3?
> - Any thoughts on #4?
> 
> <0001-WIP-HACK-SimplifyLibCalls-Disable-optimizeSinCosPi-o.patch>
> <0002-WIP-HACK-LICM-Ignore-stores-to-UndefValue-and-Consta.patch>
> <0003-WIP-IR-Remove-use-lists-from-ConstantData.patch>
We should probably improve the use-list-order2.ll test instead of deleting it.
I've attached a patch to PR24755 which might help with this ("Improve
use-list
order testing for function operands"). I think the test should fail if its
uses
of ConstantData are replaced by e.g personality functions.

vedant

> On 2016-Sep-26, at 09:02, Vedant Kumar <vsk at apple.com> wrote:
> 
>> 
>> On Sep 24, 2016, at 3:06 PM, Duncan P. N. Exon Smith <dexonsmith at
apple.com> wrote:
>> 
>> r261464 added a type called ConstantData to the Value hierarchy.  This
>> is a parent type for constants with no operands, such as i32 0 and
null.
>> 
>> Since then, I've removed most instances of iterating through the
>> use-lists of an instance of ConstantData.  I'd like to make this
>> illegal.  Since the users of ConstantData are spread across an
>> LLVMContext, most code that looks at the users is wrong.  Adding an
>> assertion should catch a lot of bugs (see r263853 and r263875) and
>> avoid some expensive walks through uninteresting code.
>> 
>> (The same is not true of Constant, generally.  A GlobalValue's
use-list
>> will local to the GlobalValue's Module.  Any ConstantVector,
>> ConstantArray, or ConstantStruct that points at a GlobalValue will also
>> be local to the same Module.  In these cases, we also need RAUW
>> support.)
>> 
>> Besides catching bugs, removing use-lists from ConstantData will
>> guarantee that the compiler output *does not* depend on the use-list
>> order of something like i32 0.
>> 
>> Finally, this should dramatically reduce the overhead of serializing
>> use-list order in bitcode.  We will no longer track the arbitrary
>> order of references to things like i32 0 and null. 
>> 
>> What's left?
>> ===========>> 
>> I just filed PR30513 to track remaining work.
>> 
>> 1. Avoid the remaining uses of ConstantData use-lists.  There are only
>>   a couple of cases left, highlighted in the WIP HACK patches attached
>>   below (0001 and 0002).
>> 
>> 2. Remove the use-lists!  Replace them with ref-counts to keep most of
>>   the use-list API functional (and minimize the size of the change).
>>   See the WIP patch below (0003).
>> 
>> 3. (Optional) Remove use-lists from other non-GlobalValue Constants
>>   that do not reference any GlobalValues.  This would require some
>>   sort of magic in, e.g., ConstantVector to conditionally have a
>>   use-list.  Call sites of API like Value::use_begin would have to
>>   check for Value::hasUseList.
> 
> Could you explain why #3 requires checking for hasListList() or e.g
> isa<ConstantVectorWithGlobalRef>() before calling use_begin()?
Same reason #2 (WIP patch 0003) requires isa<ConstantData> or
Value::hasUseList before calling use_begin.  Accessing Value::use_begin and
Value::use_end means that the call site is trying to walk through the use-list. 
Since we don't track the use-list for ConstantData, any call site that's
inferring something from an empty-looking use-list will have a bug.  IIRC, 0003
changes Value::use_begin and Value::use_end to assert that Value::hasUseList; if
it doesn't, it should.
>> 4. (Optional) Remove the ref-count from ConstantData (and, potentially,
>>   other use-list-free Constants).  This would eliminate ref-count
>>   traffic, but would also require checking at call sites before using
>>   any use-list-related API.
>> 
>> Feedback
>> =======>> 
>> - Does anyone disagree with this general direction?  Why?
>> - Any thoughts on #3?
>> - Any thoughts on #4?
>> 
>> <0001-WIP-HACK-SimplifyLibCalls-Disable-optimizeSinCosPi-o.patch>
>> <0002-WIP-HACK-LICM-Ignore-stores-to-UndefValue-and-Consta.patch>
>> <0003-WIP-IR-Remove-use-lists-from-ConstantData.patch>
> 
> We should probably improve the use-list-order2.ll test instead of deleting
it.
> I've attached a patch to PR24755 which might help with this
("Improve use-list
> order testing for function operands"). I think the test should fail if
its uses
> of ConstantData are replaced by e.g personality functions.
Agreed.  Thanks for the patch.