www.digitalmars.com         C & C++   DMDScript  

digitalmars.D - Dwarf Exception Handling question

reply Walter Bright <newshound2 digitalmars.com> writes:
I'm struggling to understand dwarf EH, and figure it's a good idea to try and 
make it binary compatible with what gdc does, or at least not gratuitously 
different. If I use gdc to compile this:

void foo1() {
         abc();
         try {
                 def();
         }
         catch(DD t) {
                 ghi(t);
         }
         catch(CC t) {
                 ghi(t);
         }
         catch {
                 mno();
         }
         jkl();
}

the code generated looks like this:

_D3eh54foo1FZv:
                 push    RBP
                 mov     RBP,RSP
                 sub     RSP,010h
                 call      abc
                 call      def
L12:            call      jkl
                 jmp short       L86
                 cmp     RDX,2
                 je      L59
                 cmp     RDX,3
                 je      L7F
                 cmp     RDX,1
                 je      L33
                 mov     RDI,RAX
                 call    _Unwind_Resume
L33:            sub     RAX,8
                 mov     RAX,[RAX]
                 mov     ESI,offset _D3eh52DD7__ClassZ
                 mov     RDI,RAX
                 call    _d_dynamic_cast
                 mov     -8[RBP],RAX
                 mov     RAX,-8[RBP]
                 mov     RDI,RAX
                 call      ghi
                 jmp short       L12
L59:            sub     RAX,8
                 mov     RAX,[RAX]
                 mov     ESI,offset _D3eh52CC7__ClassZ
                 mov     RDI,RAX
                 call    _d_dynamic_cast
                 mov     -010h[RBP],RAX
                 mov     RAX,-010h[RBP]
                 mov     RDI,RAX
                 call      ghi
                 jmp short       L12
L7F:            call      mno
                 jmp short       L12
L86:            leave
                 ret

The calls to _d_dynamic cast appear to be redundant - shouldn't the value in
RDX 
be sufficient? And why is there a 'default' call to _Unwind_Resume if RDX isn't 
an expected value? Shouldn't the personality routine simply not jump to the 
landing pad if none of the catch types are satisfied?
Nov 23 2015
next sibling parent reply Iain Buclaw via Digitalmars-d <digitalmars-d puremagic.com> writes:
On 23 November 2015 at 18:31, Walter Bright via Digitalmars-d <
digitalmars-d puremagic.com> wrote:

 I'm struggling to understand dwarf EH, and figure it's a good idea to try
 and make it binary compatible with what gdc does, or at least not
 gratuitously different. If I use gdc to compile this:

 void foo1() {
         abc();
         try {
                 def();
         }
         catch(DD t) {
                 ghi(t);
         }
         catch(CC t) {
                 ghi(t);
         }
         catch {
                 mno();
         }
         jkl();
 }

 the code generated looks like this:

 _D3eh54foo1FZv:
                 push    RBP
                 mov     RBP,RSP
                 sub     RSP,010h
                 call      abc
                 call      def
 L12:            call      jkl
                 jmp short       L86
                 cmp     RDX,2
                 je      L59
                 cmp     RDX,3
                 je      L7F
                 cmp     RDX,1
                 je      L33
                 mov     RDI,RAX
                 call    _Unwind_Resume
 L33:            sub     RAX,8
                 mov     RAX,[RAX]
                 mov     ESI,offset _D3eh52DD7__ClassZ
                 mov     RDI,RAX
                 call    _d_dynamic_cast
                 mov     -8[RBP],RAX
                 mov     RAX,-8[RBP]
                 mov     RDI,RAX
                 call      ghi
                 jmp short       L12
 L59:            sub     RAX,8
                 mov     RAX,[RAX]
                 mov     ESI,offset _D3eh52CC7__ClassZ
                 mov     RDI,RAX
                 call    _d_dynamic_cast
                 mov     -010h[RBP],RAX
                 mov     RAX,-010h[RBP]
                 mov     RDI,RAX
                 call      ghi
                 jmp short       L12
 L7F:            call      mno
                 jmp short       L12
 L86:            leave
                 ret

