[Clang][P1061] Add stuctured binding packs (#121417)

This is an implementation of P1061 Structure Bindings Introduce a Pack
without the ability to use packs outside of templates. There is a couple
o

[Clang][P1061] Add stuctured binding packs (#121417)

This is an implementation of P1061 Structure Bindings Introduce a Pack
without the ability to use packs outside of templates. There is a couple
of ways the AST could have been sliced so let me know what you think.
The only part of this change that I am unsure of is the
serialization/deserialization stuff. I followed the implementation of
other Exprs, but I do not really know how it is tested. Thank you for
your time considering this.

---------

Co-authored-by: Yanzuo Liu <zwuis@outlook.com>

show more ...

[SYCL] AST support for SYCL kernel entry point functions. (#122379)

A SYCL kernel entry point function is a non-member function or a static
member function declared with the `sycl_kernel_entry_poin

[SYCL] AST support for SYCL kernel entry point functions. (#122379)

A SYCL kernel entry point function is a non-member function or a static
member function declared with the `sycl_kernel_entry_point` attribute.
Such functions define a pattern for an offload kernel entry point
function to be generated to enable execution of a SYCL kernel on a
device. A SYCL library implementation orchestrates the invocation of
these functions with corresponding SYCL kernel arguments in response to
calls to SYCL kernel invocation functions specified by the SYCL 2020
specification.

The offload kernel entry point function (sometimes referred to as the
SYCL kernel caller function) is generated from the SYCL kernel entry
point function by a transformation of the function parameters followed
by a transformation of the function body to replace references to the
original parameters with references to the transformed ones. Exactly how
parameters are transformed will be explained in a future change that
implements non-trivial transformations. For now, it suffices to state
that a given parameter of the SYCL kernel entry point function may be
transformed to multiple parameters of the offload kernel entry point as
needed to satisfy offload kernel argument passing requirements.
Parameters that are decomposed in this way are reconstituted as local
variables in the body of the generated offload kernel entry point
function.

For example, given the following SYCL kernel entry point function
definition:
```
template<typename KernelNameType, typename KernelType>
[[clang::sycl_kernel_entry_point(KernelNameType)]]
void sycl_kernel_entry_point(KernelType kernel) {
kernel();
}
```

and the following call:
```
struct Kernel {
int dm1;
int dm2;
void operator()() const;
};
Kernel k;
sycl_kernel_entry_point<class kernel_name>(k);
```

the corresponding offload kernel entry point function that is generated
might look as follows (assuming `Kernel` is a type that requires
decomposition):
```
void offload_kernel_entry_point_for_kernel_name(int dm1, int dm2) {
Kernel kernel{dm1, dm2};
kernel();
}
```

Other details of the generated offload kernel entry point function, such
as its name and calling convention, are implementation details that need
not be reflected in the AST and may differ across target devices. For
that reason, only the transformation described above is represented in
the AST; other details will be filled in during code generation.

These transformations are represented using new AST nodes introduced
with this change. `OutlinedFunctionDecl` holds a sequence of
`ImplicitParamDecl` nodes and a sequence of statement nodes that
correspond to the transformed parameters and function body.
`SYCLKernelCallStmt` wraps the original function body and associates it
with an `OutlinedFunctionDecl` instance. For the example above, the AST
generated for the `sycl_kernel_entry_point<kernel_name>` specialization
would look as follows:
```
FunctionDecl 'sycl_kernel_entry_point<kernel_name>(Kernel)'
TemplateArgument type 'kernel_name'
TemplateArgument type 'Kernel'
ParmVarDecl kernel 'Kernel'
SYCLKernelCallStmt
CompoundStmt
<original statements>
OutlinedFunctionDecl
ImplicitParamDecl 'dm1' 'int'
ImplicitParamDecl 'dm2' 'int'
CompoundStmt
VarDecl 'kernel' 'Kernel'
<initialization of 'kernel' with 'dm1' and 'dm2'>
<transformed statements with redirected references of 'kernel'>
```

Any ODR-use of the SYCL kernel entry point function will (with future
changes) suffice for the offload kernel entry point to be emitted. An
actual call to the SYCL kernel entry point function will result in a
call to the function. However, evaluation of a `SYCLKernelCallStmt`
statement is a no-op, so such calls will have no effect other than to
trigger emission of the offload kernel entry point.

