llvm dev - Apr 2009 - [LLVMdev] how to resolve llvm exception IR?

here are the cpp file:
$ cat -n  eh1.catch.cpp
     1  #include <iostream>
     2
     3  int main()
     4  {
     5      try {
     6          throw 78;
     7      }
     8      catch (int){
     9
    10          std::cout << "at catch\n";
    11
    12      }
    13  }


LLVM-IR:

$ llvm-g++  -S -emit-llvm eh1.catch.cpp -o eh1.catch.ll

...
    46  define i32 @main() {
    47  entry:
    48          %0 = tail call i8* @__cxa_allocate_exception(i32 4)
nounwind            ; <i8*> [#uses=2]
    49          %1 = bitcast i8* %0 to i32*             ; <i32*> [#uses=1]
    50          store i32 78, i32* %1, align 4
    51          invoke void @__cxa_throw(i8* %0, i8* bitcast
(%struct.__fundamental_type_info_pseudo* @_ZTIi to i8*), void (i8*)*
null) noreturn
    52                          to label %invcont unwind label %lpad
    53
    54  invcont:                ; preds = %entry
    55          unreachable
    56
    57  bb:             ; preds = %lpad
    58          %2 = tail call i8* @__cxa_begin_catch(i8* %eh_ptr)
nounwind             ; <i8*> [#uses=0]
    59          %3 = invoke
%"struct.std::basic_ostream<char,std::char_traits<char>
>"*
@_ZStlsISt11char_traitsIcEERSt13basic_ostreamIcT_ES5_PKc(%"struct.std::basic_ostream<char,std::char_traits<char>
>"* @_ZSt4cout, i8* getelementptr ([10 x i8]* @.str, i32 0, i32 0))
    60                          to label %bb6 unwind label %lpad10
         ;
<%"struct.std::basic_ostream<char,std::char_traits<char>
>"*> [#uses=0]
    61
    62  bb6:            ; preds = %bb
    63          tail call void @__cxa_end_catch()
    64          ret i32 0
    65
    66  lpad:           ; preds = %entry
    67          %eh_ptr = tail call i8* @llvm.eh.exception()
 ; <i8*> [#uses=3]
    68          %eh_select9 = tail call i32 (i8*, i8*, ...)*
@llvm.eh.selector.i32(i8* %eh_ptr, i8* bitcast (i32 (...)*
@__gxx_personality_v0 to i8*), i8* bitcast
(%struct.__fundamental_type_info_pseudo* @_ZTIi to i8*), i8* null)
     ; <i32> [#uses=1]
    69          %eh_typeid = tail call i32 @llvm.eh.typeid.for.i32(i8*
bitcast (%struct.__fundamental_type_info_pseudo* @_ZTIi to i8*))
  ; <i32> [#uses=1]
    70          %4 = icmp eq i32 %eh_select9, %eh_typeid
 ; <i1> [#uses=1]
    71          br i1 %4, label %bb, label %Unwind
    72
    73  lpad10:         ; preds = %bb
    74          %eh_ptr11 = tail call i8* @llvm.eh.exception()
 ; <i8*> [#uses=2]
    75          %eh_select13 = tail call i32 (i8*, i8*, ...)*
@llvm.eh.selector.i32(i8* %eh_ptr11, i8* bitcast (i32 (...)*
@__gxx_personality_v0 to i8*), i8* null)             ; <i32> [#uses=0]
    76          invoke void @__cxa_end_catch()
    77                          to label %Unwind unwind label %lpad14
    78
    79  lpad14:         ; preds = %lpad10
    80          %eh_ptr15 = tail call i8* @llvm.eh.exception()
 ; <i8*> [#uses=1]
    81          %eh_select17 = tail call i32 (i8*, i8*, ...)*
@llvm.eh.selector.i32(i8* %eh_ptr15, i8* bitcast (i32 (...)*
@__gxx_personality_v0 to i8*))               ; <i32> [#uses=0]
    82          tail call void @_ZSt9terminatev() noreturn nounwind
    83          unreachable
    84
    85  Unwind:         ; preds = %lpad10, %lpad
    86          %eh_exception.0 = phi i8* [ %eh_ptr, %lpad ], [
%eh_ptr11, %lpad10 ]            ; <i8*> [#uses=1]
    87          tail call void @_Unwind_Resume(i8* %eh_exception.0)
    88          unreachable
    89  }


my  question is at line 59  can throw an exception?

here is another cpp file:
$ cat -n  eh1.try.cpp
     1  #include <iostream>
     2
     3
     4
     5  int main()
     6  {
     7      try {
     8          std::cout << "before throw\n";
     9          throw 78;
    10
    11      }
    12      catch (int){
    13
    14      }
    15  }

the generated LLVM-IR :