 The calls to _d_dynamic cast appear to be redundant - shouldn't the value
 in RDX be sufficient? And why is there a 'default' call to _Unwind_Resume
 if RDX isn't an expected value? Shouldn't the personality routine simply
 not jump to the landing pad if none of the catch types are satisfied?
Yes, the _d_dynamic_cast is redundant. This happened because the EH pointer was treated as an Object, then upcasted to the catch type via the normal convert() routines. This is what caused the unnecessary call to _d_dynamic_cast. This switch statement is generated by GCC itself, and it tries to be accommodating for all supported languages. I imagine that the default case is there to support `catch(...)` in C++ code, however D has no notion of this construct, so what is instead generated is _Unwind_Resume to tell unwind to keep looking up the call chain. In other words, the default case should never really happen in D code except in the event of a logic bug in the unwind EH personality function (or possibly corruption). If you feel it more appropriate, I don't see the harm in replacing it with whatever HLT or abort instruction you like. :-)
Nov 23 2015
parent reply Walter Bright <newshound2 digitalmars.com> writes:
On 11/23/2015 10:07 AM, Iain Buclaw via Digitalmars-d wrote:
 Yes, the _d_dynamic_cast is redundant.  This happened because the EH pointer
was
 treated as an Object, then upcasted to the catch type via the normal convert()
 routines.  This is what caused the unnecessary call to _d_dynamic_cast.

 This switch statement is generated by GCC itself, and it tries to be
 accommodating for all supported languages.  I imagine that the default case is
 there to support `catch(...)` in C++ code, however D has no notion of this
 construct, so what is instead generated is _Unwind_Resume to tell unwind to
keep
 looking up the call chain.

 In other words, the default case should never really happen in D code except in
 the event of a logic bug in the unwind EH personality function (or possibly
 corruption). If you feel it more appropriate, I don't see the harm in replacing
 it with whatever HLT or abort instruction you like. :-)
Thanks, this helps a lot, and makes me a lot more comfortable with the notion that I understand what is going on. I won't generate the cast, then, and I'll use a HLT for the default. BTW, are you working on supporting catching std::exception ? My eevil plan is to get D exceptions working completely before trying to support std::exception.
Nov 23 2015
next sibling parent reply Iain Buclaw via Digitalmars-d <digitalmars-d puremagic.com> writes:
On 23 November 2015 at 19:18, Walter Bright via Digitalmars-d <
digitalmars-d puremagic.com> wrote:

 On 11/23/2015 10:07 AM, Iain Buclaw via Digitalmars-d wrote:

 Yes, the _d_dynamic_cast is redundant.  This happened because the EH
 pointer was
 treated as an Object, then upcasted to the catch type via the normal
 convert()
 routines.  This is what caused the unnecessary call to _d_dynamic_cast.

 This switch statement is generated by GCC itself, and it tries to be
 accommodating for all supported languages.  I imagine that the default
 case is
 there to support `catch(...)` in C++ code, however D has no notion of this
 construct, so what is instead generated is _Unwind_Resume to tell unwind
 to keep
 looking up the call chain.