Additionally, as a related change inspired by code review feedback,
these changes disallow use of the `sycl_kernel_entry_point` attribute
with functions defined with a _function-try-block_. The SYCL 2020
specification prohibits the use of C++ exceptions in device functions.
Even if exceptions were not prohibited, it is unclear what the semantics
would be for an exception that escapes the SYCL kernel entry point
function; the boundary between host and device code could be an implicit
noexcept boundary that results in program termination if violated, or
the exception could perhaps be propagated to host code via the SYCL
library. Pending support for C++ exceptions in device code and clear
semantics for handling them at the host-device boundary, this change
makes use of the `sycl_kernel_entry_point` attribute with a function
defined with a _function-try-block_ an error.

show more ...

[C++20] [Modules] Support module level lookup (#122887) (#123281)

Close https://github.com/llvm/llvm-project/issues/90154

This patch is also an optimization to the lookup process to utilize the

[C++20] [Modules] Support module level lookup (#122887) (#123281)

Close https://github.com/llvm/llvm-project/issues/90154

This patch is also an optimization to the lookup process to utilize the
information provided by `export` keyword.

Previously, in the lookup process, the `export` keyword only takes part
in the check part, it doesn't get involved in the lookup process. That
said, previously, in a name lookup for 'name', we would load all of
declarations with the name 'name' and check if these declarations are
valid or not. It works well. But it is inefficient since it may load
declarations that may not be wanted.

Note that this patch actually did a trick in the lookup process instead
of bring module information to DeclarationName or considering module
information when deciding if two declarations are the same. So it may
not be a surprise to me if there are missing cases. But it is not a
regression. It should be already the case. Issue reports are welcomed.

In this patch, I tried to split the big lookup table into a lookup table
as before and a module local lookup table, which takes a combination of
the ID of the DeclContext and hash value of the primary module name as
the key. And refactored `DeclContext::lookup()` method to take the
module information. So that a lookup in a DeclContext won't load
declarations that are local to **other** modules.

And also I think it is already beneficial to split the big lookup table
since it may reduce the conflicts during lookups in the hash table.

BTW, this patch introduced a **regression** for a reachability rule in
C++20 but it was false-negative. See
'clang/test/CXX/module/module.interface/p7.cpp' for details.

This patch is not expected to introduce any other
regressions for non-c++20-modules users since the module local lookup
table should be empty for them.

show more ...

[AST] Add OriginalDC argument to ExternalASTSource::FindExternalVisibleDeclsByName (#123152)

Part for relanding https://github.com/llvm/llvm-project/pull/122887.

I split this to test where the pe

[AST] Add OriginalDC argument to ExternalASTSource::FindExternalVisibleDeclsByName (#123152)

Part for relanding https://github.com/llvm/llvm-project/pull/122887.

I split this to test where the performance regession comes from if
modules are not used.

show more ...

Revert "[C++20] [Modules] Support module level lookup (#122887)"

This reverts commit 7201cae106260aeb3e9bbbb7d5291ff30f05076a.

[C++20] [Modules] Support module level lookup (#122887)

Close https://github.com/llvm/llvm-project/issues/90154

This patch is also an optimization to the lookup process to utilize the
informatio

[C++20] [Modules] Support module level lookup (#122887)

Close https://github.com/llvm/llvm-project/issues/90154

This patch is also an optimization to the lookup process to utilize the
information provided by `export` keyword.

Previously, in the lookup process, the `export` keyword only takes part
in the check part, it doesn't get involved in the lookup process. That
said, previously, in a name lookup for 'name', we would load all of
declarations with the name 'name' and check if these declarations are
valid or not. It works well. But it is inefficient since it may load
declarations that may not be wanted.

Note that this patch actually did a trick in the lookup process instead
of bring module information to DeclarationName or considering module
information when deciding if two declarations are the same. So it may
not be a surprise to me if there are missing cases. But it is not a
regression. It should be already the case. Issue reports are welcomed.

