This CL is small, but the description can be a little long because I'm
trying to sum up the status quo for Emscripten/Wasm EH/SjLj options.
First, this CL adds an option for Wasm SjLj (`-wasm-enable-sjlj`), which
handles SjLj using Wasm EH. The implementation for this will be added as
a followup CL, but this adds the option first to do error checking.
This also adds an option for Wasm EH (`-wasm-enable-eh`), which has been
already implemented. Before we used `-exception-model=wasm` as the same
meaning as enabling Wasm EH, but after we add Wasm SjLj, it will be
possible to use Wasm EH instructions for Wasm SjLj while not enabling
EH, so going forward, to use Wasm EH, `opt` and `llc` will need this
option. This only affects `opt` and `llc` command lines and does not
affect Emscripten user interface.
Now we have two modes of EH (Emscripten/Wasm) and also two modes of SjLj
(also Emscripten/Wasm). The options corresponding to each of are:
- Emscripten EH: `-enable-emscripten-cxx-exceptions`
- Emscripten SjLj: `-enable-emscripten-sjlj`
- Wasm EH: `-wasm-enable-eh -exception-model=wasm`
`-mattr=+exception-handling`
- Wasm SjLj: `-wasm-enable-sjlj -exception-model=wasm`
`-mattr=+exception-handling`
The reason Wasm EH/SjLj's options are a little complicated are
`-exception-model` and `-mattr` are common LLVM options ane not under
our control. (`-mattr` can be omitted if it is embedded within the
bitcode file.)
And we have the following rules of the option composition:
- Emscripten EH and Wasm EH cannot be turned on at the same itme
- Emscripten SjLj and Wasm SjLj cannot be turned on at the same time
- Wasm SjLj should be used with Wasm EH
Which means we now allow these combinations:
- Emscripten EH + Emscripten SjLj: the current default in `emcc`
- Wasm EH + Emscripten SjLj:
This is allowed, but only as an interim step in which we are testing
Wasm EH but not yet have a working implementation of Wasm SjLj. This
will error out (D107687) in compile time if `setjmp` is called in a
function in which Wasm exception is used.
- Wasm EH + Wasm SjLj:
This will be the default mode later when using Wasm EH. Currently Wasm
SjLj implementation doesn't exist, so it doesn't work.
- Emscripten EH + Wasm SjLj will not work.
This CL moves these error checking routines to
`WebAssemblyPassConfig::addIRPasses`. Not sure if this is an ideal place
to do this, but I couldn't find elsewhere. Currently some checking is
done within LowerEmscriptenEHSjLj, but these checks only run if
LowerEmscriptenEHSjLj runs so it may not run when Wasm EH is used. This
moves that to `addIRPasses` and adds some more checks.
Currently LowerEmscriptenEHSjLj pass is responsible for Emscripten EH
and Emscripten SjLj. Wasm EH transformations are done in multiple
places, including WasmEHPrepare, LateEHPrepare, and CFGStackify. But in
the followup CL, LowerEmscriptenEHSjLj pass will be also responsible for
a part of Wasm SjLj transformation, because WasmSjLj will also be using
several Emscripten library functions, and we will be sharing more than
half of the transformation to do that between Emscripten SjLj and Wasm
SjLj.
Currently we have `-enable-emscripten-cxx-exceptions` and
`-enable-emscripten-sjlj` but these only work for `llc`, because for
`llc` we feed these options to the pass but when we run the pass using
`opt` the pass will be created with no options and the default options
will be used, which turns both Emscripten EH and Emscripten SjLj on.
