>
> Hey Dan, I've talked with you about this in person and on IRC, but I've
not yet laid out my thoughts on a single place, so I'll put them here.
>
> TL;DR: I really like the idea of using metadata to tag each member of a
struct with TBAA, and re-using the TBAA metadata nodes we already have. I'm
not as fond of the description of padding in the metadata node.
>
> Currently padding is really hard to represent because there is sometimes
a member of an LLVM struct which represents padding (packed structs and
cases where the frontend type requires more alignment than the datalayout
string specifies) and other times there isn't. The current proposal doesn't
entirely fix this because we still will need some way to annotate the
members of structs inserted purely for the purpose of padding.
This is not a problem in the current proposal, because it represents
padding
completely independently from the LLVM struct type.
It is a complexity of your proposal that I find unfortunate. =/ I suspect
it will also add a small amount of complexity to the consumers of the
analysis based on my work on SROA, although its possible that pass is just
unique in the questions it needs to ask.
> Further, we have the problem that sometimes what is needed is a
representation of a "hole", that is a region which is neither padding nor
part of the struct itself. The canonical example is the tail padding of a
base class where the derived class's first member has low alignent
constraints.
I don't see how a hole in a base class which isn't being used by a
subclass is
different from padding, from the optimizer's perspective. The optimizer
doesn't know about class hierarchies (unless you're proposing something
much more significant).
Because there is real data packed into that space. Because the LLVM type
system has no way to represent this, Clang turns such base classes into an
opaque array of i8 with the appropriate size, and *all* structural
information about this base class is lost. I'm trying to propose something
that is sufficiently powerful to no longer require such hacks in the
frontend.
>
> I would propose that we solve these problems by a somewhat more invasive
change, but one which will significantly simplify both LLVM and frontends
(at least Clang, I suspect other frontends):
>
> Remove non-packed struct types completely. Make LLVM structs represent a
contiguous sequence of bytes, explicitly partitioned into fields with
particular primitive types.
>
> The idea would be to make all struct types be packed[1], and to
represent padding as explicit members of the struct. These could in turn
have a "padding" TBAA metadata node which would specify that member as
being padding. This would simplify the metadata representation because
there would *always* be a member to hang the padding tag off of. It would
simplify struct layout analysis in LLVM because the difference between
alloc-size and type-size would be irrelevant. It would dramatically
simplify Clang's record layout building, which already has to fall back to
packed LLVM structs in many cases because normal structs produce offsets
that conflict with the ABI's layout requirements.
>
> Essentially, LLVM is trying to simplify ABI layout by providing a
datalayout summary description of target alignments, and building structs
with that algorithm. But unless this *exactly* matches the ABI in question,
it actually makes the job harder because now we have to try, potentially
fail, and end up with all the code to use the packed mode anyways. My
theory is that there are too many ABIs in the world (and too weird rules
within them) for us to ever really get this right at the LLVM layer.
Instead, we should force the frontend to explicitly layout the bytes as it
sees fit.
The current situation is not bleak. ABIs don't often vary that much in the
way they lay out structs and arrays, especially within a given
architecture.
Well, clearly we disagree here. ;] Have you read Clang's record layout
building code recently? Or tried to change it? It is a nightmare.
LLVM simply doesn't provide the tools to reasonably express things like
packing of low-alignment members into padding with nested sub-aggregates,
or bitfield packing.
I actually think it's kind of nice that LLVM has this native concept of
"normal"
struct layout built in. It encourages people to avoid doing their own
custom struct
layout unless they have a good reason to.
But every frontend ends up with a reason to. And they all do it
differently, and their lives are made harder and more complex by trying to
utilize LLVM's struct layout when it works, but roll their own when it
doesn't. Having looked extensively at Clang's recently, I am confident that
it would be much simpler, and produce dramatically more clear LLVM types
after my proposal. That argues for LLVM's system not being good enough, and
my contention is that there simply is not any language-neutral struct
layout system other than: the bytes in memory. So let's go back to
representing a sequence of bytes in memory as an aggregation of primitive
types.
I think your proposal would solve the original problem here, but it's not
obviously
better than the metadata approach.
I think it will be conceptually cleaner, and I think it will allow the
frontend to expose more structural information when currently it is unable
to due to that structural information not fitting into LLVM's struct layout
model.
Bytes in memory in LLVM don't have inherent types,
so optimizers can't rely on the type, or on any individual copy, to
understand the
lifetimes of data in storage allocated for padding. Consequently, a type
just
becomes a way to attach information to a copy. And it's not clear that
using a type
is better than using metadata.
Aggregate types are most useful when applied to bytes of memory as a means
of partitioning accesses to those bytes of memory into operations on
distinct primitives, and simplifying the formation of byte offset pointers
to load and the properly typed loaded values in the IR. (IE, avoiding
complex GEPs and/or bitcasts before every load).
I don't think metadata helps with that at all.
The metadata approach is nice because it separates the use cases into two
families.
On one side, copies with no metadata are simple to create, simple to
understand, and
simple to implement. On the other side, people who need more features can
add
metadata to get there, and things are more complex all around, but that's
the price
of using advanced features. This is the shape of problem that metadata was
intended
to solve.
Nothing I propose would change copies though? I'm just trying to make the
struct type able to have members for each actual primitive member that is
accessed within a range of memory...
And I'm still suggesting to use metadata to differentiate between padding
and non-padding -- i *don't* think that belongs in the type system. I think
only the *layout* belongs in the type system because that is what powers
GEPs, the primary means of computing offsets.
