caio/clang-p2996
1
0
Fork 0
Code Issues Pull Requests Actions Packages Projects Releases Wiki Activity
Files
6f912a2cd49150813ee467c079201b5ce3dfbbca
clang-p2996/llvm/test/CodeGen/WebAssembly/eh-lsda.ll
Heejin Ahn 561abd83ff [WebAssembly] Disable uses of __clang_call_terminate 
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
2021-03-04 14:26:35 -08:00

246 lines
11 KiB
LLVM
Raw Blame History

; RUN: llc < %s -disable-wasm-fallthrough-return-opt -wasm-keep-registers -exception-model=wasm -mattr=+exception-handling | FileCheck -allow-deprecated-dag-overlap %s
target datalayout = "e-m:e-p:32:32-i64:64-n32:64-S128"
target triple = "wasm32-unknown-unknown"
@_ZTIi = external constant i8*
@_ZTIf = external constant i8*
@_ZTId = external constant i8*
; Single catch (...) does not need an exception table.
;
; try {
; may_throw();
; } catch (...) {
; }
; CHECK-LABEL: test0:
; CHECK-NOT: GCC_except_table
define void @test0() personality i8* bitcast (i32 (...)* @__gxx_wasm_personality_v0 to i8*) {
entry:
invoke void @may_throw()
to label %try.cont unwind label %catch.dispatch
catch.dispatch: ; preds = %entry
%0 = catchswitch within none [label %catch.start] unwind to caller
catch.start: ; preds = %catch.dispatch
%1 = catchpad within %0 [i8* null]
%2 = call i8* @llvm.wasm.get.exception(token %1)
%3 = call i32 @llvm.wasm.get.ehselector(token %1)
%4 = call i8* @__cxa_begin_catch(i8* %2) [ "funclet"(token %1) ]
call void @__cxa_end_catch() [ "funclet"(token %1) ]
catchret from %1 to label %try.cont
try.cont: ; preds = %entry, %catch.start
ret void
}
; Exception table generation + shared action test.
;
; try {
; may_throw();
; } catch (int) {
; } catch (float) {
; } catch (double) {
; } catch (...) {
; }
;
; try {
; may_throw();
; } catch (double) {
; } catch (...) {
; }
;
; try {
; may_throw();
; } catch (int) {
; } catch (float) {
; }
;
; There are three landing pads. The second landing pad should share action table
; entries with the first landing pad because they end with the same sequence
; (double -> ...). But the third landing table cannot share action table entries
; with others, so it should create its own entries.
; CHECK-LABEL: test1:
; CHECK: .section .rodata.gcc_except_table,"",@
; CHECK-NEXT: .p2align 2
; CHECK-NEXT: GCC_except_table[[START:[0-9]+]]:
; CHECK-NEXT: .Lexception0:
; CHECK-NEXT: .int8 255 # @LPStart Encoding = omit
; CHECK-NEXT: .int8 0 # @TType Encoding = absptr
; CHECK-NEXT: .uleb128 .Lttbase0-.Lttbaseref0
; CHECK-NEXT: .Lttbaseref0:
; CHECK-NEXT: .int8 1 # Call site Encoding = uleb128
; CHECK-NEXT: .uleb128 .Lcst_end0-.Lcst_begin0
; CHECK-NEXT: .Lcst_begin0:
; CHECK-NEXT: .int8 0 # >> Call Site 0 <<
; CHECK-NEXT: # On exception at call site 0
; CHECK-NEXT: .int8 7 # Action: 4
; CHECK-NEXT: .int8 1 # >> Call Site 1 <<
; CHECK-NEXT: # On exception at call site 1
; CHECK-NEXT: .int8 3 # Action: 2
; CHECK-NEXT: .int8 2 # >> Call Site 2 <<
; CHECK-NEXT: # On exception at call site 2
; CHECK-NEXT: .int8 11 # Action: 6
; CHECK-NEXT: .Lcst_end0:
; CHECK-NEXT: .int8 1 # >> Action Record 1 <<