Now we have one more SjLj option to care for, LowerEmscriptenEHSjLj pass
needs a finer way to control these options. This CL removes those
default parameters and make LowerEmscriptenEHSjLj pass read directly
from command line options specified. So if we only run
`opt -wasm-lower-em-ehsjlj`, currently both Emscripten EH and Emscripten
SjLj will run, but with this CL, none will run unless we additionally
pass `-enable-emscripten-cxx-exceptions` or `-enable-emscripten-sjlj`,
or both. This does not affect users; this only affects our `opt` tests
because `emcc` will not call either `opt` or `llc`. As a result of this,
our existing Emscripten EH/SjLj tests gained one or both of those
options in their `RUN` lines.
Reviewed By: dschuff
Differential Revision: https://reviews.llvm.org/D107685
The data layout strings do not have any effect on llc tests and will become
misleadingly out of date as we continue to update the canonical data layout, so
remove them from the tests.
Differential Revision: https://reviews.llvm.org/D105842
Background:
Wasm EH, while using Windows EH (catchpad/cleanuppad based) IR, uses
Itanium-based libraries and ABIs with some modifications.
`__clang_call_terminate` is a wrapper generated in Clang's Itanium C++
ABI implementation. It contains this code, in C-style pseudocode:
```
void __clang_call_terminate(void *exn) {
__cxa_begin_catch(exn);
std::terminate();
}
```
So this function is a wrapper to call `__cxa_begin_catch` on the
exception pointer before termination.
In Itanium ABI, this function is called when another exception is thrown
while processing an exception. The pointer for this second, violating
exception is passed as the argument of this `__clang_call_terminate`,
which calls `__cxa_begin_catch` with that pointer and calls
`std::terminate` to terminate the program.
The spec (https://libcxxabi.llvm.org/spec.html) for `__cxa_begin_catch`
says,
```
When the personality routine encounters a termination condition, it
will call __cxa_begin_catch() to mark the exception as handled and then
call terminate(), which shall not return to its caller.
```
In wasm EH's Clang implementation, this function is called from
cleanuppads that terminates the program, which we also call terminate
pads. Cleanuppads normally don't access the thrown exception and the
wasm backend converts them to `catch_all` blocks. But because we need
the exception pointer in this cleanuppad, we generate
`wasm.get.exception` intrinsic (which will eventually be lowered to
`catch` instruction) as we do in the catchpads. But because terminate
pads are cleanup pads and should run even when a foreign exception is
thrown, so what we have been doing is:
1. In `WebAssemblyLateEHPrepare::ensureSingleBBTermPads()`, we make sure
terminate pads are in this simple shape:
```
%exn = catch
call @__clang_call_terminate(%exn)
unreachable
```
2. In `WebAssemblyHandleEHTerminatePads` pass at the end of the
pipeline, we attach a `catch_all` to terminate pads, so they will be in
this form:
```
%exn = catch
call @__clang_call_terminate(%exn)
unreachable
catch_all
call @std::terminate()
unreachable
```
In `catch_all` part, we don't have the exception pointer, so we call
`std::terminate()` directly. The reason we ran HandleEHTerminatePads at
the end of the pipeline, separate from LateEHPrepare, was it was
convenient to assume there was only a single `catch` part per `try`
during CFGSort and CFGStackify.
---
Problem:
While it thinks terminate pads could have been possibly split or calls
to `__clang_call_terminate` could have been duplicated,
`WebAssemblyLateEHPrepare::ensureSingleBBTermPads()` assumes terminate
pads contain no more than calls to `__clang_call_terminate` and
`unreachable` instruction. I assumed that because in LLVM very limited
forms of transformations are done to catchpads and cleanuppads to
maintain the scoping structure. But it turned out to be incorrect;
passes can merge cleanuppads into one, including terminate pads, as long
as the new code has a correct scoping structure. One pass that does this
I observed was `SimplifyCFG`, but there can be more. After this
transformation, a single cleanuppad can contain any number of other
instructions with the call to `__clang_call_terminate` and can span many
BBs. It wouldn't be practical to duplicate all these BBs within the
cleanuppad to generate the equivalent `catch_all` blocks, only with
calls to `__clang_call_terminate` replaced by calls to `std::terminate`.
