llvm dev - Oct 2012 - [LLVMdev] Wrong type qualifier for this pointer in case of ARM compiled binary

Hi All,
Was looking into a simple program -

class Simple
{
public:
    void fun() {}
};

int main()
{
 Simple s;
 s.fun();
 return 0;
}

When compiled using clang++ for ARM-

Expected result when we run -

print Simple::fun  in GDB is
void fun(Simple* const this)

as this should be a const pointer but in case of arm compiled binary we get
void fun(Simple* this).

Works fine when compiled using clang++ for x86.

Seems to be problem with qualifier type but was wondering how is this
target dependent?

Need input from community were can we look to fix this issue for ARM
compiled binary.

Thanks
Karthik

Hi Karthik,
> Expected result when we run -
>
> print Simple::fun  in GDB is
> void fun(Simple* const this)
>
> as this should be a const pointer but in case of arm compiled binary we get
> void fun(Simple* this).
I believe the actual type is coming from CXXMethodDecl::getThisType,
which quotes the standard as saying:

  // C++ 9.3.2p1: The type of this in a member function of a class X is X*.
  // If the member function is declared const, the type of this is const X*,
  // if the member function is declared volatile, the type of this is
  // volatile X*, and if the member function is declared const volatile,
  // the type of this is const volatile X*.

I.e. no const where you're expecting it.

I've checked the standards, and we're not making it up either: both
C++11 and C++03 say the same.
> Seems to be problem with qualifier type but was wondering how is this
> target dependent?
I think it's the gdb programs that are printing different things for
the same DWARF debug information. Dumping the information from the x86
binary gives:

<1><6b>: Abbrev Number: 6 (DW_TAG_class_type)
    <6c>   DW_AT_name        : (indirect string, offset: 0x3e): Simple
    <70>   DW_AT_byte_size   : 1
    <71>   DW_AT_decl_file   : 1
    <72>   DW_AT_decl_line   : 1
[...]
<3><7f>: Abbrev Number: 8 (DW_TAG_formal_parameter)
    <80>   DW_AT_type        : <0x86>
    <84>   DW_AT_artificial  : 1
    <84>   DW_AT_object_pointer: 1
<1><86>: Abbrev Number: 9 (DW_TAG_pointer_type)
    <87>   DW_AT_type        : <0x6b>

The "this" parameter has type "Simple *" as expected, but
also has the
attribute DW_AT_object_pointer. My guess is that the x86 gdb decides
to print it as "const" because of this, but the ARM one doesn't.

Hope this helps.

Tim.

Hi Tim,

Thanks. I just went through the function and yah as you said i looked into
the standard and it seems to be as per C++11. Was confused by the
documentation at http://msdn.microsoft.com/en-us/library/ytk2ae82.aspx which
mentioned this should be always const which may not be right as mentioned
at http://stackoverflow.com/questions/6067244/type-of-this-pointer .

Thanks for the input.

On Tue, Oct 2, 2012 at 12:34 PM, Tim Northover <t.p.northover at
gmail.com>wrote:
> Hi Karthik,
>
> > Expected result when we run -
> >
> > print Simple::fun  in GDB is
> > void fun(Simple* const this)
> >
> > as this should be a const pointer but in case of arm compiled binary
we
> get
> > void fun(Simple* this).
>
> I believe the actual type is coming from CXXMethodDecl::getThisType,
> which quotes the standard as saying:
>
>   // C++ 9.3.2p1: The type of this in a member function of a class X is X*.
>   // If the member function is declared const, the type of this is const
> X*,
>   // if the member function is declared volatile, the type of this is
>   // volatile X*, and if the member function is declared const volatile,
>   // the type of this is const volatile X*.
>
> I.e. no const where you're expecting it.
>
> I've checked the standards, and we're not making it up either: both
> C++11 and C++03 say the same.
>
> > Seems to be problem with qualifier type but was wondering how is this
> > target dependent?
>
> I think it's the gdb programs that are printing different things for
> the same DWARF debug information. Dumping the information from the x86
> binary gives:
>
> <1><6b>: Abbrev Number: 6 (DW_TAG_class_type)
>     <6c>   DW_AT_name        : (indirect string, offset: 0x3e):
Simple
>     <70>   DW_AT_byte_size   : 1
>     <71>   DW_AT_decl_file   : 1
>     <72>   DW_AT_decl_line   : 1
> [...]
> <3><7f>: Abbrev Number: 8 (DW_TAG_formal_parameter)
>     <80>   DW_AT_type        : <0x86>
>     <84>   DW_AT_artificial  : 1
>     <84>   DW_AT_object_pointer: 1
> <1><86>: Abbrev Number: 9 (DW_TAG_pointer_type)
>     <87>   DW_AT_type        : <0x6b>
>
> The "this" parameter has type "Simple *" as expected,
but also has the
> attribute DW_AT_object_pointer. My guess is that the x86 gdb decides
> to print it as "const" because of this, but the ARM one
doesn't.
>
> Hope this helps.
>
> Tim.
>
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