[llvm-dev] RFC: Generate plain !tbaa tags in place of !tbaa.struct ones

[Ivan Kosarev via llvm-dev]

On 02/11/17 05:54, Hal Finkel wrote:
>
> On 10/31/2017 05:02 AM, Ivan Kosarev wrote:
>> To clarify further, what this paper proposes is to use !tbaa for all 
>> kinds of accesses, including aggregate ones, so we don't need to 
>> bother trying to convert them when an aggregate access becomes a 
>> series of scalar accesses or vice versa. As I said, in most cases 
>> such conversions are not possible anyway, because !tbaa.struct tags 
>> do not refer to the type of the aggregate itself and only enumerate 
>> its members.
>
> That makes sense, but I don't believe you actually said *that*. I 
> agree, the fact that the tbaa.struct metadata refers to the scalar 
> types, and not the type itself, will prohibit the conversion in general.
>
> I agree that we should fix this. Given that we're planning to make 
> changes, perhaps major ones, to the TBAA representation, we should 
> account for this requirement  as part of that larger discussion. I see 
> no need for a separate thread on this particular topic in the context 
> of those larger changes.

Sure, we can discuss all the related things here. I don't mind if we 
should change the subject line to something more appropriate.

>
> That having been said, if we desire to fix this before those larger 
> changes to the TBAA representation (to handle aggregates, unions, 
> etc.), I think we should do that. In this case, however, we can/should 
> make a small enhancement that addresses this representational 
> deficiency.  We can, for example, generate tbaa.struct metadata where 
> the tbaa struct type is the first in the list. This is easy to 
> distinguish from the existing format because in the existing format 
> the first operand is an integer. This will provide the necessary 
> information to allow SROA to create struct access tags for the 
> scalarized accesses.

Generating a !tbaa tag in addition to the !tbaa.struct would probably be 
even simpler. Unfortunately, that alone wouldn't be enough as !tbaa tags 
in place of/as part of/next to !tbaa.struct tags only make sense if we 
can represent accesses to aggregates, so we need this to be done first. 
Adding support for aggregate accesses, in turn, requires us to be able 
to determine common types for aggregates plus some other things that 
imply changes to the metadata format.

If we are ready to discuss specific changes to the format, then I would 
propose the following:

* All types, including aggregate ones, to refer to their parent types. 
Otherwise, we couldn't determine the common type for a couple of 
aggregate final access types. Since all type nodes shall have this 
field, it would be natural to make it first in the list while also 
making it possible to detect the new format.

* Encode access sizes. This is essential for a better support for 
accesses to union members. This patch makes the first step in this 
direction:

https://reviews.llvm.org/D39455

but suffers from being imprecise WRT what specific part of the union 
object is accessed. Given we know the offset range (the base offset 
together with the access size), we can determine if two accesses to 
members of the same union type actually overlap.

* Encode type sizes. This, provided in addition to access sizes, is what 
we could use to support member arrays. As of now, we describe all 
accesses to array elements, including elements of member arrays, as if 
they were standalone objects, because there is no way to represent an 
access to a range of array elements or a specific array element. This 
means all TBAA information about the base lvalue gets lost.

Once we can encode the information about the size of the element type 
and the size of the access, we can determine the range of potentially 
accessed elements. This is how it is supposed work in terms of TBAA 
metadata:

struct S {
   int i[7];
};

; The good old int. 4 bytes in size. Mutable.
!3 = !{!1, i64 4, i64 0, !"int"}

; A structure containing an array of seven ints.
; We know the element type is int because we explicitly refer to the 
node of that type.
; We know there are 7 of them because the size of the field is 28 and 
the size of the type is 4.
!5 = !{!1, i64 28, i64 0, !"_ZTS1S", !3, i64 0, i64 28}

; An access to all or any of the array elements in S.
!7 = !{!5, !3, i64 0, i64 28}

; An access to the element with index 2.
!9 = !{!5, !3, i64 8, i64 4}

The same principle can be applied to describing accesses to slices of 
scalarized aggregates.

* We would need to generalize the concept of type identifiers. Strings 
containing mangled names are not enough because tagged types in C do not 
have mangled names and there are local types in C++ with no linkage and 
there are tagged types that have no names. As a result, in some cases 
like this one:

https://bugs.llvm.org/show_bug.cgi?id=8572

we count different types to be the same type.

If we can allow type identifiers to be metadata nodes of unspecified 
content, then we could generate as detailed identifiers as we need to 
support the rules of the input language. For example, C's structures and 
unions are subject to the type compatibility rules that respect things 
like widths of bit fields and the order of members. And for local types 
in C++ we could produce 'distinct' metadata nodes.

