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Reid Kleckner via llvm-dev

2020-Jan-26 23:12 UTC

[llvm-dev] [RFC] Replacing inalloca with llvm.call.setup and preallocated

Hello all,

A few years ago, I added the inalloca feature to LLVM IR so that Clang
could be C++ ABI compatible with MSVC on 32-bit x86. The feature works, but
there is room for improvement. I recently took the time to write up a
design using token values that will hopefully be better named and easier to
work with and around.

For the technical details of the proposal, I've written up the RFC in
Markdown here:
https://github.com/rnk/llvm-project/blob/call-setup-docs/llvm/docs/CallSetup.md
I've pasted the text below if you want to quote and reply on the list.

The main question I have for the community is, given that it is infeasible
to upgrade inalloca to llvm.call.setup, can we drop support for the old IR?
So far as I am aware, Clang has been the only user of this attribute, and
only for i*86-windows-msvc targets. The main goal of adding llvm.call.setup
is to get rid of inalloca, so if all LLVM IR transforms have to keep
knowing about it, there is no reason to add a new way to do the same thing.

Thanks,
Reid

The last RFC: http://lists.llvm.org/pipermail/llvm-dev/2013-July/064218.html

Current inalloca docs: https://llvm.org/docs/InAlloca.html

----------------






# RFC: Replace inalloca with llvm.call.setup and preallocated

In order to pass non-trivially copyable objects by value in a way that is
compatible with the Visual C++ compiler on 32-bit x86, Clang has to be able
to
separate the allocation of call argument memory from the call site. The
`inalloca` feature was added to LLVM for this purpose. However, this feature
usually results in inefficient code, and is often incompatible with
otherwise
straightforward LLVM IR transforms. Therefore, I would like to get rid of it
and replace it with `llvm.call.setup`, which I hope will be more
maintainable
in the long run.

These are some of the drawbacks of inalloca that I want to fix with the new
IR:
- Blocks most interprocedural prototype changing transforms: dead argument
  elimination, argument promotion, application of fastcc, etc.
- Blocks alias analysis: Unrelated args in memory are hard to analyze
- Blocks function attribute inference for unrelated parameters
- Hard for frontends to use because one argument can affect every other
argument

Since inalloca was added, the token type was added to LLVM. Transforms are
not
allowed to obscure the definition of a token value, and tokens can be used
to
create single-entry, multi-exit regions in LLVM IR. This allows the
creation of
a call setup operation that must remain paired with its call site throughout
mid-level optimization. That guarantee allows the backend to look at the
call
site when emitting the call setup instructions. If the IR for a call setup
forms a proper region, that can also help the backend perform frame pointer
elimination in many cases.

## New IR Features

Here is the list of new IR features I think this requires:

Intrinsics:
- `token @llvm.call.setup(i32 %numArgs)`
- `i8* @llvm.call.alloc(token %site, i32 %argIdx)`
- `void @llvm.call.teardown(token)`

Attributes:
- `preallocated(<ty>)`, similar to byval, but no copy

Bundles:
- `[ "callsetup"(token %site) ]`

Verifier rules:
- `llvm.call.setup` must have exactly one corresponding call site
- call site must have equal number of preallocated args to `llvm.call.setup`

## Intended Usage

Here is an example of LLVM IR for an unpacked call site:

```llvm
%cs = call token @llvm.call.setup(i32 3)
%m0 = call i8* @llvm.call.alloc(token %cs, i32 0)  ;; allocates {i32}
call void @llvm.memset*(i8* %m0, i32 0, i32 4)
%m1 = call i8* @llvm.call.alloc(token %cs, i32 1)  ;; allocates {i32, i32}
call void @llvm.memset*(i8* %m1, i32 0, i32 8)
%m2 = call i8* @llvm.call.alloc(token %cs, i32 2)  ;; allocates {i32, i32,
i32}
call void @llvm.memset*(i8* %m2, i32 0, i32 12)
call void @use_callsetup(
    {i32}* preallocated({i32}) %m1,
    i32 13,
    {i32,i32}* preallocated({i32,i32}) %m2,
    i32 42,
    {i32,i32,i32}* preallocated({i32,i32,i32}) %m3)
    [ "callsetup"(token %cs) ]
```

Many transforms will need to be made aware of the new verifier guarantees,
but
they should only block optimizations on preallocated arguments. The goal is
that unrelated arguments, such as the i32 arguments above, remain
unaffected.
DAE, for example, is free to eliminate the plain integers, but it cannot
eliminate a preallocated argument without adjusting the call.setup.

