561abd83ff
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
278 lines
9.8 KiB
LLVM
278 lines
9.8 KiB
LLVM
; RUN: opt < %s -winehprepare -demote-catchswitch-only -wasmehprepare -S | FileCheck %s
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target datalayout = "e-m:e-p:32:32-i64:64-n32:64-S128"
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target triple = "wasm32-unknown-unknown"
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; CHECK: @__wasm_lpad_context = external global { i32, i8*, i32 }
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@_ZTIi = external constant i8*
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%struct.Temp = type { i8 }
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; A single 'catch (int)' clause.
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; A wasm.catch() call, wasm.lsda() call, and personality call to generate a
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; selector should all be genereated after the catchpad.
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;
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; void foo();
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; void test0() {
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; try {
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; foo();
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; } catch (int) {
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; }
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; }
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define void @test0() personality i8* bitcast (i32 (...)* @__gxx_wasm_personality_v0 to i8*) {
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; CHECK-LABEL: @test0()
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entry:
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invoke void @foo()
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to label %try.cont unwind label %catch.dispatch
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catch.dispatch: ; preds = %entry
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%0 = catchswitch within none [label %catch.start] unwind to caller
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catch.start: ; preds = %catch.dispatch
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%1 = catchpad within %0 [i8* bitcast (i8** @_ZTIi to i8*)]
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%2 = call i8* @llvm.wasm.get.exception(token %1)
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%3 = call i32 @llvm.wasm.get.ehselector(token %1)
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%4 = call i32 @llvm.eh.typeid.for(i8* bitcast (i8** @_ZTIi to i8*))
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%matches = icmp eq i32 %3, %4
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br i1 %matches, label %catch, label %rethrow
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; CHECK: catch.start:
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; CHECK-NEXT: %[[CATCHPAD:.*]] = catchpad
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; CHECK-NEXT: %[[EXN:.*]] = call i8* @llvm.wasm.catch(i32 0)
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; CHECK-NEXT: call void @llvm.wasm.landingpad.index(token %[[CATCHPAD]], i32 0)
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; CHECK-NEXT: store i32 0, i32* getelementptr inbounds ({ i32, i8*, i32 }, { i32, i8*, i32 }* @__wasm_lpad_context, i32 0, i32 0)
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; CHECK-NEXT: %[[LSDA:.*]] = call i8* @llvm.wasm.lsda()
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; CHECK-NEXT: store i8* %[[LSDA]], i8** getelementptr inbounds ({ i32, i8*, i32 }, { i32, i8*, i32 }* @__wasm_lpad_context, i32 0, i32 1)
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; CHECK-NEXT: call i32 @_Unwind_CallPersonality(i8* %[[EXN]]) {{.*}} [ "funclet"(token %[[CATCHPAD]]) ]
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; CHECK-NEXT: %[[SELECTOR:.*]] = load i32, i32* getelementptr inbounds ({ i32, i8*, i32 }, { i32, i8*, i32 }* @__wasm_lpad_context, i32 0, i32 2)
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; CHECK: icmp eq i32 %[[SELECTOR]]
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catch: ; preds = %catch.start
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%5 = call i8* @__cxa_begin_catch(i8* %2) [ "funclet"(token %1) ]
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call void @__cxa_end_catch() [ "funclet"(token %1) ]
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catchret from %1 to label %try.cont
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; CHECK: catch:
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; CHECK-NEXT: call i8* @__cxa_begin_catch(i8* %[[EXN]])
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rethrow: ; preds = %catch.start
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call void @llvm.wasm.rethrow() [ "funclet"(token %1) ]
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unreachable
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try.cont: ; preds = %entry, %catch
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ret void
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}
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; Two try-catches.
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; For the catchpad with a single 'catch (...)', only a wasm.catch() call should
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; be generated after the catchpad; wasm.landingpad.index() and personality call
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; should NOT be generated. For the other catchpad, the argument of
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; wasm.landingpad.index() should be not 1 but 0.