In this patch, I tried to split the big lookup table into a lookup table
as before and a module local lookup table, which takes a combination of
the ID of the DeclContext and hash value of the primary module name as
the key. And refactored `DeclContext::lookup()` method to take the
module information. So that a lookup in a DeclContext won't load
declarations that are local to **other** modules.

And also I think it is already beneficial to split the big lookup table
since it may reduce the conflicts during lookups in the hash table.

BTW, this patch introduced a **regression** for a reachability rule in
C++20 but it was false-negative. See
'clang/test/CXX/module/module.interface/p7.cpp' for details.

This patch is not expected to introduce any other
regressions for non-c++20-modules users since the module local lookup
table should be empty for them.

---

On the API side, this patch unfortunately add a maybe-confusing argument
`Module *NamedModule` to
`ExternalASTSource::FindExternalVisibleDeclsByName()`. People may think
we can get the information from the first argument `const DeclContext
*DC`. But sadly there are declarations (e.g., namespace) can appear in
multiple different modules as a single declaration. So we have to add
additional information to indicate this.

show more ...

[clang][NFC]add static for internal linkage function (#117482)

Detected by misc-use-internal-linkage

[AST] Remove unused includes (NFC) (#116549)

Identified with misc-include-cleaner.

[C++20][Modules] Relax ODR check in unnamed modules (#111160)

Summary:
Option `-fskip-odr-check-in-gmf` is set by default and I think it is
what most of C++ developers want. But in header units, C

[C++20][Modules] Relax ODR check in unnamed modules (#111160)

Summary:
Option `-fskip-odr-check-in-gmf` is set by default and I think it is
what most of C++ developers want. But in header units, Clang ODR
checking is too strict, making them hard to use, as seen in the example
in the diff. This diff relaxes ODR checks for unnamed modules to match
GMF ODR checking.

Test Plan: check-clang

show more ...

Return available function types for BindingDecls. (#102196)

Only return nullptr when we don't have an available QualType.

Reland [C++20] [Modules] [Itanium ABI] Generate the vtable in the mod… (#102287)

Reland https://github.com/llvm/llvm-project/pull/75912

The differences of this PR between
https://github.com/llvm

Reland [C++20] [Modules] [Itanium ABI] Generate the vtable in the mod… (#102287)

Reland https://github.com/llvm/llvm-project/pull/75912

The differences of this PR between
https://github.com/llvm/llvm-project/pull/75912 are:

- Fixed a regression in `Decl::isInAnotherModuleUnit()` in DeclBase.cpp
pointed by @mizvekov and add the corresponding test.
- Fixed the regression in windows
https://github.com/llvm/llvm-project/issues/97447. The changes are in
`CodeGenModule::getVTableLinkage` from
`clang/lib/CodeGen/CGVTables.cpp`. According to the feedbacks from MSVC
devs, the linkage of vtables won't affected by modules. So I simply
skipped the case for MSVC.

Given this is more or less fundamental to the use of modules. I hope we
can backport this to 19.x.

show more ...

[C++20] [Modules] Skip ODR checks in implicit global modules

Previously we skipped the ODR checks in explicit global modules. And due
to similar reasons, we should skip the ODR checks in implicit gl

[C++20] [Modules] Skip ODR checks in implicit global modules

Previously we skipped the ODR checks in explicit global modules. And due
to similar reasons, we should skip the ODR checks in implicit global
modules too.

show more ...

[Clang] Remove `IDNS_Ordinary` flag in `IndirectFieldDecl::IdentifierNamespace` (#100525)

There is a `IDNS_Ordinary` flag in
`IndirectFieldDecl::IdentifierNamespace` so that members in nested
anon

[Clang] Remove `IDNS_Ordinary` flag in `IndirectFieldDecl::IdentifierNamespace` (#100525)

There is a `IDNS_Ordinary` flag in
`IndirectFieldDecl::IdentifierNamespace` so that members in nested
anonymous struct/union can be found as ordinary identifiers.

```c
struct S {
struct { int x; };
// Previous behaviour: `x` in previous line is found
// Expected: nothing is found
int arr[sizeof(x)];
};
```

This PR fixes this issue.