The next most important thing to keep in mind is how this interacts with
exception handling. This is where `llvm.call.teardown` comes into play. The
idea is that, in a call region, clang should push an exceptional cleanup
onto
the cleanup stack. Here is what the IR for the previous example would look
like, assuming each argument has a constructor that may throw:

```llvm
%cs = call token @llvm.call.setup(i32 3)
%m0 = call i8* @llvm.call.alloc(token %cs, i32 0)
invoke void @ctor0(i8* %m0)
  to label %cont0 unwind label %cleanupCall

cont0:
%m1 = call i8* @llvm.call.alloc(token %cs, i32 1)
invoke void @ctor1(i8* %m1)
  to label %cont1 unwind label %cleanup0

cont1:
%m2 = call i8* @llvm.call.alloc(token %cs, i32 2)
invoke void @ctor2(i8* %m2)
  to label %cont2 unwind label %cleanup1

cont2:
  call void @use_callsetup(i8* preallocated %m1,
                           i32 13,
                           i8* preallocated %m2,
                           i32 42,
                           i8* preallocated %m3)
      [ "callsetup"(token %cs) ]


cleanup1:
  %cl1 = cleanuppad unwind to caller
  call void @dtor(i8* %m1) [ "funclet"(token %cl1) ]
  cleanupret %cl1 to label %cleanup0

cleanup0:
  %cl0 = cleanuppad unwind to caller
  call void @dtor(i8% %m0) [ "funclet"(token %cl2) ]
  cleanupret %cl0 to label %cleanupCall

cleanupCall:
  %clC = cleanuppad unwind to caller
  call void @llvm.call.teardown(token %cs)  ;; Or llvm.call.cleanup?
  cleanupret %clC to caller
```

Generally, cleanups to tear down a call setup region are not needed if
control
cannot return to the current function. The cleanupCall block is an example
of
such an unnecessary cleanup. However, to make things easy for the inliner,
the
frontend is required to emit these cleanups. Prior to code generation, the
WinEHPrepare pass can remove any unneeded argument memory cleanups.

## Canonical Form

`llvm.call.setup` is designed for compatibility, not for performance or
analyzability. Mid-level optimization passes should generally try to remove
these intrinsics and attributes when possible. When it is possible to change
the prototype of a function using these attributes, call sites should be
canonicalized in the following way:

- Create a static alloca of appropriate type for each preallocated argument
- Replace all uses of `llvm.call.alloc` with the corresponding new alloca
- Insert `lifetime.start` for each static alloca at the original alloc site,
  and `llvm.lifetime.end` after the call and at `llvm.call.teardown`.
- Remove all setup, alloc, teardown intrinsics, and remove the
`preallocated`
  attributes from caller and callee.

GlobalOpt seems like a logical place for this transform. This form should
enable downstream optimizations, and reduce the number of transforms that
need
to be taught that `llvm.call.alloc` creates stack memory.

## Inliner Considerations

When inlining a call site that uses `llvm.call.setup`, the inliner should
also
make the transforms described above, only without adjusting the callee's
prototype.

## Corner cases

### Catching an exception within a call region

If an exception is thrown and caught within the call setup region, the newly
established SP must be saved into the EH record when a call is setup.
Consider
the case below of inlining try / catch into a call region:

```c++
struct Foo {
  Foo(int x);
  Foo(const Foo &o);
  ~Foo();
  int x, y, z;
};
void use_callsetup(int, Foo, Foo);
int maythrow2();
static inline int maythrow() {
  try { return maythrow2(); } catch (int) {}
  return 0;
}
Foo getf();
int main() {
  use_callsetup(maythrow(), getf(), getf());
}
```

The backend should be able to detect whether SP must be saved or not by
checking if there are any invokes reachable along normal paths from the call
setup that do not first reach the call itself, or a normal
`llvm.call.teardown`.

