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Kaylor, Andrew

2014-Nov-24 20:12 UTC

[LLVMdev] RFC: How to represent SEH (try / except) in LLVM IR

Hi Reid,

I've been working on the outlining code and have a prototype that produces
what I want for a simple case.

Now I'm thinking about the heuristics for recognizing the various logical
pieces for C++ exception handling code and removing them once they’ve been
cloned.  I've been working from various comments you've made earlier in
this thread, and I'd like to run something by you to make sure we're on
the same page.

Starting from a C++ function that looks like this:

void do_some_thing(int &i)
{
  Outer outer;
  try {
    Middle middle;
    if (i == 1) {
        do_thing_one();
    }
    else {
        Inner inner;
        do_thing_two();
    }
  }
  catch (int en) {
    i = -1;
  }
}


I'll have IR that looks more or less like this:

; Function Attrs: uwtable
define void @_Z13do_some_thingRi(i32* dereferenceable(4) %i) #0 {
entry:
  %i.addr = alloca i32*, align 8
  %outer = alloca %class.Outer, align 1
  %middle = alloca %class.Middle, align 1
  %exn.slot = alloca i8*
  %ehselector.slot = alloca i32
  %inner = alloca %class.Inner, align 1
  %en = alloca i32, align 4
  store i32* %i, i32** %i.addr, align 8
  call void @_ZN5OuterC1Ev(%class.Outer* %outer)
  invoke void @_ZN6MiddleC1Ev(%class.Middle* %middle)
          to label %invoke.cont unwind label %lpad

invoke.cont:                                      ; preds = %entry
  %0 = load i32** %i.addr, align 8
  %1 = load i32* %0, align 4
  %cmp = icmp eq i32 %1, 1
  br i1 %cmp, label %if.then, label %if.else

if.then:                                          ; preds = %invoke.cont
  invoke void @_Z12do_thing_onev()
          to label %invoke.cont2 unwind label %lpad1

invoke.cont2:                                     ; preds = %if.then
  br label %if.end

; From 'entry' invoke of Middle constructor
;   outer needs post-catch cleanup
lpad:                                             ; preds = %if.end, %entry
  %2 = landingpad { i8*, i32 } personality i8* bitcast (i32 (...)*
@__CxxFrameHandler3 to i8*)
          cleanup
          catch i8* bitcast (i8** @_ZTIi to i8*)
  %3 = extractvalue { i8*, i32 } %2, 0
  store i8* %3, i8** %exn.slot
  %4 = extractvalue { i8*, i32 } %2, 1
  store i32 %4, i32* %ehselector.slot
  ; No pre-catch cleanup for this landingpad
  br label %catch.dispatch

; From 'if.then' invoke of do_thing_one()
; Or from 'if.else' invoke of Inner constructor
; Or from 'invoke.cont5 invoke of Inner destructor
;   middle needs pre-catch cleanup
;   outer needs post-catch cleanup
lpad1:                                            ; preds = %invoke.cont5,
%if.else, %if.then
  %5 = landingpad { i8*, i32 } personality i8* bitcast (i32 (...)*
@__CxxFrameHandler3 to i8*)
          cleanup
          catch i8* bitcast (i8** @_ZTIi to i8*)
  %6 = extractvalue { i8*, i32 } %5, 0
  store i8* %6, i8** %exn.slot
  %7 = extractvalue { i8*, i32 } %5, 1
  store i32 %7, i32* %ehselector.slot
  ; Branch to shared label to do pre-catch cleanup
  br label %ehcleanup

if.else:                                          ; preds = %invoke.cont
  invoke void @_ZN5InnerC1Ev(%class.Inner* %inner)
          to label %invoke.cont3 unwind label %lpad1

invoke.cont3:                                     ; preds = %if.else
  invoke void @_Z12do_thing_twov()
          to label %invoke.cont5 unwind label %lpad4

invoke.cont5:                                     ; preds = %invoke.cont3
  invoke void @_ZN5InnerD1Ev(%class.Inner* %inner)
          to label %invoke.cont6 unwind label %lpad1

invoke.cont6:                                     ; preds = %invoke.cont5
  br label %if.end

