As there is now certain areas where we now have the possibility of
having either a ModuleOp or GPUModuleOp and both of these modules can
have DataLayout's and we may require utilising the DataLayout utilities
in these areas I've taken the liberty of trying to extend them for use
with both.
Those with more knowledge of how they wish the GPUModuleOp's to interact
with their parent ModuleOp's DataLayout may have further alterations
they wish to make in the future, but for the moment, it'll simply
utilise the basic data layout construction which I believe combines
parent and child datalayouts from the ModuleOp and GPUModuleOp. If there
is no GPUModuleOp DataLayout it should default to the parent ModuleOp.
It's worth noting there is some weirdness if you have two module
operations defining builtin dialect DataLayout Entries, it appears the
combinatorial functionality for DataLayouts doesn't support the merging
of these.
This behaviour is useful for areas like:
https://github.com/llvm/llvm-project/pull/119585/files#diff-19fc4bcb38829d085e25d601d344bbd85bf7ef749ca359e348f4a7c750eae89dR1412
where we have a crossroads between the two different module operations.
In the context of C/Fortran interoperability (BIND(C)), it is possible
to give the VALUE attribute to a BIND(C) derived type dummy, which
according to Fortran 2018 18.3.6 - 2. (4) implies that it must be passed
like the equivalent C structure value. The way C structure value are
passed is ABI dependent.
LLVM does not implement the C struct ABI passing for LLVM aggregate type
arguments. It is up to the front-end, like clang is doing, to split the
struct into registers or pass the struct on the stack (llvm "byval") as
required by the target ABI.
So the logic for C struct passing sits in clang. Using it from flang
requires setting up a lot of clang context and to bridge FIR/MLIR
representation to clang AST representation for function signatures (in
both directions). It is a non trivial task.
See
https://stackoverflow.com/questions/39438033/passing-structs-by-value-in-llvm-ir/75002581#75002581.
Since BIND(C) struct are rather limited as opposed to generic C struct
(e.g. no bit fields). It is easier to provide a limited implementation
of it for the case that matter to Fortran.
This patch:
- Updates the generic target rewrite pass to keep track of both the new
argument type and attributes. The motivation for this is to be able to
tell if a previously marshalled argument is passed in memory (it is a C
pointer), or if it is being passed on the stack (has the byval llvm
attributes).
- Adds an entry point in the target specific codegen to marshal struct
arguments, and use it in the generic target rewrite pass.
- Implements limited support for the X86-64 case. So far, the support
allows telling if a struct must be passed in register or on the stack,
and to deal with the stack case. The register case is left TODO in this
patch.
The X86-64 ABI implemented is the System V ABI for AMD64 version 1.0
Preliminary patch to change lowering/code generation to use
llvm::DataLayout information instead of generating "sizeof" GEP (see
https://github.com/llvm/llvm-project/issues/71507).
Fortran Semantic analysis needs to know about the target type size and
alignment to deal with common blocks, and intrinsics like
C_SIZEOF/TRANSFER. This information should be obtained from the
llvm::DataLayout so that it is consistent during the whole compilation
flow.
This change is changing flang-new and bbc drivers to:
1. Create the llvm::TargetMachine so that the data layout of the target
can be obtained before semantics.
2. Sharing bbc/flang-new set-up of the
SemanticConstext.targetCharateristics from the llvm::TargetMachine. For
now, the actual part that set-up the Fortran type size and alignment
from the llvm::DataLayout is left TODO so that this change is mostly an
NFC impacting the drivers.
3. Let the lowering bridge set-up the mlir::Module datalayout attributes
since it is doing it for the target attribute, and that allows the llvm
data layout information to be available during lowering.
For flang-new, the changes are code shuffling: the `llvm::TargetMachine`
instance is moved to `CompilerInvocation` class so that it can be used
to set-up the semantic contexts. `setMLIRDataLayout` is moved to
`flang/Optimizer/Support/DataLayout.h` (it will need to be used from
codegen pass for fir-opt target independent testing.)), and the code
setting-up semantics targetCharacteristics is moved to
`Tools/TargetSetup.h` so that it can be shared with bbc.
As a consequence, LLVM targets must be registered when running
semantics, and it is not possible to run semantics for a target that is
not registered with the -triple option (hence the power pc specific
modules can only be built if the PowerPC target is available.