### Non-exceptional call region exit

It is possible, using statement expressions, to exit a call setup region
with
normal control flow. In this case, the Clang should emit a normal cleanup to
call `llvm.eh.teardown`. Consider:

```c++
#define RETURN_IF_ERR(maybeError) \
  ({                              \
    auto val = maybeError;        \
    if (val.isError())            \
      return val.getError();      \
    val.getValue;                 \
  })
ErrorOr<int> mayFail();
void use_callsetup(int, Foo);
void f(bool cond) {
  use_callsetup(RETURN_IF_ERR(mayFail()), Foo());
}
```

### Inlining non-EH code into EH code

If exceptions are disabled, the frontend should not emit exceptional
cleanups
to teardown the call region. However, this code could be inlined into a
function that uses exceptions, and the caller could catch an exception
thrown
through the non-EH code. Generally this should be impossible, because the
calls
will be marked nounwind, and it is UB to throw an exception through a
nounwind
call site. However, SEH allows exceptions to be thrown through nounwind call
sites. Consider:

```c++
int mayThrow();
void use_callsetup(int, Foo);
static inline void inlineMe() {
  void use_callsetup(mayThrow(), Foo());
}
void f(int numRetries) {
  for (int i = 0; i < numRetries; i++) {
    __try {
      inlineMe();
    } __except (1) {
    }
  }
}
```

Unless the compiler inserts stack adjustments along the unwind path, this
well
setup call argument memory for every loop iteration, and never clean it up.
This could result in code generation bugs or excessive stack memory use. The
two options are:
1. Refuse to inline functions containing call regions through invoke call
sites
1. Teach the inliner to synthesize llvm.call.teardown cleanups inside call
   regions
1. Do nothing, the frontend already disables inlining into `__try`

Given that this corner case seems specific to SEH, the third option seems
most
reasonable.

## Implementation Steps

In the spirit of incremental development, I think the implementation could
be
broken down into the following patch series:
- IR: LLVM IR intrinsics and attributes
- Clang: IRGen, under -cc1 flag
- X86: backend implementation: inefficient, no EH
- Transforms: Update inliner and globalopt, audit other transforms
- X86: MSVC C++ EH SP management
- ... test it on Chrome with -cc1 flag
- Clang: Remove cc1 flag and inalloca IRGen logic
- ... announce inalloca removal from LLVM IR
- inalloca removal

## Backwards compatibility

It may be possible to upgrade some bitcode from `inalloca` to
`llvm.call.setup`, but in cases with complex control flow where the
allocation
site does not dominate the call, it will not be straightforward. Therefore,
I
think we should make an exception to LLVM's usual policy of bitcode
backwards
compatibility, and drop support for `inalloca`. The `inalloca` attribute was
generally a source of bugs, and was only used for code targetting
`i*86-windows-msvc`. To the best of my knowledge, there are no users of LLVM
who archive bitcode for that platform and expect to be able to upgrade it
for
use with future LLVM versions.
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Eli Friedman via llvm-dev

2020-Jan-28 00:31 UTC

head link

[llvm-dev] [RFC] Replacing inalloca with llvm.call.setup and preallocated

I assume by “drop support”, you mean reject it in the bitcode reader/IR parser? 
We can’t reasonably support a complex feature like inalloca if nobody is testing
it. If we can’t reasonably upgrade it, and we don’t think there are any users
other than clang targeting 32-bit Windows, probably dropping support is best.

More details comments on the proposal:

“llvm.call.setup must have exactly one corresponding call site”: Normal IR rules
would allow cloning the call site (in jump threading), or erasing the call site
(if there’s a noreturn call in an argument).  What’s the benefit of enforcing
this rule, as opposed to just saying all the call sites must have the same
signature?

The proposal doesn’t address what happens if llvm.call.setup is called while
there’s another llvm.call.setup still active.  Is it legal to call
llvm.call.setup in a loop?  Or should nested llvm.call.setup calls have the
parent callsetup token as an operand?