; From 'invoke.cont3' invoke of do_something_two()
;   middle and inner need pre-catch cleanup
;   outer needs post-catch cleanup
lpad4:                                            ; preds = %invoke.cont3
  %8 = landingpad { i8*, i32 } personality i8* bitcast (i32 (...)*
@__CxxFrameHandler3 to i8*)
          cleanup
          catch i8* bitcast (i8** @_ZTIi to i8*)
  %9 = extractvalue { i8*, i32 } %8, 0
  store i8* %9, i8** %exn.slot
  %10 = extractvalue { i8*, i32 } %8, 1
  store i32 %10, i32* %ehselector.slot
  ; Pre-catch cleanup begins here, but will continue at ehcleanup
  invoke void @_ZN5InnerD1Ev(%class.Inner* %inner)
          to label %invoke.cont7 unwind label %terminate.lpad

invoke.cont7:                                     ; preds = %lpad4
  br label %ehcleanup

if.end:                                           ; preds = %invoke.cont6,
%invoke.cont2
  invoke void @_ZN6MiddleD1Ev(%class.Middle* %middle)
          to label %invoke.cont8 unwind label %lpad

invoke.cont8:                                     ; preds = %if.end
  br label %try.cont

; Pre-catch cleanup for lpad1
; Continuation of pre-catch cleanup for lpad4
ehcleanup:                                        ; preds = %invoke.cont7,
%lpad1
  invoke void @_ZN6MiddleD1Ev(%class.Middle* %middle)
          to label %invoke.cont9 unwind label %terminate.lpad

invoke.cont9:                                     ; preds = %ehcleanup
  br label %catch.dispatch

; Catch dispatch for lpad, lpad1 and lpad4
catch.dispatch:                                   ; preds = %invoke.cont9, %lpad
  %sel = load i32* %ehselector.slot
  %11 = call i32 @llvm.eh.typeid.for(i8* bitcast (i8** @_ZTIi to i8*)) #4
  %matches = icmp eq i32 %sel, %11
  br i1 %matches, label %catch, label %ehcleanup10

catch:                                            ; preds = %catch.dispatch
  %exn = load i8** %exn.slot
  %12 = call i8* @__cxa_begin_catch(i8* %exn) #4
  %13 = bitcast i8* %12 to i32*
  %14 = load i32* %13, align 4
  store i32 %14, i32* %en, align 4
  %15 = load i32** %i.addr, align 8
  store i32 -1, i32* %15, align 4
  call void @__cxa_end_catch() #4
  br label %try.cont

try.cont:                                         ; preds = %catch,
%invoke.cont8
  call void @_ZN5OuterD1Ev(%class.Outer* %outer)
  ret void

; Post catch cleanup for lpad, lpad1
ehcleanup10:                                      ; preds = %catch.dispatch
  invoke void @_ZN5OuterD1Ev(%class.Outer* %outer)
          to label %invoke.cont11 unwind label %terminate.lpad

invoke.cont11:                                    ; preds = %ehcleanup10
  br label %eh.resume

eh.resume:                                        ; preds = %invoke.cont11
  %exn12 = load i8** %exn.slot
  %sel13 = load i32* %ehselector.slot
  %lpad.val = insertvalue { i8*, i32 } undef, i8* %exn12, 0
  %lpad.val14 = insertvalue { i8*, i32 } %lpad.val, i32 %sel13, 1
  resume { i8*, i32 } %lpad.val14

terminate.lpad:                                   ; preds = %ehcleanup10,
%ehcleanup, %lpad4
  %16 = landingpad { i8*, i32 } personality i8* bitcast (i32 (...)*
@__CxxFrameHandler3 to i8*)
          catch i8* null
  %17 = extractvalue { i8*, i32 } %16, 0
  call void @__clang_call_terminate(i8* %17) #5
  unreachable
}



If I've understood your intentions correctly, we'll have an outlining
pass that transforms the above IR to this:

%struct.do_some_thing.captureblock = type { %class.Outer, %class.Middle,
%class.Inner, %i32* }

