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* [Bug python/29011] New: Python Value.dynamic_cast does not work as expected
@ 2022-03-30 23:55 evan_l00 at qq dot com
2022-03-31 15:38 ` [Bug python/29011] " ssbssa at sourceware dot org
` (5 more replies)
0 siblings, 6 replies; 7+ messages in thread
From: evan_l00 at qq dot com @ 2022-03-30 23:55 UTC (permalink / raw)
To: gdb-prs
https://sourceware.org/bugzilla/show_bug.cgi?id=29011
Bug ID: 29011
Summary: Python Value.dynamic_cast does not work as expected
Product: gdb
Version: 11.1
Status: UNCONFIRMED
Severity: normal
Priority: P2
Component: python
Assignee: unassigned at sourceware dot org
Reporter: evan_l00 at qq dot com
Target Milestone: ---
The document says that Value.dynamic_cast works like dynamic cast in C++, but
in the following experiment, it shows that the Value.cast works in C++, and
Value.dynamic_cast gives some value I don't understand.
I have asked on the stackoverflow in
https://stackoverflow.com/questions/71644182/gdb-python-api-dynamic-cast-not-work-as-expected,
but received a comment that says this might be a bug in the gdb. So I open this
ticket.
This is the sample code
class Base {
public:
virtual void p() {}
int i =3;
};
class Base2 {
public:
virtual void f() {}
int i2 =4;
};
class Derived: public Base, public Base2{
public:
void p() override{}
int t =4;
};
int main() {
auto* p = new Derived();
Base2* pb2 = p;
Base* pb = p;
return 0;
}
Run the code, and print the values of p, pb, and pb2
(gdb) p p
$1 = (Derived *) 0x613c20
(gdb) p pb //
$2 = (Base *) 0x613c20
(gdb) p pb2
$3 = (Base2 *) 0x613c30
Then use python script to print the value as follows, the Value.dynamic_cast
will give
(gdb) py print(ppb2.dynamic_cast(ppb2.dynamic_type))
0x400808 <vtable for Derived+16>
And the Value.cast gives the correct derived pointer,
(gdb) py print(ppb2.cast(ppb2.dynamic_type))
0x613c20
pp, ppb, and ppb2 are the corresponding values but in Python env, got from the
following code.
(gdb) py pp = gdb.parse_and_eval('p')
(gdb) py print(pp)
0x613c20
(gdb) py ppb2 = gdb.parse_and_eval('pb2')
(gdb) py print(ppb2)
0x613c30
(gdb) py print(ppb2.cast(ppb2.dynamic_type))
0x613c20
(gdb) py print(ppb2.dynamic_cast(ppb2.dynamic_type))
0x400808 <vtable for Derived+16>
Why Value.dynamic_cast will give 0x400808 <vtable for Derived+16> instead of
0x613c20 (the derived pointer)?
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* [Bug python/29011] Python Value.dynamic_cast does not work as expected
2022-03-30 23:55 [Bug python/29011] New: Python Value.dynamic_cast does not work as expected evan_l00 at qq dot com
@ 2022-03-31 15:38 ` ssbssa at sourceware dot org
2022-03-31 17:09 ` simark at simark dot ca
` (4 subsequent siblings)
5 siblings, 0 replies; 7+ messages in thread
From: ssbssa at sourceware dot org @ 2022-03-31 15:38 UTC (permalink / raw)
To: gdb-prs
https://sourceware.org/bugzilla/show_bug.cgi?id=29011
Hannes Domani <ssbssa at sourceware dot org> changed:
What |Removed |Added
----------------------------------------------------------------------------
CC| |ssbssa at sourceware dot org
--- Comment #1 from Hannes Domani <ssbssa at sourceware dot org> ---
You also get the same result without Python:
(gdb) p p
$1 = (Derived *) 0x5d20c0
(gdb) p pb
$2 = (Base *) 0x5d20c0
(gdb) p pb2
$3 = (Base2 *) 0x5d20d0
(gdb) p dynamic_cast<Derived*>(p)
$4 = (Derived *) 0x13f4e8800 <vtable for Derived+16>
(gdb) p dynamic_cast<Derived*>(pb)
$5 = (Derived *) 0x13f4e8800 <vtable for Derived+16>
(gdb) p dynamic_cast<Derived*>(pb2)
$6 = (Derived *) 0x13f4e8800 <vtable for Derived+16>