Fixes #31295.

show more ...

[Clang][AST] Move NamespaceDecl bits to DeclContext (#98567)

Currently, `NamespaceDecl` has a member `AnonOrFirstNamespaceAndFlags`
which stores a few pieces of data:
- a bit indicating whether th

[Clang][AST] Move NamespaceDecl bits to DeclContext (#98567)

Currently, `NamespaceDecl` has a member `AnonOrFirstNamespaceAndFlags`
which stores a few pieces of data:
- a bit indicating whether the namespace was declared `inline`, and
- a bit indicating whether the namespace was declared as a
_nested-namespace-definition_, and
- a pointer a `NamespaceDecl` that either stores:
- a pointer to the first declaration of that namespace if the
declaration is no the first declaration, or
- a pointer to the unnamed namespace that inhabits the namespace
otherwise.

`Redeclarable` already stores a pointer to the first declaration of an
entity, so it's unnecessary to store this in `NamespaceDecl`.
`DeclContext` has 8 bytes in which various bitfields can be stored for a
declaration, so it's not necessary to store these in `NamespaceDecl`
either. We only need to store a pointer to the unnamed namespace that
inhabits the first declaration of a namespace. This patch moves the two
bits currently stored in `NamespaceDecl` to `DeclContext`, and only
stores a pointer to the unnamed namespace that inhabits a namespace in
the first declaration of that namespace. Since `getOriginalNamespace`
always returns the same `NamespaceDecl` as `getFirstDecl`, this function
is removed to avoid confusion.

show more ...

[NFC] [Modules] Introduce 'DeclBase::isInNamedModule' interface

This patch introduces DeclBase::isInNamedModule API to ease the use
of modules slightly.

Revert "[C++20] [Modules] [Itanium ABI] Generate the vtable in the module unit of dynamic classes (#75912)"

This reverts commit 18f3bcbb13ca83d33223b00761d8cddf463e9ffb, 15bb02650e26875c48889053d6a9

Revert "[C++20] [Modules] [Itanium ABI] Generate the vtable in the module unit of dynamic classes (#75912)"

This reverts commit 18f3bcbb13ca83d33223b00761d8cddf463e9ffb, 15bb02650e26875c48889053d6a9697444583721 and
99873b35da7ecb905143c8a6b8deca4d4416f1a9.

See the post commit message in
https://github.com/llvm/llvm-project/pull/75912 to see the reasons.

show more ...

[NFC] [Serialization] Unify how LocalDeclID can be created

Now we can create a LocalDeclID directly with an integer without
verifying. It may be hard to refactor if we want to change the way we
seri

[NFC] [Serialization] Unify how LocalDeclID can be created

Now we can create a LocalDeclID directly with an integer without
verifying. It may be hard to refactor if we want to change the way we
serialize DeclIDs (See https://github.com/llvm/llvm-project/pull/95897).
Also it is hard for us to debug if someday someone construct a
LocalDeclID with an incorrect value.

So in this patch, I tried to unify the way we can construct a
LocalDeclID in ASTReader, where we will construct the LocalDeclID from
the serialized data. Also, now we can verify the constructed LocalDeclID
sooner in the new interface.

show more ...

[HLSL] Strict Availability Diagnostics (#93860)

Implements HLSL availability diagnostics' strict mode.

HLSL availability diagnostics emits errors or warning when unavailable
shader APIs are used

[HLSL] Strict Availability Diagnostics (#93860)

Implements HLSL availability diagnostics' strict mode.

HLSL availability diagnostics emits errors or warning when unavailable
shader APIs are used. Unavailable shader APIs are APIs that are exposed
in HLSL code but are not available in the target shader stage or shader
model version.

In the strict mode the compiler emits an error when an unavailable API
is found in any function regardless of whether it is reachable from the
shader entry point or not. This mode is enabled by
``-fhlsl-strict-availability``.

See HLSL Availability Diagnostics design doc
[here](https://github.com/llvm/llvm-project/blob/main/clang/docs/HLSL/AvailabilityDiagnostics.rst)
for more details.