 In other words, the default case should never really happen in D code
 except in
 the event of a logic bug in the unwind EH personality function (or
 possibly
 corruption). If you feel it more appropriate, I don't see the harm in
 replacing
 it with whatever HLT or abort instruction you like. :-)
Thanks, this helps a lot, and makes me a lot more comfortable with the notion that I understand what is going on. I won't generate the cast, then, and I'll use a HLT for the default. BTW, are you working on supporting catching std::exception ? My eevil plan is to get D exceptions working completely before trying to support std::exception.
Actively? No. First comes 2.067, which will have to be updated *as is* without the visitor conversions. I will need to adjust *my* personality function to be more graceful for handling foreign exceptions. After that, I can look into C++ typeinfo mangling. It's just normal C++ mangling with a _ZT prefix, no? :-) Also, I notice that you are using an older version of GDC. That version you're using doesn't support exception chaining. To get that working I had to invent a callback __gdc_begin_catch() which cleans up the chained exceptions list.
Nov 23 2015
parent Walter Bright <newshound2 digitalmars.com> writes:
On 11/23/2015 10:31 AM, Iain Buclaw via Digitalmars-d wrote:
 I will need to adjust *my* personality function to be more graceful for
handling
 foreign exceptions.  After that, I can look into C++ typeinfo mangling.  It's
 just normal C++ mangling with a _ZT prefix, no? :-)
I think so, but I haven't looked.
 Also, I notice that you are using an older version of GDC.
It's the one I installed recently on Ubuntu with: sudo apt-get install gdc
 That version you're
 using doesn't support exception chaining.  To get that working I had to invent
a
 callback __gdc_begin_catch() which cleans up the chained exceptions list.
Could you please invent it as Boost licensed code, so I can simply incorporate it?
Nov 23 2015
prev sibling parent reply Jacob Carlborg <doob me.com> writes:
On 2015-11-23 19:18, Walter Bright wrote:

 My eevil plan is to get D exceptions working completely before trying to
 support std::exception.
Is the idea to replace the existing exception handling mechanism for D code that don't interact with C++ as well? -- /Jacob Carlborg
Nov 23 2015
parent deadalnix <deadalnix gmail.com> writes:
On Tuesday, 24 November 2015 at 07:52:13 UTC, Jacob Carlborg 
wrote:
 On 2015-11-23 19:18, Walter Bright wrote:

 My eevil plan is to get D exceptions working completely before 
 trying to
 support std::exception.
Is the idea to replace the existing exception handling mechanism for D code that don't interact with C++ as well?
Walter is moving DMD to use libunwind.
Nov 24 2015
prev sibling parent reply David Nadlinger <code klickverbot.at> writes:
On Monday, 23 November 2015 at 17:31:51 UTC, Walter Bright wrote:
 The calls to _d_dynamic cast appear to be redundant - shouldn't 
 the value in RDX be sufficient?
This is indeed the case, but of course you need to know in which order the compiler backend wrote the type info entries to the tables. Maybe at the time there didn't exist an intrinsic for that in GDC or something like that. This is how the function looks in LDC (on OS X, but the libunwind "client"-side code is very similar): --- 0000000100000a40 <__D5test24foo1FZv>: 100000a40: 53 push rbx 100000a41: e8 ca fe ff ff call 100000910 <__D4test3abcFZv> 100000a46: e8 d5 fe ff ff call 100000920 <__D4test3defFZv> 100000a4b: 5b pop rbx 100000a4c: e9 ef fe ff ff jmp 100000940 <__D4test3jklFZv> 100000a51: 48 89 c3 mov rbx,rax 100000a54: 83 fa 03 cmp edx,0x3 100000a57: 74 05 je 100000a5e <__D5test24foo1FZv+0x1e> 100000a59: 83 fa 02 cmp edx,0x2 100000a5c: 75 0f jne 100000a6d <__D5test24foo1FZv+0x2d> 100000a5e: e8 5d ff 00 00 call 1000109c0 <__d_eh_enter_catch> 100000a63: 48 8b 3b mov rdi,QWORD PTR [rbx] 100000a66: e8 c5 fe ff ff call 100000930 <__D4test3ghiFC4test2CCZv> 100000a6b: eb de jmp 100000a4b <__D5test24foo1FZv+0xb> 100000a6d: 83 fa 01 cmp edx,0x1 100000a70: 75 10 jne 100000a82 <__D5test24foo1FZv+0x42> 100000a72: e8 49 ff 00 00 call 1000109c0 <__d_eh_enter_catch> 100000a77: e8 d4 fe ff ff call 100000950 <__D4test3mnoFZv> 100000a7c: 5b pop rbx 100000a7d: e9 be fe ff ff jmp 100000940 <__D4test3jklFZv> 100000a82: 48 89 df mov rdi,rbx 100000a85: e8 16 ff 00 00 call 1000109a0 <__d_eh_resume_unwind> ---
 And why is there a 'default' call to _Unwind_Resume if RDX 
 isn't an expected value? Shouldn't the personality routine 
 simply not jump to the landing pad if none of the catch types 
 are satisfied?
The reason LDC emits the __d_eh_resume_unwind call all the time is because it is needed as soon as there is a cleanup involved anyway, and so far I was just too lazy to optimize the extra branch away in the special case that there are no cleanups. — David
Nov 23 2015
next sibling parent Iain Buclaw via Digitalmars-d <digitalmars-d puremagic.com> writes:
On 23 November 2015 at 19:32, David Nadlinger via Digitalmars-d <
digitalmars-d puremagic.com> wrote:

 On Monday, 23 November 2015 at 17:31:51 UTC, Walter Bright wrote:

 And why is there a 'default' call to _Unwind_Resume if RDX isn't an
 expected value? Shouldn't the personality routine simply not jump to the
 landing pad if none of the catch types are satisfied?
The reason LDC emits the __d_eh_resume_unwind call all the time is because it is needed as soon as there is a cleanup involved anyway, and so far I was just too lazy to optimize the extra branch away in the special case that there are no cleanups.
We seem to be doing very similar things here.
Nov 23 2015
prev sibling parent reply Walter Bright <newshound2 digitalmars.com> writes:
On 11/23/2015 10:32 AM, David Nadlinger wrote:
 This is how the function looks in LDC (on OS X, but the libunwind "client"-side
 code is very similar):

 ---
 0000000100000a40 <__D5test24foo1FZv>:
     100000a40:   53                      push   rbx
     100000a41:   e8 ca fe ff ff          call   100000910 <__D4test3abcFZv>
     100000a46:   e8 d5 fe ff ff          call   100000920 <__D4test3defFZv>
     100000a4b:   5b                      pop    rbx
     100000a4c:   e9 ef fe ff ff          jmp    100000940 <__D4test3jklFZv>
     100000a51:   48 89 c3                mov    rbx,rax
     100000a54:   83 fa 03                cmp    edx,0x3
     100000a57:   74 05                   je     100000a5e
<__D5test24foo1FZv+0x1e>
     100000a59:   83 fa 02                cmp    edx,0x2
     100000a5c:   75 0f                   jne    100000a6d
<__D5test24foo1FZv+0x2d>
     100000a5e:   e8 5d ff 00 00          call   1000109c0 <__d_eh_enter_catch>
     100000a63:   48 8b 3b                mov    rdi,QWORD PTR [rbx]
     100000a66:   e8 c5 fe ff ff          call   100000930
 <__D4test3ghiFC4test2CCZv>
     100000a6b:   eb de                   jmp    100000a4b
<__D5test24foo1FZv+0xb>
     100000a6d:   83 fa 01                cmp    edx,0x1
     100000a70:   75 10                   jne    100000a82
<__D5test24foo1FZv+0x42>
     100000a72:   e8 49 ff 00 00          call   1000109c0 <__d_eh_enter_catch>
     100000a77:   e8 d4 fe ff ff          call   100000950 <__D4test3mnoFZv>
     100000a7c:   5b                      pop    rbx
     100000a7d:   e9 be fe ff ff          jmp    100000940 <__D4test3jklFZv>
     100000a82:   48 89 df                mov    rdi,rbx
     100000a85:   e8 16 ff 00 00          call   1000109a0
<__d_eh_resume_unwind>
 ---
The code looks quite good. I've been trying to adjust things, however, so there are no pushes and pops in the code, trying to preallocate everything needed in the function prolog.
 And why is there a 'default' call to _Unwind_Resume if RDX isn't an expected
 value? Shouldn't the personality routine simply not jump to the landing pad if
 none of the catch types are satisfied?
The reason LDC emits the __d_eh_resume_unwind call all the time is because it is needed as soon as there is a cleanup involved anyway, and so far I was just too lazy to optimize the extra branch away in the special case that there are no cleanups.
dmd rewrites try-catch-finally into try-{try-catch}-finally, which makes it easier to generate code, because fewer special cases and fewer bugs. I've become a big fan of that technique :-)
Nov 23 2015
next sibling parent reply deadalnix <deadalnix gmail.com> writes:
On Monday, 23 November 2015 at 21:05:29 UTC, Walter Bright wrote:
 dmd rewrites try-catch-finally into try-{try-catch}-finally, 