Unless we do more complicated transformation to split those calls to
`__clang_call_terminate` into a separate cleanuppad, it is tricky to
solve.
---
Solution (?):
This CL just disables the generation and use of `__clang_call_terminate`
and calls `std::terminate()` directly in its place.
The possible downside of this approach can be, because the Itanium ABI
intended to "mark" the violating exception handled, we don't do that
anymore. What `__cxa_begin_catch` actually does is increment the
exception's handler count and decrement the uncaught exception count,
which in my opinion do not matter much given that we are about to
terminate the program anyway. Also it does not affect info like stack
traces that can be possibly shown to developers.
And while we use a variant of Itanium EH ABI, we can make some
deviations if we choose to; we are already different in that in the
current version of the EH spec we don't support two-phase unwinding. We
can possibly consider a more complicated transformation later to
reenable this, but I don't think that has high priority.
Changes in this CL contains:
- In Clang, we don't generate a call to `wasm.get.exception()` intrinsic
and `__clang_call_terminate` function in terminate pads anymore; we
simply generate calls to `std::terminate()`, which is the default
implementation of `CGCXXABI::emitTerminateForUnexpectedException`.
- Remove `WebAssembly::ensureSingleBBTermPads() function and
`WebAssemblyHandleEHTerminatePads` pass, because terminate pads are
already `catch_all` now (because they don't need the exception
pointer) and we don't need these transformations anymore.
- Change tests to use `std::terminate` directly. Also removes tests that
tested `LateEHPrepare::ensureSingleBBTermPads` and
`HandleEHTerminatePads` pass.
- Drive-by fix: Add some function attributes to EH intrinsic
declarations
Fixes https://github.com/emscripten-core/emscripten/issues/13582.
Reviewed By: dschuff, tlively
Differential Revision: https://reviews.llvm.org/D97834
This implements basic instructions for the new spec.
- Adds new versions of instructions: `catch`, `catch_all`, and `rethrow`
- Adds support for instruction selection for the new instructions
- `catch` needs a custom routine for the same reason `throw` needs one,
to encode `__cpp_exception` tag symbol.
- Updates `WebAssembly::isCatch` utility function to include `catch_all`
and Change code that compares an instruction's opcode with `catch` to
use that function.
- LateEHPrepare
- Previously in LateEHPrepare we added `catch` instruction to both
`catchpad`s (for user catches) and `cleanuppad`s (for destructors).
In the new version `catch` is generated from `llvm.catch` intrinsic
in instruction selection phase, so we only need to add `catch_all`
to the beginning of cleanup pads.
- `catch` is generated from instruction selection, but we need to
hoist the `catch` instruction to the beginning of every EH pad,
because `catch` can be in the middle of the EH pad or even in a
split BB from it after various code transformations.
- Removes `addExceptionExtraction` function, which was used to
generate `br_on_exn` before.
- CFGStackfiy: Deletes `fixUnwindMismatches` function. Running this
function on the new instruction causes crashes, and the new version
will be added in a later CL, whose contents will be completely
different. So deleting the whole function will make the diff easier to
read.
- Reenables all disabled tests in exception.ll and eh-lsda.ll and a
single basic test in cfg-stackify-eh.ll.
- Updates existing tests to use the new assembly format. And deletes
`br_on_exn` instructions from the tests and FileCheck lines.
Reviewed By: dschuff, tlively
Differential Revision: https://reviews.llvm.org/D94040
Clang generates `wasm.get.exception` and `wasm.get.ehselector`
intrinsics, which respectively return a caught exception value (a
pointer to some C++ exception struct) and a selector (an integer value
that tells which C++ `catch` clause the current exception matches, or
does not match any).