(gdb) p dynamic_cast<Base*>(p)
$7 = (Base *) 0x5d20c0
(gdb) p dynamic_cast<Base*>(pb)
$8 = (Base *) 0x13f4e8800 <vtable for Derived+16>
(gdb) p dynamic_cast<Base*>(pb2)
$9 = (Base *) 0x5d20c0
(gdb) p dynamic_cast<Base2*>(p)
$10 = (Base2 *) 0x5d20d0
(gdb) p dynamic_cast<Base2*>(pb)
$11 = (Base2 *) 0x5d20d0
(gdb) p dynamic_cast<Base2*>(pb2)
$12 = (Base2 *) 0x13f4e8800 <vtable for Derived+16>
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* [Bug python/29011] Python Value.dynamic_cast does not work as expected
2022-03-30 23:55 [Bug python/29011] New: Python Value.dynamic_cast does not work as expected evan_l00 at qq dot com
2022-03-31 15:38 ` [Bug python/29011] " ssbssa at sourceware dot org
@ 2022-03-31 17:09 ` simark at simark dot ca
2022-04-02 16:00 ` tromey at sourceware dot org
` (3 subsequent siblings)
5 siblings, 0 replies; 7+ messages in thread
From: simark at simark dot ca @ 2022-03-31 17:09 UTC (permalink / raw)
To: gdb-prs
https://sourceware.org/bugzilla/show_bug.cgi?id=29011
Simon Marchi <simark at simark dot ca> changed:
What |Removed |Added
----------------------------------------------------------------------------
CC| |simark at simark dot ca
--- Comment #2 from Simon Marchi <simark at simark dot ca> ---
Looks similar to https://sourceware.org/bugzilla/show_bug.cgi?id=28907
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* [Bug python/29011] Python Value.dynamic_cast does not work as expected
2022-03-30 23:55 [Bug python/29011] New: Python Value.dynamic_cast does not work as expected evan_l00 at qq dot com
2022-03-31 15:38 ` [Bug python/29011] " ssbssa at sourceware dot org
2022-03-31 17:09 ` simark at simark dot ca
@ 2022-04-02 16:00 ` tromey at sourceware dot org
2023-07-31 14:45 ` [Bug c++/29011] " tromey at sourceware dot org
` (2 subsequent siblings)
5 siblings, 0 replies; 7+ messages in thread
From: tromey at sourceware dot org @ 2022-04-02 16:00 UTC (permalink / raw)
To: gdb-prs
https://sourceware.org/bugzilla/show_bug.cgi?id=29011
Tom Tromey <tromey at sourceware dot org> changed:
What |Removed |Added
----------------------------------------------------------------------------
CC| |tromey at sourceware dot org
--- Comment #3 from Tom Tromey <tromey at sourceware dot org> ---
(In reply to Simon Marchi from comment #2)
> Looks similar to https://sourceware.org/bugzilla/show_bug.cgi?id=28907
That one is about casts without RTTI, which is done differently.
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* [Bug c++/29011] Python Value.dynamic_cast does not work as expected
2022-03-30 23:55 [Bug python/29011] New: Python Value.dynamic_cast does not work as expected evan_l00 at qq dot com
` (2 preceding siblings ...)
2022-04-02 16:00 ` tromey at sourceware dot org
@ 2023-07-31 14:45 ` tromey at sourceware dot org
2023-12-11 14:53 ` cvs-commit at gcc dot gnu.org
2023-12-11 14:55 ` ssbssa at sourceware dot org
5 siblings, 0 replies; 7+ messages in thread
From: tromey at sourceware dot org @ 2023-07-31 14:45 UTC (permalink / raw)
To: gdb-prs
https://sourceware.org/bugzilla/show_bug.cgi?id=29011
Tom Tromey <tromey at sourceware dot org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Component|python |c++
--- Comment #4 from Tom Tromey <tromey at sourceware dot org> ---
Recategorizing since it happens without Python
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* [Bug c++/29011] Python Value.dynamic_cast does not work as expected
2022-03-30 23:55 [Bug python/29011] New: Python Value.dynamic_cast does not work as expected evan_l00 at qq dot com
` (3 preceding siblings ...)