Fixes #90096

show more ...

[C++20] [Modules] [Itanium ABI] Generate the vtable in the module unit of dynamic classes (#75912)

Close https://github.com/llvm/llvm-project/issues/70585 and reflect
https://github.com/itanium-cxx

[C++20] [Modules] [Itanium ABI] Generate the vtable in the module unit of dynamic classes (#75912)

Close https://github.com/llvm/llvm-project/issues/70585 and reflect
https://github.com/itanium-cxx-abi/cxx-abi/issues/170.

The significant change of the patch is: for dynamic classes attached to
module units, we generate the vtable to the attached module units
directly and the key functions for such classes is meaningless.

show more ...

[clang] Replace X && isa<Y>(X) with isa_and_nonnull<Y>(X). NFC (#94987)

This addresses a clang-tidy suggestion.

[serialization] no transitive decl change (#92083)

Following of https://github.com/llvm/llvm-project/pull/86912

The motivation of the patch series is that, for a module interface unit
`X`, when the

[serialization] no transitive decl change (#92083)

Following of https://github.com/llvm/llvm-project/pull/86912

The motivation of the patch series is that, for a module interface unit
`X`, when the dependent modules of `X` changes, if the changes is not
relevant with `X`, we hope the BMI of `X` won't change. For the specific
patch, we hope if the changes was about irrelevant declaration changes,
we hope the BMI of `X` won't change. **However**, I found the patch
itself is not very useful in practice, since the adding or removing
declarations, will change the state of identifiers and types in most
cases.

That said, for the most simple example,

```
// partA.cppm
export module m:partA;

// partA.v1.cppm
export module m:partA;
export void a() {}

// partB.cppm
export module m:partB;
export void b() {}

// m.cppm
export module m;
export import :partA;
export import :partB;

// onlyUseB;
export module onlyUseB;
import m;
export inline void onluUseB() {
b();
}
```

the BMI of `onlyUseB` will change after we change the implementation of
`partA.cppm` to `partA.v1.cppm`. Since `partA.v1.cppm` introduces new
identifiers and types (the function prototype).

So in this patch, we have to write the tests as:

```
// partA.cppm
export module m:partA;
export int getA() { ... }
export int getA2(int) { ... }

// partA.v1.cppm
export module m:partA;
export int getA() { ... }
export int getA(int) { ... }
export int getA2(int) { ... }

// partB.cppm
export module m:partB;
export void b() {}

// m.cppm
export module m;
export import :partA;
export import :partB;

// onlyUseB;
export module onlyUseB;
import m;
export inline void onluUseB() {
b();
}
```

so that the new introduced declaration `int getA(int)` doesn't introduce
new identifiers and types, then the BMI of `onlyUseB` can keep
unchanged.

While it looks not so great, the patch should be the base of the patch
to erase the transitive change for identifiers and types since I don't
know how can we introduce new types and identifiers without introducing
new declarations. Given how tightly the relationship between
declarations, types and identifiers, I think we can only reach the ideal
state after we made the series for all of the three entties.

The design of the patch is similar to
https://github.com/llvm/llvm-project/pull/86912, which extends the
32-bit DeclID to 64-bit and use the higher bits to store the module file
index and the lower bits to store the Local Decl ID.

A slight difference is that we only use 48 bits to store the new DeclID
since we try to use the higher 16 bits to store the module ID in the
prefix of Decl class. Previously, we use 32 bits to store the module ID
and 32 bits to store the DeclID. I don't want to allocate additional
space so I tried to make the additional space the same as 64 bits. An
potential interesting thing here is about the relationship between the
module ID and the module file index. I feel we can get the module file
index by the module ID. But I didn't prove it or implement it. Since I
want to make the patch itself as small as possible. We can make it in
the future if we want.

Another change in the patch is the new concept Decl Index, which means
the index of the very big array `DeclsLoaded` in ASTReader. Previously,
the index of a loaded declaration is simply the Decl ID minus
PREDEFINED_DECL_NUMs. So there are some places they got used
ambiguously. But this patch tried to split these two concepts.