46  define i32 @main() {
    47  entry:
    48          %0 = invoke
%"struct.std::basic_ostream<char,std::char_traits<char>
>"*
@_ZStlsISt11char_traitsIcEERSt13basic_ostreamIcT_ES5_PKc(%"struct.std::basic_ostream<char,std::char_traits<char>
>"* @_ZSt4cout, i8* getelementptr ([14 x i8]* @.str, i32 0, i32 0))
    49                          to label %invcont unwind label %lpad
         ;
<%"struct.std::basic_ostream<char,std::char_traits<char>
>"*> [#uses=0]
    50
    51  invcont:                ; preds = %entry
    52          %1 = tail call i8* @__cxa_allocate_exception(i32 4)
nounwind            ; <i8*> [#uses=2]
    53          %2 = bitcast i8* %1 to i32*             ; <i32*> [#uses=1]
    54          store i32 78, i32* %2, align 4
    55          invoke void @__cxa_throw(i8* %1, i8* bitcast
(%struct.__fundamental_type_info_pseudo* @_ZTIi to i8*), void (i8*)*
null) noreturn
    56                          to label %invcont1 unwind label %lpad
    57
    58  invcont1:               ; preds = %invcont
    59          unreachable
    60
    61  bb:             ; preds = %lpad
    62          %3 = tail call i8* @__cxa_begin_catch(i8* %eh_ptr)
nounwind             ; <i8*> [#uses=0]
    63          tail call void @__cxa_end_catch()
    64          ret i32 0
    65
    66  lpad:           ; preds = %invcont, %entry
    67          %eh_ptr = tail call i8* @llvm.eh.exception()
 ; <i8*> [#uses=3]
    68          %eh_select = tail call i32 (i8*, i8*, ...)*
@llvm.eh.selector.i32(i8* %eh_ptr, i8* bitcast (i32 (...)*
@__gxx_personality_v0 to i8*), i8* bitcast
(%struct.__fundamental_type_info_pseudo* @_ZTIi to i8*), i8* null)
      ; <i32> [#uses=1]
    69          %eh_typeid = tail call i32 @llvm.eh.typeid.for.i32(i8*
bitcast (%struct.__fundamental_type_info_pseudo* @_ZTIi to i8*))
  ; <i32> [#uses=1]
    70          %4 = icmp eq i32 %eh_select, %eh_typeid         ; <i1>
[#uses=1]
    71          br i1 %4, label %bb, label %Unwind
    72
    73  Unwind:         ; preds = %lpad
    74          tail call void @_Unwind_Resume(i8* %eh_ptr)
    75          unreachable
    76  }

here at line 48 can throw an exception?

my another problem :  are there any different of above two  llmv-ir
on  handle c++ exception  ?


best regards
zhangzw
2009-4-28

On Apr 27, 2009, at 7:37 PM, zhengjian zhang wrote:
>
>    59          %3 = invoke
> %"struct.std::basic_ostream<char,std::char_traits<char>
>"*
> @_ZStlsISt11char_traitsIcEERSt13basic_ostreamIcT_ES5_PKc
> (%"struct.std::basic_ostream<char,std::char_traits<char>
>> "* @_ZSt4cout, i8* getelementptr ([10 x i8]* @.str, i32 0, i32 0))
> my  question is at line 59  can throw an exception?
>
Yes. The 'invoke' instruction indicates a call which can return either  
via the normal return sequence or by throwing an exception.
> my another problem :  are there any different of above two  llmv-ir
> on  handle c++ exception  ?
Different in what way? I'm not sure I understand your question. In  
neither case will an exception thrown from the << operator be caught.  
In the first case, the << operator is not in a try block, and in both  
cases, only an exception of integer type is caught, while the <<  
operator throws exceptions (when enabled) of type ios::exception.

Regards,
  Jim

Hi zhangzw,

...
>     59          %3 = invoke
> %"struct.std::basic_ostream<char,std::char_traits<char>
>"*
>
@_ZStlsISt11char_traitsIcEERSt13basic_ostreamIcT_ES5_PKc(%"struct.std::basic_ostream<char,std::char_traits<char>
> >"* @_ZSt4cout, i8* getelementptr ([10 x i8]* @.str, i32 0, i32 0))
>     60                          to label %bb6 unwind label %lpad10
...
> my  question is at line 59  can throw an exception?
I don't know if it can really throw an exception or not.  Presumably it
could run out of memory?  But it's not marked as not throwing any exceptions
("nounwind") so LLVM has to assume that it might.  If LLVM had access
to
the definition of @_ZStlsISt11char_traitsIcEERSt13basic_ostreamIcT_ES5_PKc
then it might be able to prove that it doesn't throw any exceptions, at
which
point the invoke would be turned into an ordinary call.  If you know that
this routine cannot throw exceptions, then it can be marked as nothrow in
some the C++ header somewhere (and you should send a patch to the libstdc++
people).  By the way, the reason an invoke is needed here is to ensure that
@__cxa_end_catch() will be called on the original exception before the new
exception is propagated out.

...
>     48          %0 = invoke
> %"struct.std::basic_ostream<char,std::char_traits<char>
>"*
>
@_ZStlsISt11char_traitsIcEERSt13basic_ostreamIcT_ES5_PKc(%"struct.std::basic_ostream<char,std::char_traits<char>
> >"* @_ZSt4cout, i8* getelementptr ([14 x i8]* @.str, i32 0, i32 0))
...
> here at line 48 can throw an exception?
Same basic answer.
> my another problem :  are there any different of above two  llmv-ir
> on  handle c++ exception  ?
They should result in the same output when the program is run, assuming
that the exception throwing routine doesn't run out of memory and abort,
in which case you might see a message for the second version but not for
the first.

Ciao,

Duncan.

hi,

     I'm   doing to make llvm backend support sjlj-eh!   I have try to
use the llvm-IR to generate the sjlj-eh code,
but I'm totally despair !
    my major problem is that llvm-ir , I mean exception instrinstics
are enough  for codegen the sjlj-eh code?   May I need to modify the
llvm-gcc ?
or  someone can give some advice about  llvm-backend 's support sjlj-eh ?


best regards
zhangzw