Is there some way we can allow optimizations if we can’t modify the callee, but
we can prove nothing captures the address of the preallocated region before the
call?  I guess under the current proposal we could transform
preallocated->byval, but that isn’t very exciting.

How does this interact with other dynamic stack allocations?  Should we switch
VLAs to use a similar mechanism?  (The problems with dynamic alloca in general
aren’t as terrible, but it might still benefit: for example, it’s much easier to
transform a dynamic allocation into a static allocation.)

“If an exception is thrown and caught within the call setup region, the newly
established SP must be saved into the EH record when a call is setup.”  What
makes this case special vs. what we currently implement?  Is this currently
broken?  Or is it related to supporting frame pointer elimination?
-Eli

From: llvm-dev <llvm-dev-bounces at lists.llvm.org> On Behalf Of Reid
Kleckner via llvm-dev
Sent: Sunday, January 26, 2020 3:12 PM
To: llvm-dev <llvm-dev at lists.llvm.org>
Subject: [EXT] [llvm-dev] [RFC] Replacing inalloca with llvm.call.setup and
preallocated

Hello all,

A few years ago, I added the inalloca feature to LLVM IR so that Clang could be
C++ ABI compatible with MSVC on 32-bit x86. The feature works, but there is room
for improvement. I recently took the time to write up a design using token
values that will hopefully be better named and easier to work with and around.

For the technical details of the proposal, I've written up the RFC in
Markdown here:
https://github.com/rnk/llvm-project/blob/call-setup-docs/llvm/docs/CallSetup.md
I've pasted the text below if you want to quote and reply on the list.

The main question I have for the community is, given that it is infeasible to
upgrade inalloca to llvm.call.setup, can we drop support for the old IR? So far
as I am aware, Clang has been the only user of this attribute, and only for
i*86-windows-msvc targets. The main goal of adding llvm.call.setup is to get rid
of inalloca, so if all LLVM IR transforms have to keep knowing about it, there
is no reason to add a new way to do the same thing.

Thanks,
Reid

The last RFC: http://lists.llvm.org/pipermail/llvm-dev/2013-July/064218.html
Current inalloca docs: https://llvm.org/docs/InAlloca.html

----------------






# RFC: Replace inalloca with llvm.call.setup and preallocated

In order to pass non-trivially copyable objects by value in a way that is
compatible with the Visual C++ compiler on 32-bit x86, Clang has to be able to
separate the allocation of call argument memory from the call site. The
`inalloca` feature was added to LLVM for this purpose. However, this feature
usually results in inefficient code, and is often incompatible with otherwise
straightforward LLVM IR transforms. Therefore, I would like to get rid of it
and replace it with `llvm.call.setup`, which I hope will be more maintainable
in the long run.

These are some of the drawbacks of inalloca that I want to fix with the new IR:
- Blocks most interprocedural prototype changing transforms: dead argument
  elimination, argument promotion, application of fastcc, etc.
- Blocks alias analysis: Unrelated args in memory are hard to analyze
- Blocks function attribute inference for unrelated parameters
- Hard for frontends to use because one argument can affect every other argument

Since inalloca was added, the token type was added to LLVM. Transforms are not
allowed to obscure the definition of a token value, and tokens can be used to
create single-entry, multi-exit regions in LLVM IR. This allows the creation of
a call setup operation that must remain paired with its call site throughout
mid-level optimization. That guarantee allows the backend to look at the call
site when emitting the call setup instructions. If the IR for a call setup
forms a proper region, that can also help the backend perform frame pointer
elimination in many cases.