; Uncaught exception cleanup for lpad, lpad1 and lpad4
define void @do_some_thing_cleanup0(i8* %eh_ptrs, i8* %rbp) #0 {
entry:
  %capture.block = call @llvm.eh.get_capture_block(@_Z13do_some_thingRi , %rbp)
  %outer = getelementptr inbounds %struct.do_some_this.captureblock*
%capture.block, i32 0, i32 0
  invoke void @_ZN5OuterD1Ev(%class.Outer* %outer)
          to label %invoke.cont unwind label %terminate.lpad

invoke.cont:
  ret void

terminate.lpad:                                   ; preds = %ehcleanup10,
%ehcleanup, %lpad4
  %0 = landingpad { i8*, i32 } personality i8* bitcast (i32 (...)*
@__CxxFrameHandler3 to i8*)
          catch i8* null
  %1 = extractvalue { i8*, i32 } %0, 0
  call void @__clang_call_terminate(i8* %1) #5
  unreachable
}

; Catch handler for _ZTIi
define i8* @do_some_thing_catch0(i8* %eh_ptrs, i8* %rbp) #0 {
entry:
  %capture.block = call @llvm.eh.get_capture_block(@_Z13do_some_thingRi , %rbp)
  %i.addr = getelementptr inbounds %struct.do_some_this.captureblock*
%capture.block, i32 0, i32 4
  %1 = load i32** %i.addr, align 8
  store i32 -1, i32* %1, align 4
  ret i8* blockaddress(@_Z13do_some_thingRi, %try.cont)
}

; Outlined pre-catch cleanup handler for lpad1
define void @do_some_thing_cleanup1(i8* %eh_ptrs, i8* %rbp) #0 {
entry:
  %capture.block = call @llvm.eh.get_capture_block(@_Z13do_some_thingRi, %rbp)
  ; Outlined from 'ehcleanup'
  %middle = getelementptr inbounds %struct.do_some_this.captureblock*
%capture.block, i32 0, i32 1
  invoke void @_ZN6MiddleD1Ev(%class.Middle* %middle)
          to label %invoke.cont unwind label %terminate.lpad

invoke.cont:
  ret void

terminate.lpad:                                   ; preds = %ehcleanup10,
%ehcleanup, %lpad4
  %0 = landingpad { i8*, i32 } personality i8* bitcast (i32 (...)*
@__CxxFrameHandler3 to i8*)
          catch i8* null
  %1 = extractvalue { i8*, i32 } %0, 0
  call void @__clang_call_terminate(i8* %1) #5
  unreachable
}

; Outlined pre-catch cleanup handler for 'lpad4'
define void @do_some_thing_cleanup2(i8* %eh_ptrs, i8* %rbp) #0 {
entry:
  %capture.block = call @llvm.eh.get_capture_block(@_Z13do_some_thingRi , %rbp)
  ; Outlined from 'lpad4'
  %inner = getelementptr inbounds %struct.do_some_this.captureblock*
%capture.block, i32 0, i32 2
  invoke void @_ZN5InnerD1Ev(%class.Inner* %inner)
          to label %invoke.cont unwind label %terminate.lpad

invoke.cont:                                     ; preds = %entry
  ; Outlined from 'ehcleanup'
  %middle = getelementptr inbounds %struct.do_some_this.captureblock*
%capture.block, i32 0, i32 1
  invoke void @_ZN6MiddleD1Ev(%class.Middle* %middle)
          to label %invoke.cont1 unwind label %terminate.lpad

invoke.cont1:
  ret void

terminate.lpad:                                   ; preds = %ehcleanup10,
%ehcleanup, %lpad4
  %0 = landingpad { i8*, i32 } personality i8* bitcast (i32 (...)*
@__CxxFrameHandler3 to i8*)
          catch i8* null
  %1 = extractvalue { i8*, i32 } %0, 0
  call void @__clang_call_terminate(i8* %1) #5
  unreachable
}


; Function Attrs: uwtable
define void @_Z13do_some_thingRi(i32* dereferenceable(4) %i) #0 {
entry:
  %capture.block = alloca %struct.do_some_thing.capture.block, align 1
  %i_addr = getelementptr inbounds %struct.do_some_thing_capture_block*
%capture_block, i32 0, i32 3
  store i32* %i, i32** %i_addr, align 8
  llvm.eh.set_capture_block
  %eh.cont.label = alloca i8*
  %en = alloca i32, align 4
  store i32* %i, i32** %i.addr, align 8
  %outer = getelementptr inbounds %struct.do_some_thing.capture.block*
%capture.block, i32 0, i32 0
  call void @_ZN5OuterC1Ev(%class.Outer* %outer)
  %middle = getelementptr inbounds %struct.do_some_thing.capture.block*
%capture.block, i32 0, i32 1
  invoke void @_ZN6MiddleC1Ev(%class.Middle* %middle)
          to label %invoke.cont unwind label %lpad