As https://github.com/llvm/llvm-project/pull/86912 did, the change will
increase the on-disk PCM file sizes. As the declaration ID may be the
most IDs in the PCM file, this can have the biggest impact on the size.
In my experiments, this change will bring 6.6% increase of the on-disk
PCM size. No compile-time performance regression observed. Given the
benefits in the motivation example, I think the cost is worthwhile.

show more ...

Revert "[serialization] no transitive decl change (#92083)"

This reverts commit 5c104879c1a98eeb845c03e7c45206bd48e88f0c.

The ArmV7 bot is complaining the change breaks the alignment.

[serialization] no transitive decl change (#92083)

Following of https://github.com/llvm/llvm-project/pull/86912

The motivation of the patch series is that, for a module interface unit
`X`, when the

[serialization] no transitive decl change (#92083)

Following of https://github.com/llvm/llvm-project/pull/86912

The motivation of the patch series is that, for a module interface unit
`X`, when the dependent modules of `X` changes, if the changes is not
relevant with `X`, we hope the BMI of `X` won't change. For the specific
patch, we hope if the changes was about irrelevant declaration changes,
we hope the BMI of `X` won't change. **However**, I found the patch
itself is not very useful in practice, since the adding or removing
declarations, will change the state of identifiers and types in most
cases.

That said, for the most simple example,

```
// partA.cppm
export module m:partA;

// partA.v1.cppm
export module m:partA;
export void a() {}

// partB.cppm
export module m:partB;
export void b() {}

// m.cppm
export module m;
export import :partA;
export import :partB;

// onlyUseB;
export module onlyUseB;
import m;
export inline void onluUseB() {
b();
}
```

the BMI of `onlyUseB` will change after we change the implementation of
`partA.cppm` to `partA.v1.cppm`. Since `partA.v1.cppm` introduces new
identifiers and types (the function prototype).

So in this patch, we have to write the tests as:

```
// partA.cppm
export module m:partA;
export int getA() { ... }
export int getA2(int) { ... }

// partA.v1.cppm
export module m:partA;
export int getA() { ... }
export int getA(int) { ... }
export int getA2(int) { ... }

// partB.cppm
export module m:partB;
export void b() {}

// m.cppm
export module m;
export import :partA;
export import :partB;

// onlyUseB;
export module onlyUseB;
import m;
export inline void onluUseB() {
b();
}
```

so that the new introduced declaration `int getA(int)` doesn't introduce
new identifiers and types, then the BMI of `onlyUseB` can keep
unchanged.

While it looks not so great, the patch should be the base of the patch
to erase the transitive change for identifiers and types since I don't
know how can we introduce new types and identifiers without introducing
new declarations. Given how tightly the relationship between
declarations, types and identifiers, I think we can only reach the ideal
state after we made the series for all of the three entties.

The design of the patch is similar to
https://github.com/llvm/llvm-project/pull/86912, which extends the
32-bit DeclID to 64-bit and use the higher bits to store the module file
index and the lower bits to store the Local Decl ID.

A slight difference is that we only use 48 bits to store the new DeclID
since we try to use the higher 16 bits to store the module ID in the
prefix of Decl class. Previously, we use 32 bits to store the module ID
and 32 bits to store the DeclID. I don't want to allocate additional
space so I tried to make the additional space the same as 64 bits. An
potential interesting thing here is about the relationship between the
module ID and the module file index. I feel we can get the module file
index by the module ID. But I didn't prove it or implement it. Since I
want to make the patch itself as small as possible. We can make it in
the future if we want.

Another change in the patch is the new concept Decl Index, which means
the index of the very big array `DeclsLoaded` in ASTReader. Previously,
the index of a loaded declaration is simply the Decl ID minus
PREDEFINED_DECL_NUMs. So there are some places they got used
ambiguously. But this patch tried to split these two concepts.

As https://github.com/llvm/llvm-project/pull/86912 did, the change will
increase the on-disk PCM file sizes. As the declaration ID may be the
most IDs in the PCM file, this can have the biggest impact on the size.
In my experiments, this change will bring 6.6% increase of the on-disk
PCM size. No compile-time performance regression observed. Given the
benefits in the motivation example, I think the cost is worthwhile.

show more ...