## New IR Features

Here is the list of new IR features I think this requires:

Intrinsics:
- `token @llvm.call.setup(i32 %numArgs)`
- `i8* @llvm.call.alloc(token %site, i32 %argIdx)`
- `void @llvm.call.teardown(token)`

Attributes:
- `preallocated(<ty>)`, similar to byval, but no copy

Bundles:
- `[ "callsetup"(token %site) ]`

Verifier rules:
- `llvm.call.setup` must have exactly one corresponding call site
- call site must have equal number of preallocated args to `llvm.call.setup`

## Intended Usage

Here is an example of LLVM IR for an unpacked call site:

```llvm
%cs = call token @llvm.call.setup(i32 3)
%m0 = call i8* @llvm.call.alloc(token %cs, i32 0)  ;; allocates {i32}
call void @llvm.memset*(i8* %m0, i32 0, i32 4)
%m1 = call i8* @llvm.call.alloc(token %cs, i32 1)  ;; allocates {i32, i32}
call void @llvm.memset*(i8* %m1, i32 0, i32 8)
%m2 = call i8* @llvm.call.alloc(token %cs, i32 2)  ;; allocates {i32, i32, i32}
call void @llvm.memset*(i8* %m2, i32 0, i32 12)
call void @use_callsetup(
    {i32}* preallocated({i32}) %m1,
    i32 13,
    {i32,i32}* preallocated({i32,i32}) %m2,
    i32 42,
    {i32,i32,i32}* preallocated({i32,i32,i32}) %m3)
    [ "callsetup"(token %cs) ]
```

Many transforms will need to be made aware of the new verifier guarantees, but
they should only block optimizations on preallocated arguments. The goal is
that unrelated arguments, such as the i32 arguments above, remain unaffected.
DAE, for example, is free to eliminate the plain integers, but it cannot
eliminate a preallocated argument without adjusting the call.setup.

The next most important thing to keep in mind is how this interacts with
exception handling. This is where `llvm.call.teardown` comes into play. The
idea is that, in a call region, clang should push an exceptional cleanup onto
the cleanup stack. Here is what the IR for the previous example would look
like, assuming each argument has a constructor that may throw:

```llvm
%cs = call token @llvm.call.setup(i32 3)
%m0 = call i8* @llvm.call.alloc(token %cs, i32 0)
invoke void @ctor0(i8* %m0)
  to label %cont0 unwind label %cleanupCall

cont0:
%m1 = call i8* @llvm.call.alloc(token %cs, i32 1)
invoke void @ctor1(i8* %m1)
  to label %cont1 unwind label %cleanup0

cont1:
%m2 = call i8* @llvm.call.alloc(token %cs, i32 2)
invoke void @ctor2(i8* %m2)
  to label %cont2 unwind label %cleanup1

cont2:
  call void @use_callsetup(i8* preallocated %m1,
                           i32 13,
                           i8* preallocated %m2,
                           i32 42,
                           i8* preallocated %m3)
      [ "callsetup"(token %cs) ]


cleanup1:
  %cl1 = cleanuppad unwind to caller
  call void @dtor(i8* %m1) [ "funclet"(token %cl1) ]
  cleanupret %cl1 to label %cleanup0

cleanup0:
  %cl0 = cleanuppad unwind to caller
  call void @dtor(i8% %m0) [ "funclet"(token %cl2) ]
  cleanupret %cl0 to label %cleanupCall

cleanupCall:
  %clC = cleanuppad unwind to caller
  call void @llvm.call.teardown(token %cs)  ;; Or llvm.call.cleanup?
  cleanupret %clC to caller
```

Generally, cleanups to tear down a call setup region are not needed if control
cannot return to the current function. The cleanupCall block is an example of
such an unnecessary cleanup. However, to make things easy for the inliner, the
frontend is required to emit these cleanups. Prior to code generation, the
WinEHPrepare pass can remove any unneeded argument memory cleanups.

## Canonical Form

`llvm.call.setup` is designed for compatibility, not for performance or
analyzability. Mid-level optimization passes should generally try to remove
these intrinsics and attributes when possible. When it is possible to change
the prototype of a function using these attributes, call sites should be
canonicalized in the following way:

- Create a static alloca of appropriate type for each preallocated argument
- Replace all uses of `llvm.call.alloc` with the corresponding new alloca
- Insert `lifetime.start` for each static alloca at the original alloc site,
  and `llvm.lifetime.end` after the call and at `llvm.call.teardown`.
- Remove all setup, alloc, teardown intrinsics, and remove the `preallocated`
  attributes from caller and callee.

