Re: [PATCH] PR59170 make pretty printers check for singular iterators

Re: [PATCH] PR59170 make pretty printers check for singular iterators

[Jan Kratochvil]

On Fri, 16 Dec 2016 13:33:52 +0100, Jonathan Wakely wrote:
> We don't do auto-deref for std::shared_ptr or std::unique_ptr, even
> though we know the object they point to definitely is live and safe to
> access, and that's because those types have pointer semantics not
> reference semantics.

This is wrong std::shared_ptr or std::unique_ptr is not auto-dereferenced for
GDB printing.  But it may be more a GDB problem, not libstdc++ pretty printers
problem.

For example glib pretty printers already auto-dereference data structures:
	5  GList* list = NULL;
	(gdb) p/r list
	$1 = (GList *) 0x607a00
	(gdb) p list
	$2 = 0x607a00 = {0x400810}
	/usr/share/glib-2.0/gdb/glib.py
	    if type.code == gdb.TYPE_CODE_PTR:
		type = type.target().unqualified()
		t = str(type)
		if t == "GList":
		    return GListPrinter(val, "GList")

But that is more a GDB bug that should be solved even for generic pointers.
Currently while traversing through data structures one has to randomly add or
remove '*' from the beginning of the GDB print expression:

	1	class E { int a[1000]; int i=42; } ee;
	2	class D { E *e=ⅇ } dd;
	3	class C { D &d=dd; } cc;
	4	class B { C *c=&cc; } bb;
	5	int main() {}
	(gdb) p bb
	$1 = {c = 0x601030 <cc>}
	(gdb) p bb.c
	$2 = (C *) 0x601030 <cc>
Oops, I need to add a dereference:
	(gdb) p *bb.c
	$3 = {d = @0x601028}
	(gdb) p *bb.c.d
	No symbol "operator*" in current context.
Oops, I need to remove a dereference:
	(gdb) p bb.c.d
	$4 = (D &) @0x601028: {e = 0x601060 <ee>}
	(gdb) p bb.c.d.e
	$5 = (E *) 0x601060 <ee>
Oops, I need to add a dereference:
	(gdb) p *bb.c.d.e
	$6 = {a = {0 <repeats 1000 times>}, i = 42}
	(gdb) p *bb.c.d.e.i
	Cannot access memory at address 0x2a
Oops, I need to remove a dereference:
	(gdb) p bb.c.d.e.i
	$7 = 42

This is probably solved in some clickable front-end interfaces.


Jan

Re: [PATCH] PR59170 make pretty printers check for singular iterators

[Jonathan Wakely]

On 16/12/16 14:06 +0100, Jan Kratochvil wrote:
>On Fri, 16 Dec 2016 13:33:52 +0100, Jonathan Wakely wrote:
>> We don't do auto-deref for std::shared_ptr or std::unique_ptr, even
>> though we know the object they point to definitely is live and safe to
>> access, and that's because those types have pointer semantics not
>> reference semantics.
>
>This is wrong std::shared_ptr or std::unique_ptr is not auto-dereferenced for
>GDB printing.  But it may be more a GDB problem, not libstdc++ pretty printers
>problem.
>
>For example glib pretty printers already auto-dereference data structures:
>	5  GList* list = NULL;
>	(gdb) p/r list
>	$1 = (GList *) 0x607a00
>	(gdb) p list
>	$2 = 0x607a00 = {0x400810}
>	/usr/share/glib-2.0/gdb/glib.py
>	    if type.code == gdb.TYPE_CODE_PTR:
>		type = type.target().unqualified()
>		t = str(type)
>		if t == "GList":
>		    return GListPrinter(val, "GList")
>
>But that is more a GDB bug that should be solved even for generic pointers.
>Currently while traversing through data structures one has to randomly add or
>remove '*' from the beginning of the GDB print expression:
>
>	1	class E { int a[1000]; int i=42; } ee;
>	2	class D { E *e=ⅇ } dd;
>	3	class C { D &d=dd; } cc;
>	4	class B { C *c=&cc; } bb;
>	5	int main() {}
>	(gdb) p bb
>	$1 = {c = 0x601030 <cc>}
>	(gdb) p bb.c
>	$2 = (C *) 0x601030 <cc>
>Oops, I need to add a dereference:
>	(gdb) p *bb.c
>	$3 = {d = @0x601028}
>	(gdb) p *bb.c.d
>	No symbol "operator*" in current context.
>Oops, I need to remove a dereference:
>	(gdb) p bb.c.d
>	$4 = (D &) @0x601028: {e = 0x601060 <ee>}

Wat?

bb.c.d is not a valid expression.

B::c is a pointer, it should be bb.c->d

Is it GDB policy to make invalid expressions like that "work"?

Because I'm not comfortable emulating that in the libstdc++ printers.


>	(gdb) p bb.c.d.e
>	$5 = (E *) 0x601060 <ee>
>Oops, I need to add a dereference:
>	(gdb) p *bb.c.d.e
>	$6 = {a = {0 <repeats 1000 times>}, i = 42}
>	(gdb) p *bb.c.d.e.i
>	Cannot access memory at address 0x2a
>Oops, I need to remove a dereference:
>	(gdb) p bb.c.d.e.i
>	$7 = 42

I had no idea this even worked, I'd have used bb.c->d.e->i because
that's the correct expression for accessing that variable.

Re: [PATCH] PR59170 make pretty printers check for singular iterators

[Jan Kratochvil]

On Fri, 16 Dec 2016 14:17:32 +0100, Jonathan Wakely wrote:
> On 16/12/16 14:06 +0100, Jan Kratochvil wrote:
> > 	(gdb) p bb.c.d
> > 	$4 = (D &) @0x601028: {e = 0x601060 <ee>}
> 
> Wat?
> 
> bb.c.d is not a valid expression.
> 
> B::c is a pointer, it should be bb.c->d
> 
> Is it GDB policy to make invalid expressions like that "work"?

Yes, this is a GDB extension from the times of plain C.  In C++ it became
a problem.  That was not the topic of this example for the initial
dereferencing operator.


> I had no idea this even worked, I'd have used bb.c->d.e->i because
> that's the correct expression for accessing that variable.

There was a plan to make the 'compile' project backward compatible with these
confusing GDB universal dot operators.


Jan