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;
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; void foo();
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; void test1() {
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; try {
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; foo();
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; } catch (...) {
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; }
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; try {
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; foo();
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; } catch (int) {
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; }
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; }
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define void @test1() personality i8* bitcast (i32 (...)* @__gxx_wasm_personality_v0 to i8*) {
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; CHECK-LABEL: @test1()
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entry:
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invoke void @foo()
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to label %try.cont unwind label %catch.dispatch
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catch.dispatch: ; preds = %entry
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%0 = catchswitch within none [label %catch.start] unwind to caller
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catch.start: ; preds = %catch.dispatch
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%1 = catchpad within %0 [i8* null]
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%2 = call i8* @llvm.wasm.get.exception(token %1)
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%3 = call i32 @llvm.wasm.get.ehselector(token %1)
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%4 = call i8* @__cxa_begin_catch(i8* %2) [ "funclet"(token %1) ]
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call void @__cxa_end_catch() [ "funclet"(token %1) ]
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catchret from %1 to label %try.cont
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; CHECK: catch.start:
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; CHECK-NEXT: catchpad within %0 [i8* null]
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; CHECK-NOT: call void @llvm.wasm.landingpad.index
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; CHECK-NOT: store {{.*}} @__wasm_lpad_context
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; CHECK-NOT: call i8* @llvm.wasm.lsda()
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; CHECK-NOT: call i32 @_Unwind_CallPersonality
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; CHECK-NOT: load {{.*}} @__wasm_lpad_context
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try.cont: ; preds = %entry, %catch.start
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invoke void @foo()
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to label %try.cont7 unwind label %catch.dispatch2
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catch.dispatch2: ; preds = %try.cont
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%5 = catchswitch within none [label %catch.start3] unwind to caller
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catch.start3: ; preds = %catch.dispatch2
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%6 = catchpad within %5 [i8* bitcast (i8** @_ZTIi to i8*)]
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%7 = call i8* @llvm.wasm.get.exception(token %6)
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%8 = call i32 @llvm.wasm.get.ehselector(token %6)
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%9 = call i32 @llvm.eh.typeid.for(i8* bitcast (i8** @_ZTIi to i8*))
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%matches = icmp eq i32 %8, %9
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br i1 %matches, label %catch4, label %rethrow
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; CHECK: catch.start3:
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; CHECK: call void @llvm.wasm.landingpad.index(token %{{.+}}, i32 0)
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catch4: ; preds = %catch.start3
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%10 = call i8* @__cxa_begin_catch(i8* %7) [ "funclet"(token %6) ]
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call void @__cxa_end_catch() [ "funclet"(token %6) ]
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catchret from %6 to label %try.cont7
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rethrow: ; preds = %catch.start3
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call void @llvm.wasm.rethrow() [ "funclet"(token %6) ]
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unreachable
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try.cont7: ; preds = %try.cont, %catch4
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ret void
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}
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; PHI demotion test. Only the phi before catchswitch should be demoted; the phi
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; before cleanuppad should NOT.
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;
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; void foo();
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; int bar(int) noexcept;
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; struct Temp {
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; ~Temp() {}
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; };
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;
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; void test2() {
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; int num;
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; try {
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; Temp t;
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; num = 1;
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; foo();
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; num = 2;
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; foo();
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; } catch (...) {
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; bar(num);
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; }
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; try {
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; foo();
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; num = 1;
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; foo();
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; num = 2;
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; } catch (...) {
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; bar(num);
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; }
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; }
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define void @test2() personality i8* bitcast (i32 (...)* @__gxx_wasm_personality_v0 to i8*) {
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; CHECK-LABEL: @test2
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entry:
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%t = alloca %struct.Temp, align 1
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invoke void @foo()
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to label %invoke.cont unwind label %ehcleanup
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invoke.cont: ; preds = %entry
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invoke void @foo()
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to label %invoke.cont1 unwind label %ehcleanup