invoke.cont:                                      ; preds = %entry
  %0 = load i32** %i.addr, align 8
  %1 = load i32* %0, align 4
  %cmp = icmp eq i32 %1, 1
  br i1 %cmp, label %if.then, label %if.else

if.then:                                          ; preds = %invoke.cont
  invoke void @_Z12do_thing_onev()
          to label %invoke.cont2 unwind label %lpad1

invoke.cont2:                                     ; preds = %if.then
  br label %if.end

; From 'entry' invoke of Middle constructor
;   outer needs post-catch cleanup
lpad:                                             ; preds = %if.end, %entry
  %2 = landingpad { i8*, i32 } personality i8* bitcast (i32 (...)*
@__CxxFrameHandler3 to i8*)
          cleanup
          catch i8* bitcast (i8** @_ZTIi to i8*)
  %eh.cont.label = call i8* (...)* @llvm.eh.outlined_handlers(
      i8* @_ZTIi, i8* (i8*, i8*)* @do_some_thing_catch0,
      void (i8*, i8*)* @do_some_thing_cleanup0)
  indirectbr i8* %eh.cont.label

; From 'if.then' invoke of do_thing_one()
; Or from 'if.else' invoke of Inner constructor
; Or from 'invoke.cont5 invoke of Inner destructor
;   middle needs pre-catch cleanup
;   outer needs post-catch cleanup
lpad1:                                            ; preds = %invoke.cont5,
%if.else, %if.then
  %5 = landingpad { i8*, i32 } personality i8* bitcast (i32 (...)*
@__CxxFrameHandler3 to i8*)
          cleanup
          catch i8* bitcast (i8** @_ZTIi to i8*)
  %eh.cont.label = call i8* (...)* @llvm.eh.outlined_handlers(
      void (i8*, i8*)* @do_some_thing_cleanup1,
      i8* @_ZTIi, i8* (i8*, i8*)* @do_some_thing_catch0,
      void (i8*, i8*)* @do_some_thing_cleanup0)
  indirectbr i8* %eh.cont.label

if.else:                                          ; preds = %invoke.cont
  %inner = getelementptr inbounds %struct.do_some_thing.capture.block*
%capture.block, i32 0, i32 2
  invoke void @_ZN5InnerC1Ev(%class.Inner* %inner)
          to label %invoke.cont3 unwind label %lpad1

invoke.cont3:                                     ; preds = %if.else
  invoke void @_Z12do_thing_twov()
          to label %invoke.cont5 unwind label %lpad4

invoke.cont5:                                     ; preds = %invoke.cont3
  invoke void @_ZN5InnerD1Ev(%class.Inner* %inner)
          to label %invoke.cont6 unwind label %lpad1

invoke.cont6:                                     ; preds = %invoke.cont5
  br label %if.end

; From 'invoke.cont3' invoke of do_something_two()
;   middle and inner need pre-catch cleanup
;   outer needs post-catch cleanup
lpad4:                                            ; preds = %invoke.cont3
  %8 = landingpad { i8*, i32 } personality i8* bitcast (i32 (...)*
@__CxxFrameHandler3 to i8*)
          cleanup
          catch i8* bitcast (i8** @_ZTIi to i8*)
  %eh.cont.label = call i8* (...)* @llvm.eh.outlined_handlers(
      void (i8*, i8*)* @do_some_thing_cleanup2,
      i8* @_ZTIi, i8* (i8*, i8*)* @do_some_thing_catch0,
      void (i8*, i8*)* @do_some_thing_cleanup0)
  indirectbr i8* %eh.cont.label

if.end:                                           ; preds = %invoke.cont6,
%invoke.cont2
  invoke void @_ZN6MiddleD1Ev(%class.Middle* %middle)
          to label %invoke.cont8 unwind label %lpad

invoke.cont8:                                     ; preds = %if.end
  br label %try.cont

try.cont:                                         ; preds = %catch,
%invoke.cont8
  call void @_ZN5OuterD1Ev(%class.Outer* %outer)
  ret void
}


Does that look about like what you’d expect?