Thanks,


>
> Thanks again,
> Hal
>
>>
>> As of today, there is no adequate representation that could be used 
>> to describe accesses to individual slices of scalarized aggregates. 
>> But at very least we could decorate accesses to slices with tags of 
>> their aggregates. That requires aggregate accesses to be described 
>> the same way we describe scalar accesses.
>>
>> To provide some background on importance of this matter, SROA and 
>> passing flattened parameters/returning values in clang together 
>> produce 75% of all undecorated loads and 90% if all undecorated 
>> stores on the LLVM code base under -O1.
>>
>>
>> On 31/10/17 10:44, Ivan Kosarev wrote:
>>> In short, the problem with !tbaa.struct is that in most cases it 
>>> cannot be converted to !tbaa. For transformations like SROA this 
>>> means they cannot propagate !tbaa.struct tags for 
>>> aggregate-accessing instructions like memcpy() calls to the 
>>> resulting loads and stores.
>>>
>>>
>>> On 31/10/17 05:11, Hal Finkel wrote:
>>>>
>>>> On 10/18/2017 05:49 AM, Ivan Kosarev via llvm-dev wrote:
>>>>> Hello,
>>>>>
>>>>> The motivation behind this proposal is to make it possible to
>>>>> propagate TBAA information through scalarizing transformations,
>>>>> such as SROA, that rewrite accesses to aggregates, e.g., memcpy()
>>>>> calls, into accesses to scalars, that is, load and store
>>>>> instructions.
>>>>>
>>>>> Currently, we decorate instructions that initialize and copy
>>>>> aggregates with !tbaa.struct tags that generally cannot be
>>>>> transformed to !tbaa tags. For this reason every time we
>>>>> scalarize an aggregate, we leave resulting loads and stores
>>>>> undecorated, meaning optimization of such instructions cannot
>>>>> benefit from TBAA.
>>>>
>>>> I understand that we'd like to improve TBAA in a number of 
>>>> different ways, and that probably involves replacing !tbaa.struct 
>>>> with an enhanced !tbaa representation, but I don't understand the 
>>>> point of this proposal. As you state, there are lots of places that 
>>>> could use !tbaa.struct information, such as SROA, but don't. Is the 
>>>> current !tbaa.struct missing some information that is essential for 
>>>> that purpose?
>>>>
>>>>  -Hal
>>>>
>>>>>
>>>>> Furthermore, our analysis indicates that the only place where
>>>>> !tbaa.struct tags may potentially impact code generation is
>>>>> simplification of memcpy() and memmove() calls, see
>>>>> SimplifyMemTransfer() in InstCombineCalls.cpp. Ironically, what
>>>>> the code that makes that sole use of such tags is trying to do is
>>>>> to construct a !tbaa tag from the information encoded in the
>>>>> given !tbaa.struct tag. Note that it can only do that if the
>>>>> !tbaa.struct tag describes a structure with a single member of a
>>>>> scalar type.
>>>>>
>>>>> Here's how we propose to resolve the issue in terms of specific
>>>>> steps:
>>>>>
>>>>> 1. Extend the TBAA facilities in clang to support aggregate types
>>>>>    as final access types. This patch:
>>>>>
>>>>>    [CodeGen] Propagate may-alias'ness of lvalues with TBAA info
>>>>>    https://reviews.llvm.org/D39008
>>>>>
>>>>>    implements this for the needs of fixing issues with
>>>>>    propagation of TBAA information, which in turn is necessary to
>>>>>    support TBAA for unions. So once this patch is committed, this
>>>>>    step is done.
>>>>>
>>>>> 2. Generate !tbaa tags in addition to !tbaa.struct tags for
>>>>>    aggregate accesses.
>>>>>
>>>>> 3. Fix the TBAA analysis to support aggregate accesses as
>>>>>    explained in this proposal:
>>>>>
>>>>>    RFC: Resolving TBAA issues
>>>>> http://lists.llvm.org/pipermail/llvm-dev/2017-August/116652.html
>>>>>
>>>>> 4. Switch the code that simplifies memcpy() and memmove() calls
>>>>>    to !tbaa tags.
>>>>>
>>>>> 5. Remove the support for !tbaa.struct tags.
>>>>>
>>>>>    I guess we would want to remove all code which only purpose is
>>>>>    to generate such tags, but I'm not sure what we should do with
>>>>>    the MD_tbaa_struct enumerator. Some possible options:
>>>>>    a) Leave it as is, just don't use it.
>>>>>    b) Rename to something like MD_unused.
>>>>>    c) Remove, but do not change the values of other MD_*
>>>>>       enumerators.
>>>>>    d) Remove and adjust values of other MD_* enumerators
>>>>>       respectively.
>>>>>    Or, maybe we want some multi-stage plan here?
>>>>>
>>>>> Further steps are supposed to include things like fixing SROA to
>>>>> propagate TBAA information.
>>>>>
>>>>> Any feedback is highly appreciated.
>>>>>
>>>>> Thanks,
>>>>>
>>>>
>>>
>>
>

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