; CHECK-NEXT: # Catch TypeInfo 1
; CHECK-NEXT: .int8 0 # No further actions
; CHECK-NEXT: .int8 2 # >> Action Record 2 <<
; CHECK-NEXT: # Catch TypeInfo 2
; CHECK-NEXT: .int8 125 # Continue to action 1
; CHECK-NEXT: .int8 3 # >> Action Record 3 <<
; CHECK-NEXT: # Catch TypeInfo 3
; CHECK-NEXT: .int8 125 # Continue to action 2
; CHECK-NEXT: .int8 4 # >> Action Record 4 <<
; CHECK-NEXT: # Catch TypeInfo 4
; CHECK-NEXT: .int8 125 # Continue to action 3
; CHECK-NEXT: .int8 3 # >> Action Record 5 <<
; CHECK-NEXT: # Catch TypeInfo 3
; CHECK-NEXT: .int8 0 # No further actions
; CHECK-NEXT: .int8 4 # >> Action Record 6 <<
; CHECK-NEXT: # Catch TypeInfo 4
; CHECK-NEXT: .int8 125 # Continue to action 5
; CHECK-NEXT: .p2align 2
; CHECK-NEXT: # >> Catch TypeInfos <<
; CHECK-NEXT: .int32 _ZTIi # TypeInfo 4
; CHECK-NEXT: .int32 _ZTIf # TypeInfo 3
; CHECK-NEXT: .int32 _ZTId # TypeInfo 2
; CHECK-NEXT: .int32 0 # TypeInfo 1
; CHECK-NEXT: .Lttbase0:
; CHECK-NEXT: .p2align 2
; CHECK-NEXT: .LGCC_except_table_end[[END:[0-9]+]]:
; CHECK-NEXT: .size GCC_except_table[[START]], .LGCC_except_table_end[[END]]-GCC_except_table[[START]]
define void @test1() personality i8* bitcast (i32 (...)* @__gxx_wasm_personality_v0 to i8*) {
entry:
invoke void @may_throw()
to label %try.cont unwind label %catch.dispatch
catch.dispatch: ; preds = %entry
%0 = catchswitch within none [label %catch.start] unwind to caller
catch.start: ; preds = %catch.dispatch
%1 = catchpad within %0 [i8* bitcast (i8** @_ZTIi to i8*), i8* bitcast (i8** @_ZTIf to i8*), i8* bitcast (i8** @_ZTId to i8*), i8* null]
%2 = call i8* @llvm.wasm.get.exception(token %1)
%3 = call i32 @llvm.wasm.get.ehselector(token %1)
%4 = call i32 @llvm.eh.typeid.for(i8* bitcast (i8** @_ZTIi to i8*))
%matches = icmp eq i32 %3, %4
br i1 %matches, label %catch10, label %catch.fallthrough
catch10: ; preds = %catch.start
%5 = call i8* @__cxa_begin_catch(i8* %2) [ "funclet"(token %1) ]
%6 = bitcast i8* %5 to i32*
%7 = load i32, i32* %6, align 4
call void @__cxa_end_catch() [ "funclet"(token %1) ]
catchret from %1 to label %try.cont
try.cont: ; preds = %entry, %catch, %catch4, %catch7, %catch10
invoke void @may_throw()
to label %try.cont23 unwind label %catch.dispatch14
catch.dispatch14: ; preds = %try.cont
%8 = catchswitch within none [label %catch.start15] unwind to caller
catch.start15: ; preds = %catch.dispatch14
%9 = catchpad within %8 [i8* bitcast (i8** @_ZTId to i8*), i8* null]
%10 = call i8* @llvm.wasm.get.exception(token %9)
%11 = call i32 @llvm.wasm.get.ehselector(token %9)
%12 = call i32 @llvm.eh.typeid.for(i8* bitcast (i8** @_ZTId to i8*))
%matches16 = icmp eq i32 %11, %12
%13 = call i8* @__cxa_begin_catch(i8* %10) [ "funclet"(token %9) ]
br i1 %matches16, label %catch20, label %catch17
catch20: ; preds = %catch.start15
%14 = bitcast i8* %13 to double*
%15 = load double, double* %14, align 8
call void @__cxa_end_catch() [ "funclet"(token %9) ]
catchret from %9 to label %try.cont23
try.cont23: ; preds = %try.cont, %catch17, %catch20
invoke void @may_throw()
to label %try.cont36 unwind label %catch.dispatch25