2023-07-31 14:45 ` [Bug c++/29011] " tromey at sourceware dot org
@ 2023-12-11 14:53 ` cvs-commit at gcc dot gnu.org
2023-12-11 14:55 ` ssbssa at sourceware dot org
5 siblings, 0 replies; 7+ messages in thread
From: cvs-commit at gcc dot gnu.org @ 2023-12-11 14:53 UTC (permalink / raw)
To: gdb-prs
https://sourceware.org/bugzilla/show_bug.cgi?id=29011
--- Comment #5 from Sourceware Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Hannes Domani <ssbssa@sourceware.org>:
https://sourceware.org/git/gitweb.cgi?p=binutils-gdb.git;h=0238b543f3c294fc8512021a40b708e8ddc72bb2
commit 0238b543f3c294fc8512021a40b708e8ddc72bb2
Author: Hannes Domani <ssbssa@yahoo.de>
Date: Tue Mar 29 20:05:06 2022 +0200
Fix dynamic_cast
PR29011 notes that dynamic_cast does not work correctly if
classes with virtual methods are involved, some of the results
wrongly point into the vtable of the derived class:
```
(gdb) p vlr
$1 = (VirtualLeftRight *) 0x162240
(gdb) p vl
$2 = (VirtualLeft *) 0x162240
(gdb) p vr
$3 = (VirtualRight *) 0x162250
(gdb) p dynamic_cast<VirtualLeftRight*>(vlr)
$4 = (VirtualLeftRight *) 0x13fab89b0 <vtable for VirtualLeftRight+16>
(gdb) p dynamic_cast<VirtualLeftRight*>(vl)
$5 = (VirtualLeftRight *) 0x13fab89b0 <vtable for VirtualLeftRight+16>
(gdb) p dynamic_cast<VirtualLeftRight*>(vr)
$6 = (VirtualLeftRight *) 0x13fab89b0 <vtable for VirtualLeftRight+16>
(gdb) p dynamic_cast<VirtualLeft*>(vlr)
$7 = (VirtualLeft *) 0x162240
(gdb) p dynamic_cast<VirtualLeft*>(vl)
$8 = (VirtualLeft *) 0x13fab89b0 <vtable for VirtualLeftRight+16>
(gdb) p dynamic_cast<VirtualLeft*>(vr)
$9 = (VirtualLeft *) 0x162240
(gdb) p dynamic_cast<VirtualRight*>(vlr)
$10 = (VirtualRight *) 0x162250
(gdb) p dynamic_cast<VirtualRight*>(vl)
$11 = (VirtualRight *) 0x162250
(gdb) p dynamic_cast<VirtualRight*>(vr)
$12 = (VirtualRight *) 0x13fab89b0 <vtable for VirtualLeftRight+16>
```
For the cases where the dynamic_cast type is the same as the
original type, it used the ARG value for the result, which in
case of pointer types was already the dereferenced value.
And the TEM value at the value address was created with the
pointer/reference type, not the actual class type.
With these fixed, the dynamic_cast results make more sense:
```
(gdb) p vlr
$1 = (VirtualLeftRight *) 0x692240
(gdb) p vl
$2 = (VirtualLeft *) 0x692240
(gdb) p vr
$3 = (VirtualRight *) 0x692250
(gdb) p dynamic_cast<VirtualLeftRight*>(vlr)
$4 = (VirtualLeftRight *) 0x692240
(gdb) p dynamic_cast<VirtualLeftRight*>(vl)
$5 = (VirtualLeftRight *) 0x692240
(gdb) p dynamic_cast<VirtualLeftRight*>(vr)
$6 = (VirtualLeftRight *) 0x692240
(gdb) p dynamic_cast<VirtualLeft*>(vlr)
$7 = (VirtualLeft *) 0x692240
(gdb) p dynamic_cast<VirtualLeft*>(vl)
$8 = (VirtualLeft *) 0x692240
(gdb) p dynamic_cast<VirtualLeft*>(vr)
$9 = (VirtualLeft *) 0x692240
(gdb) p dynamic_cast<VirtualRight*>(vlr)
$10 = (VirtualRight *) 0x692250
(gdb) p dynamic_cast<VirtualRight*>(vl)
$11 = (VirtualRight *) 0x692250
(gdb) p dynamic_cast<VirtualRight*>(vr)
$12 = (VirtualRight *) 0x692250
```
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=29011
Approved-By: Tom Tromey <tom@tromey.com>
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* [Bug c++/29011] Python Value.dynamic_cast does not work as expected
2022-03-30 23:55 [Bug python/29011] New: Python Value.dynamic_cast does not work as expected evan_l00 at qq dot com
` (4 preceding siblings ...)
2023-12-11 14:53 ` cvs-commit at gcc dot gnu.org
@ 2023-12-11 14:55 ` ssbssa at sourceware dot org
5 siblings, 0 replies; 7+ messages in thread
From: ssbssa at sourceware dot org @ 2023-12-11 14:55 UTC (permalink / raw)
To: gdb-prs
https://sourceware.org/bugzilla/show_bug.cgi?id=29011
Hannes Domani <ssbssa at sourceware dot org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Status|UNCONFIRMED |RESOLVED
Target Milestone|--- |15.1
Resolution|--- |FIXED
--- Comment #6 from Hannes Domani <ssbssa at sourceware dot org> ---
Fixed.
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2022-03-30 23:55 [Bug python/29011] New: Python Value.dynamic_cast does not work as expected evan_l00 at qq dot com
2022-03-31 15:38 ` [Bug python/29011] " ssbssa at sourceware dot org
2022-03-31 17:09 ` simark at simark dot ca
2022-04-02 16:00 ` tromey at sourceware dot org
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