Revert "[serialization] no transitive decl change (#92083)"

This reverts commit 97c866f6c86456b3316006e6beff47e68a81c00a.

This fails on 32bit machines. See
https://github.com/llvm/llvm-project/pull

Revert "[serialization] no transitive decl change (#92083)"

This reverts commit 97c866f6c86456b3316006e6beff47e68a81c00a.

This fails on 32bit machines. See
https://github.com/llvm/llvm-project/pull/92083

show more ...

[serialization] no transitive decl change (#92083)

Following of https://github.com/llvm/llvm-project/pull/86912

The motivation of the patch series is that, for a module interface unit
`X`, when the

[serialization] no transitive decl change (#92083)

Following of https://github.com/llvm/llvm-project/pull/86912

The motivation of the patch series is that, for a module interface unit
`X`, when the dependent modules of `X` changes, if the changes is not
relevant with `X`, we hope the BMI of `X` won't change. For the specific
patch, we hope if the changes was about irrelevant declaration changes,
we hope the BMI of `X` won't change. **However**, I found the patch
itself is not very useful in practice, since the adding or removing
declarations, will change the state of identifiers and types in most
cases.

That said, for the most simple example,

```
// partA.cppm
export module m:partA;

// partA.v1.cppm
export module m:partA;
export void a() {}

// partB.cppm
export module m:partB;
export void b() {}

// m.cppm
export module m;
export import :partA;
export import :partB;

// onlyUseB;
export module onlyUseB;
import m;
export inline void onluUseB() {
b();
}
```

the BMI of `onlyUseB` will change after we change the implementation of
`partA.cppm` to `partA.v1.cppm`. Since `partA.v1.cppm` introduces new
identifiers and types (the function prototype).

So in this patch, we have to write the tests as:

```
// partA.cppm
export module m:partA;
export int getA() { ... }
export int getA2(int) { ... }

// partA.v1.cppm
export module m:partA;
export int getA() { ... }
export int getA(int) { ... }
export int getA2(int) { ... }

// partB.cppm
export module m:partB;
export void b() {}

// m.cppm
export module m;
export import :partA;
export import :partB;

// onlyUseB;
export module onlyUseB;
import m;
export inline void onluUseB() {
b();
}
```

so that the new introduced declaration `int getA(int)` doesn't introduce
new identifiers and types, then the BMI of `onlyUseB` can keep
unchanged.

While it looks not so great, the patch should be the base of the patch
to erase the transitive change for identifiers and types since I don't
know how can we introduce new types and identifiers without introducing
new declarations. Given how tightly the relationship between
declarations, types and identifiers, I think we can only reach the ideal
state after we made the series for all of the three entties.

The design of the patch is similar to
https://github.com/llvm/llvm-project/pull/86912, which extends the
32-bit DeclID to 64-bit and use the higher bits to store the module file
index and the lower bits to store the Local Decl ID.

A slight difference is that we only use 48 bits to store the new DeclID
since we try to use the higher 16 bits to store the module ID in the
prefix of Decl class. Previously, we use 32 bits to store the module ID
and 32 bits to store the DeclID. I don't want to allocate additional
space so I tried to make the additional space the same as 64 bits. An
potential interesting thing here is about the relationship between the
module ID and the module file index. I feel we can get the module file
index by the module ID. But I didn't prove it or implement it. Since I
want to make the patch itself as small as possible. We can make it in
the future if we want.

Another change in the patch is the new concept Decl Index, which means
the index of the very big array `DeclsLoaded` in ASTReader. Previously,
the index of a loaded declaration is simply the Decl ID minus
PREDEFINED_DECL_NUMs. So there are some places they got used
ambiguously. But this patch tried to split these two concepts.

As https://github.com/llvm/llvm-project/pull/86912 did, the change will
increase the on-disk PCM file sizes. As the declaration ID may be the
most IDs in the PCM file, this can have the biggest impact on the size.
In my experiments, this change will bring 6.6% increase of the on-disk
PCM size. No compile-time performance regression observed. Given the
benefits in the motivation example, I think the cost is worthwhile.

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