I just have a few questions.

I'm pretty much just guessing at how you intended the
llvm.eh.set_capture_block intrinsic to work.  It wasn't clear to me if I
just needed to set it where the structure was created or if it would need to be
set anywhere an exception might be thrown.  The answer is probably related to my
next question.

In the above example I created a single capture block for the entire function. 
That works reasonably well for a simple case like this and corresponds to the
co-location of the allocas in the original IR, but for functions with more
complex structures and multiple try blocks it could get ugly.  Do you have ideas
for how to handle that?

For C++ exception handling, we need cleanup code that executes before the catch
handlers and cleanup code that excutes in the case on uncaught exceptions.  I
think both of these need to be outlined for the MSVC environment. Do you think
we need a stub handler to be inserted in cases where no actual cleanup is
performed?

I didn't do that in the mock-up above, but it seems like it would simplify
things.  Basically, I'm imagining a final pattern that looks like this:

lpad:
  %eh_vals = landingpad { i8*, i32 } personality i8* bitcast (i32 (...)*
@__CxxFrameHandler3 to i8*)
      cleanup
      catch i8* @typeid1
      catch i8* @typeid2
      ...
  %label = call i8* (...)* @llvm.eh.outlined_handlers(
      void (i8*, i8*)* @<pre-catch cleanup function>,
      i8* @typeid1, i8* (i8*, i8*)* @<typeid1 catch function>,
      i8* @typeid2, i8* (i8*, i8*)* @<typeid2 catch function>,
      ...
      void (i8*, i8*)* @<uncaught exception cleanup function>)
  indirectbr i8* %label


Finally, how do you see this meshing with SEH?  As I understand it, both the
exception handlers and the cleanup code in that case execute in the original
function context and only the filter handlers need to be outlined.  I suppose
the outlining pass can look at the personality function and change its behavior
accordingly.  Is that what you were thinking?

-Andy

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Kaylor, Andrew

2014-Nov-25 23:09 UTC

head link

[LLVMdev] RFC: How to represent SEH (try / except) in LLVM IR

> We should also think about how to call std::terminate when cleanup dtors
throw. The current representation for Itanium is inefficient. As a strawman, I
propose making @__clang_call_terminate an intrinsic:…

That sounds like a good starting point.

> Chandler expressed strong concerns about this design, however, as
@llvm.eh.get_capture_block adds an ordering constraint on CodeGen. Once you add
this intrinsic, we *have* to do frame layout of @_Z13do_some_thingRi *before* we
can emit code for all the callers of @llvm.eh.get_capture_block. Today, this is
easy, because module order defines emission order, but in the great glorious
future, codegen will hopefully be parallelized, and then we've inflicted
this horrible constraint on the innocent.
> His suggestion to break the ordering dependence was to lock down the frame
offset of the capture block to always be some fixed offset known by the target
(ie ebp - 4 on x86, if we like that).
Chandler probably has a better feel for this sort of thing than I do.  I can’t
think of a reason offhand why that wouldn’t work, but it makes me a little
nervous.

What would that look like in the IR?  Would we use the same intrinsics and just
lower them to use the known location?

I’ll think about this, but for now I’m happy to just proceed with the belief
that it’s a solvable problem either way.
>> For C++ exception handling, we need cleanup code that executes before
the catch handlers and cleanup code that excutes in the case on uncaught
exceptions.  I think both of these need to be outlined for the MSVC environment.
Do you think we need a stub handler to be inserted in cases where no actual
cleanup is performed?
> I think it's actually harder than that, once you consider nested trys:
> void f() {
>  try {
>    Outer outer;
>    try {
>      Inner inner;
>      g();
>    } catch (int) {
>      // ~Inner gets run first
>    }
>  } catch (float) {
>    // ~Inner gets run first
>    // ~Outer gets run next
>  }
>  // uncaught exception? Run ~Inner then ~Outer.
> }
I took a look at the IR that’s generated for this example.  I see what you mean.
So there is potentially cleanup code before and after every catch handler,
right?

Do you happen to know offhand what that looks like in the .xdata for the
_CxxFrameHandler3 function?

-Andy

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