WasmEHPrepare is a pass that does some IR-level preparation before
instruction selection. Previously one of things we did in this pass was
to convert `wasm.get.exception` intrinsic calls to
`wasm.extract.exception` intrinsics. Their semantics were the same
except `wasm.extract.exception` did not have a token argument. We
maintained these two separate intrinsics with the same semantics because
instruction selection couldn't handle token arguments. This
`wasm.extract.exception` intrinsic was later converted to
`extract_exception` instruction in instruction selection, which was a
pseudo instruction to implement `br_on_exn`. Because `br_on_exn` pushed
an extracted value onto the value stack after the `end` instruction of a
`block`, but LLVM does not have a way of modeling that kind of behavior,
so this pseudo instruction was used to pull an extracted value out of
thin air, like this:
```
block $l0
...
br_on_exn $cpp_exception $l0
...
end
extract_exception ;; pushes values onto the stack
```
In the new spec, we don't need this pseudo instruction anymore because
`catch` itself returns a value and we don't have `br_on_exn` anymore. In
the spec `catch` returns multiple values (like `br_on_exn`), but here we
assume it only returns a single i32, which is sufficient to support C++.
So this renames `wasm.get.exception` intrinsic to `wasm.catch`. Because
this CL does not yet contain instruction selection for `wasm.catch`
intrinsic, all `RUN` lines in exception.ll, eh-lsda.ll, and
cfg-stackify-eh.ll, and a single `RUN` line in wasm-eh.cpp (which is an
end-to-end test from C++ source to assembly) fail. So this CL
temporarily disables those `RUN` lines, and for those test files without
any valid remaining `RUN` lines, adds a dummy `RUN` line to make them
pass. These tests will be reenabled in later CLs.
Reviewed By: dschuff, tlively
Differential Revision: https://reviews.llvm.org/D94039
Summary:
This adds support for LSDA (exception table) generation for wasm EH.
Wasm EH mostly follows the structure of Itanium-style exception tables,
with one exception: a call site table entry in wasm EH corresponds to
not a call site but a landing pad.
In wasm EH, the VM is responsible for stack unwinding. After an
exception occurs and the stack is unwound, the control flow is
transferred to wasm 'catch' instruction by the VM, after which the
personality function is called from the compiler-generated code. (Refer
to WasmEHPrepare pass for more information on this part.)
This patch:
- Changes wasm.landingpad.index intrinsic to take a token argument, to
make this 1:1 match with a catchpad instruction
- Stores landingpad index info and catch type info MachineFunction in
before instruction selection
- Lowers wasm.lsda intrinsic to an MCSymbol pointing to the start of an
exception table
- Adds WasmException class with overridden methods for table generation
- Adds support for LSDA section in Wasm object writer
Reviewers: dschuff, sbc100, rnk
Subscribers: mgorny, jgravelle-google, sunfish, llvm-commits
Differential Revision: https://reviews.llvm.org/D52748
llvm-svn: 345345
Summary:
This adds support for LSDA (exception table) generation for wasm EH.
Wasm EH mostly follows the structure of Itanium-style exception tables,
with one exception: a call site table entry in wasm EH corresponds to
not a call site but a landing pad.
In wasm EH, the VM is responsible for stack unwinding. After an
exception occurs and the stack is unwound, the control flow is
transferred to wasm 'catch' instruction by the VM, after which the
personality function is called from the compiler-generated code. (Refer
to WasmEHPrepare pass for more information on this part.)
This patch:
- Changes wasm.landingpad.index intrinsic to take a token argument, to
make this 1:1 match with a catchpad instruction
- Stores landingpad index info and catch type info MachineFunction in
before instruction selection
- Lowers wasm.lsda intrinsic to an MCSymbol pointing to the start of an
exception table
- Adds WasmException class with overridden methods for table generation
- Adds support for LSDA section in Wasm object writer
Reviewers: dschuff, sbc100, rnk
Subscribers: mgorny, jgravelle-google, sunfish, llvm-commits
Differential Revision: https://reviews.llvm.org/D52748
llvm-svn: 344575
Heejin Ahn