GlobalOpt seems like a logical place for this transform. This form should
enable downstream optimizations, and reduce the number of transforms that need
to be taught that `llvm.call.alloc` creates stack memory.

## Inliner Considerations

When inlining a call site that uses `llvm.call.setup`, the inliner should also
make the transforms described above, only without adjusting the callee's
prototype.

## Corner cases

### Catching an exception within a call region

If an exception is thrown and caught within the call setup region, the newly
established SP must be saved into the EH record when a call is setup. Consider
the case below of inlining try / catch into a call region:

```c++
struct Foo {
  Foo(int x);
  Foo(const Foo &o);
  ~Foo();
  int x, y, z;
};
void use_callsetup(int, Foo, Foo);
int maythrow2();
static inline int maythrow() {
  try { return maythrow2(); } catch (int) {}
  return 0;
}
Foo getf();
int main() {
  use_callsetup(maythrow(), getf(), getf());
}
```

The backend should be able to detect whether SP must be saved or not by
checking if there are any invokes reachable along normal paths from the call
setup that do not first reach the call itself, or a normal `llvm.call.teardown`.

### Non-exceptional call region exit

It is possible, using statement expressions, to exit a call setup region with
normal control flow. In this case, the Clang should emit a normal cleanup to
call `llvm.eh.teardown`. Consider:

```c++
#define RETURN_IF_ERR(maybeError) \
  ({                              \
    auto val = maybeError;        \
    if (val.isError())            \
      return val.getError();      \
    val.getValue;                 \
  })
ErrorOr<int> mayFail();
void use_callsetup(int, Foo);
void f(bool cond) {
  use_callsetup(RETURN_IF_ERR(mayFail()), Foo());
}
```

### Inlining non-EH code into EH code

If exceptions are disabled, the frontend should not emit exceptional cleanups
to teardown the call region. However, this code could be inlined into a
function that uses exceptions, and the caller could catch an exception thrown
through the non-EH code. Generally this should be impossible, because the calls
will be marked nounwind, and it is UB to throw an exception through a nounwind
call site. However, SEH allows exceptions to be thrown through nounwind call
sites. Consider:

```c++
int mayThrow();
void use_callsetup(int, Foo);
static inline void inlineMe() {
  void use_callsetup(mayThrow(), Foo());
}
void f(int numRetries) {
  for (int i = 0; i < numRetries; i++) {
    __try {
      inlineMe();
    } __except (1) {
    }
  }
}
```

Unless the compiler inserts stack adjustments along the unwind path, this well
setup call argument memory for every loop iteration, and never clean it up.
This could result in code generation bugs or excessive stack memory use. The
two options are:
1. Refuse to inline functions containing call regions through invoke call sites
1. Teach the inliner to synthesize llvm.call.teardown cleanups inside call
   regions
1. Do nothing, the frontend already disables inlining into `__try`

Given that this corner case seems specific to SEH, the third option seems most
reasonable.

## Implementation Steps

In the spirit of incremental development, I think the implementation could be
broken down into the following patch series:
- IR: LLVM IR intrinsics and attributes
- Clang: IRGen, under -cc1 flag
- X86: backend implementation: inefficient, no EH
- Transforms: Update inliner and globalopt, audit other transforms
- X86: MSVC C++ EH SP management
- ... test it on Chrome with -cc1 flag
- Clang: Remove cc1 flag and inalloca IRGen logic
- ... announce inalloca removal from LLVM IR
- inalloca removal

## Backwards compatibility

It may be possible to upgrade some bitcode from `inalloca` to
`llvm.call.setup`, but in cases with complex control flow where the allocation
site does not dominate the call, it will not be straightforward. Therefore, I
think we should make an exception to LLVM's usual policy of bitcode
backwards
compatibility, and drop support for `inalloca`. The `inalloca` attribute was
generally a source of bugs, and was only used for code targetting
`i*86-windows-msvc`. To the best of my knowledge, there are no users of LLVM
who archive bitcode for that platform and expect to be able to upgrade it for
use with future LLVM versions.
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Reid Kleckner via llvm-dev