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invoke.cont1: ; preds = %invoke.cont
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%call = call %struct.Temp* @_ZN4TempD2Ev(%struct.Temp* %t)
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br label %try.cont
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ehcleanup: ; preds = %invoke.cont, %entry
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%num.0 = phi i32 [ 2, %invoke.cont ], [ 1, %entry ]
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%0 = cleanuppad within none []
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%call2 = call %struct.Temp* @_ZN4TempD2Ev(%struct.Temp* %t) [ "funclet"(token %0) ]
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cleanupret from %0 unwind label %catch.dispatch
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; CHECK: ehcleanup:
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; CHECK-NEXT: = phi
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catch.dispatch: ; preds = %ehcleanup
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%1 = catchswitch within none [label %catch.start] unwind to caller
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catch.start: ; preds = %catch.dispatch
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%2 = catchpad within %1 [i8* null]
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%3 = call i8* @llvm.wasm.get.exception(token %2)
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%4 = call i32 @llvm.wasm.get.ehselector(token %2)
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%5 = call i8* @__cxa_begin_catch(i8* %3) [ "funclet"(token %2) ]
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call void @bar(i32 %num.0) [ "funclet"(token %2) ]
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call void @__cxa_end_catch() [ "funclet"(token %2) ]
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catchret from %2 to label %try.cont
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try.cont: ; preds = %catch.start, %invoke.cont1
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invoke void @foo()
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to label %invoke.cont3 unwind label %catch.dispatch5
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invoke.cont3: ; preds = %try.cont
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invoke void @foo()
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to label %try.cont10 unwind label %catch.dispatch5
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catch.dispatch5: ; preds = %invoke.cont3, %try.cont
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%num.1 = phi i32 [ 2, %invoke.cont3 ], [ 1, %try.cont ]
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%6 = catchswitch within none [label %catch.start6] unwind to caller
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; CHECK: catch.dispatch5:
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; CHECK-NOT: = phi
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catch.start6: ; preds = %catch.dispatch5
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%7 = catchpad within %6 [i8* null]
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%8 = call i8* @llvm.wasm.get.exception(token %7)
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%9 = call i32 @llvm.wasm.get.ehselector(token %7)
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%10 = call i8* @__cxa_begin_catch(i8* %8) [ "funclet"(token %7) ]
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call void @bar(i32 %num.1) [ "funclet"(token %7) ]
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call void @__cxa_end_catch() [ "funclet"(token %7) ]
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catchret from %7 to label %try.cont10
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try.cont10: ; preds = %invoke.cont3, %catch.start6
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ret void
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}
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; Tests if instructions after a call to @llvm.wasm.throw are deleted and the
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; BB's dead children are deleted.
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; CHECK-LABEL: @test3
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define i32 @test3(i1 %b, i8* %p) {
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entry:
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br i1 %b, label %bb.true, label %bb.false
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; CHECK: bb.true:
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; CHECK-NEXT: call void @llvm.wasm.throw(i32 0, i8* %p)
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; CHECK-NEXT: unreachable
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bb.true: ; preds = %entry
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call void @llvm.wasm.throw(i32 0, i8* %p)
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br label %bb.true.0
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; CHECK-NOT: bb.true.0
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bb.true.0: ; preds = %bb.true
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br label %merge
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; CHECK: bb.false
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bb.false: ; preds = %entry
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br label %merge
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; CHECK: merge
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merge: ; preds = %bb.true.0, %bb.false
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ret i32 0
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}
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declare void @foo()
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declare void @bar(i32)
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declare %struct.Temp* @_ZN4TempD2Ev(%struct.Temp* returned)
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declare i32 @__gxx_wasm_personality_v0(...)
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; Function Attrs: nounwind
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declare i8* @llvm.wasm.get.exception(token) #0
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; Function Attrs: nounwind
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declare i32 @llvm.wasm.get.ehselector(token) #0
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; Function Attrs: nounwind
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declare i32 @llvm.eh.typeid.for(i8*) #0
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; Function Attrs: noreturn
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declare void @llvm.wasm.throw(i32, i8*) #1
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; Function Attrs: noreturn
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declare void @llvm.wasm.rethrow() #1
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declare i8* @__cxa_begin_catch(i8*)
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declare void @__cxa_end_catch()
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declare void @_ZSt9terminatev()
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attributes #0 = { nounwind }
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attributes #1 = { noreturn }
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; CHECK-DAG: declare void @llvm.wasm.landingpad.index(token, i32 immarg)
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; CHECK-DAG: declare i8* @llvm.wasm.lsda()
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; CHECK-DAG: declare i32 @_Unwind_CallPersonality(i8*)
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