 which makes it easier to generate code, because fewer special 
 cases and fewer bugs. I've become a big fan of that technique 
 :-)
Wouldn't that makes unwinding slower because one need to go through several landing pads ?
Nov 23 2015
parent reply Walter Bright <newshound2 digitalmars.com> writes:
On 11/23/2015 1:32 PM, deadalnix wrote:
 On Monday, 23 November 2015 at 21:05:29 UTC, Walter Bright wrote:
 dmd rewrites try-catch-finally into try-{try-catch}-finally, which makes it
 easier to generate code, because fewer special cases and fewer bugs. I've
 become a big fan of that technique :-)
Wouldn't that makes unwinding slower because one need to go through several landing pads ?
Yes. But if you're choked by unwinding speed, you're doing it wrong.
Nov 23 2015
parent deadalnix <deadalnix gmail.com> writes:
On Monday, 23 November 2015 at 21:38:39 UTC, Walter Bright wrote:
 On 11/23/2015 1:32 PM, deadalnix wrote:
 On Monday, 23 November 2015 at 21:05:29 UTC, Walter Bright 
 wrote:
 dmd rewrites try-catch-finally into try-{try-catch}-finally, 
 which makes it
 easier to generate code, because fewer special cases and 
 fewer bugs. I've
 become a big fan of that technique :-)
Wouldn't that makes unwinding slower because one need to go through several landing pads ?
Yes. But if you're choked by unwinding speed, you're doing it wrong.
I don't think this is a good reason for making it twice as slow :)
Nov 23 2015
prev sibling parent reply David Nadlinger <code klickverbot.at> writes:
On Monday, 23 November 2015 at 21:05:29 UTC, Walter Bright wrote:
 The code looks quite good. I've been trying to adjust things, 
 however, so there are no pushes and pops in the code, trying to 
 preallocate everything needed in the function prolog.
Wouldn't you still need to restore the stack before leaving the function (tail call in this case)? For a single register, push/pop is probably still cheaper than setting up RBP and having the extra mov/sub.
 dmd rewrites try-catch-finally into try-{try-catch}-finally, 
 which makes it easier to generate code, because fewer special 
 cases and fewer bugs. I've become a big fan of that technique
Except that we actually need to flatten all the nesting into a single landing pad anyway. How would you do this in DMD? I didn't realize you could even have multiple EH table entries attached to a single code location. — David
Nov 23 2015
parent Walter Bright <newshound2 digitalmars.com> writes:
On 11/23/2015 4:36 PM, David Nadlinger wrote:
 On Monday, 23 November 2015 at 21:05:29 UTC, Walter Bright wrote:
 The code looks quite good. I've been trying to adjust things, however, so
 there are no pushes and pops in the code, trying to preallocate everything
 needed in the function prolog.
Wouldn't you still need to restore the stack before leaving the function (tail call in this case)?
Yes.
 dmd rewrites try-catch-finally into try-{try-catch}-finally, which makes it
 easier to generate code, because fewer special cases and fewer bugs. I've
 become a big fan of that technique
Except that we actually need to flatten all the nesting into a single landing pad anyway.
I don't know why that would be true. gdc generates multiple landing pads.
 How would you do this in DMD? I didn't realize you could even have
 multiple EH table entries attached to a single code location.
You can't. The most deeply nested one gets the table entry.
Nov 23 2015