2020-Jan-28 00:57 UTC

head link

[llvm-dev] [RFC] Replacing inalloca with llvm.call.setup and preallocated

On Mon, Jan 27, 2020 at 4:31 PM Eli Friedman <efriedma at quicinc.com>
wrote:
> I assume by “drop support”, you mean reject it in the bitcode reader/IR
> parser?  We can’t reasonably support a complex feature like inalloca if
> nobody is testing it. If we can’t reasonably upgrade it, and we don’t think
> there are any users other than clang targeting 32-bit Windows, probably
> dropping support is best.
>
That's a good point. There are already enough lightly tested features in
LLVM. There's no reason to leave another one lying around like a trap for
the first unsuspecting user to try it.

More details comments on the proposal:>
>
>
> “llvm.call.setup must have exactly one corresponding call site”: Normal IR
> rules would allow cloning the call site (in jump threading), or erasing the
> call site (if there’s a noreturn call in an argument).  What’s the benefit
> of enforcing this rule, as opposed to just saying all the call sites must
> have the same signature?
>
I think we could cope with unreachable code elimination deleting a paired
call site (zero or one), but code duplication creating a second call site
could be problematic. The call setup doesn't describe the prototype of the
main call site, so if there were multiple call sites, the backend would
have to pick one call site arbitrarily or compare the call sites when
setting up the call. If there are zero call sites, the backend can create
static allocas of the appropriate type to satisfy the allocations. Of
course, an IR pass (instcombine?) should do this transform first if it sees
it. Maybe we could have CGP take care of it, too.

> The proposal doesn’t address what happens if llvm.call.setup is called
> while there’s another llvm.call.setup still active.  Is it legal to call
> llvm.call.setup in a loop?  Or should nested llvm.call.setup calls have the
> parent callsetup token as an operand?
>
Nested setup is OK, but the verifier rule that there must be a paired call
site should make it impossible to do in a loop. I guess we should have some
rule to reject the following:
%cs1 = llvm.call.setup()
%cs2 = llvm.call.setup()
call void @cs1() [ "callsetup"(token %cs1) ]
call void @cs2() [ "callsetup"(token %cs2) ]

> Is there some way we can allow optimizations if we can’t modify the
> callee, but we can prove nothing captures the address of the preallocated
> region before the call?  I guess under the current proposal we could
> transform preallocated->byval, but that isn’t very exciting.
>
I suppose we could say that the combo of byval+preallocated just means
`byval`, and teach transforms that that's OK.

> How does this interact with other dynamic stack allocations?  Should we
> switch VLAs to use a similar mechanism?  (The problems with dynamic alloca
> in general aren’t as terrible, but it might still benefit: for example,
> it’s much easier to transform a dynamic allocation into a static
> allocation.)
>
VLAs could use something like this, but they are generally of unknown size
while call sites have a known fixed size. I think that makes them pretty
different.

> “If an exception is thrown and caught within the call setup region, the
> newly established SP must be saved into the EH record when a call is
> setup.”  What makes this case special vs. what we currently implement?  Is
> this currently broken?  Or is it related to supporting frame pointer
> elimination?
>
I think of it as a special case because you can't write this in standard
C++. Today, I think we leak stack memory in this case. There's no
correctness issue because we copy SP into its own virtual register at the
point of the alloca, and arguments are addressed relative to the vreg. What
I had in mind for the new system is that we make some kind of fixed stack
object that uses pre-computed SP offsets, assuming there are no dynamic
allocas in the function. This would be a problem for a program that does:

setup call 1
store call 1 arg 0
try {
  setup call 2
  throw exception
  call 2
} catch (...) {}
; call 2's frame is still on the stack
store call 1 arg 1 ; SP offset would be incorrect
call 1
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llvm dev - Jan 2020 - [RFC] Replacing inalloca with llvm.call.setup and preallocated

[llvm-dev] [RFC] Replacing inalloca with llvm.call.setup and preallocated

[llvm-dev] [RFC] Replacing inalloca with llvm.call.setup and preallocated

[llvm-dev] [RFC] Replacing inalloca with llvm.call.setup and preallocated