catch.dispatch25: ; preds = %try.cont23
%16 = catchswitch within none [label %catch.start26] unwind to caller
catch.start26: ; preds = %catch.dispatch25
%17 = catchpad within %16 [i8* bitcast (i8** @_ZTIi to i8*), i8* bitcast (i8** @_ZTIf to i8*)]
%18 = call i8* @llvm.wasm.get.exception(token %17)
%19 = call i32 @llvm.wasm.get.ehselector(token %17)
%20 = call i32 @llvm.eh.typeid.for(i8* bitcast (i8** @_ZTIi to i8*))
%matches27 = icmp eq i32 %19, %20
br i1 %matches27, label %catch33, label %catch.fallthrough28
catch33: ; preds = %catch.start26
%21 = call i8* @__cxa_begin_catch(i8* %18) [ "funclet"(token %17) ]
%22 = bitcast i8* %21 to i32*
%23 = load i32, i32* %22, align 4
call void @__cxa_end_catch() [ "funclet"(token %17) ]
catchret from %17 to label %try.cont36
catch.fallthrough28: ; preds = %catch.start26
%24 = call i32 @llvm.eh.typeid.for(i8* bitcast (i8** @_ZTIf to i8*))
%matches29 = icmp eq i32 %19, %24
br i1 %matches29, label %catch30, label %rethrow
catch30: ; preds = %catch.fallthrough28
%25 = call i8* @__cxa_begin_catch(i8* %18) [ "funclet"(token %17) ]
%26 = bitcast i8* %25 to float*
%27 = load float, float* %26, align 4
call void @__cxa_end_catch() [ "funclet"(token %17) ]
catchret from %17 to label %try.cont36
rethrow: ; preds = %catch.fallthrough28
call void @__cxa_rethrow() [ "funclet"(token %17) ]
unreachable
try.cont36: ; preds = %try.cont23, %catch30, %catch33
ret void
catch17: ; preds = %catch.start15
call void @__cxa_end_catch() [ "funclet"(token %9) ]
catchret from %9 to label %try.cont23
catch.fallthrough: ; preds = %catch.start
%28 = call i32 @llvm.eh.typeid.for(i8* bitcast (i8** @_ZTIf to i8*))
%matches1 = icmp eq i32 %3, %28
br i1 %matches1, label %catch7, label %catch.fallthrough2
catch7: ; preds = %catch.fallthrough
%29 = call i8* @__cxa_begin_catch(i8* %2) [ "funclet"(token %1) ]
%30 = bitcast i8* %29 to float*
%31 = load float, float* %30, align 4
call void @__cxa_end_catch() [ "funclet"(token %1) ]
catchret from %1 to label %try.cont
catch.fallthrough2: ; preds = %catch.fallthrough
%32 = call i32 @llvm.eh.typeid.for(i8* bitcast (i8** @_ZTId to i8*))
%matches3 = icmp eq i32 %3, %32
%33 = call i8* @__cxa_begin_catch(i8* %2) [ "funclet"(token %1) ]
br i1 %matches3, label %catch4, label %catch
catch4: ; preds = %catch.fallthrough2
%34 = bitcast i8* %33 to double*
%35 = load double, double* %34, align 8
call void @__cxa_end_catch() [ "funclet"(token %1) ]
catchret from %1 to label %try.cont
catch: ; preds = %catch.fallthrough2
call void @__cxa_end_catch() [ "funclet"(token %1) ]
catchret from %1 to label %try.cont
}
declare void @may_throw()
; Function Attrs: nounwind
declare i32 @llvm.eh.typeid.for(i8*) #0
; Function Attrs: nounwind
declare i8* @llvm.wasm.get.exception(token) #0
; Function Attrs: nounwind
declare i32 @llvm.wasm.get.ehselector(token) #0
declare void @__cxa_rethrow()
declare i8* @__cxa_begin_catch(i8*)
declare void @__cxa_end_catch()
declare i32 @__gxx_wasm_personality_v0(...)
attributes #0 = { nounwind }
Reference in New Issue View Git Blame Copy Permalink