llvm dev - Jun 2015 - [LLVMdev] No location info for artificial arguments: cannot step into class methods

Hi David,

I got a problem related to the change made in r188651 - not including 
file/line info for artificial arguments like "this" and
"self".
Would you be able to take a look at it, please?

I am compiling the code for ARM linux with fast ISel disabled: -mllvm 
-fast-isel=0. The problem is that it's not possible to step intothe 
constructor, destructor or a method of a class while debugging. Clang 
generates correct .loc in function prologues, but then,  due to r188651, 
inserts a debug info location with zeroed line number (.loc 1 0 0) for 
"this", which misleads the debugger.

Could you explain the reasoning behind this change, please?

This hasn't been noticed before due to the fast ISel omitting location 
info for arguments, so the correct location from the prologue is used. 
It is probably yet another problem to handle.

Thanks in advance for any help.
Kind regards,
Oleg
-------------- next part --------------
An HTML attachment was scrubbed...
URL:
<http://lists.llvm.org/pipermail/llvm-dev/attachments/20150625/710b0bae/attachment.html>

On Wed, Jun 24, 2015 at 2:28 PM, Oleg Ranevskyy <llvm.mail.list at
gmail.com>
wrote:
>  Hi David,
>
> I got a problem related to the change made in r188651 - not including
> file/line info for artificial arguments like "this" and
"self".
> Would you be able to take a look at it, please?
>
Could you provide a simple test case? (& ideally: does this reproduce on
X86, a platform I've easy access to (& if it doesn't, I'd be
curious to
understand why not...))

> I am compiling the code for ARM linux with fast ISel disabled: -mllvm
> -fast-isel=0. The problem is that it's not possible to step into the
> constructor, destructor or a method of a class while debugging. Clang
> generates correct .loc in function prologues, but then,  due to r188651,
> inserts a debug info location with zeroed line number (.loc 1 0 0) for
> "this", which misleads the debugger.
>
> Could you explain the reasoning behind this change, please?
>
The reason for the change, if I recall correctly (perhaps I mentioned in
the commit message, I'm not sure) was to reduce debug info size for the
debug_info section - I don't believe the intent was to have any impact on
the line table.

(artificial variables, like "this" don't have a location they are
declared
on, so there was/is no need to have a DW_AT_decl_file/line in the
debug_info describing their declaration location)

>
> This hasn't been noticed before due to the fast ISel omitting location
> info for arguments, so the correct location from the prologue is used. It
> is probably yet another problem to handle.
>
> Thanks in advance for any help.
> Kind regards,
> Oleg
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL:
<http://lists.llvm.org/pipermail/llvm-dev/attachments/20150624/790d1725/attachment.html>

> On Wed, Jun 24, 2015 at 2:28 PM, Oleg Ranevskyy 
> <llvm.mail.list at gmail.com <mailto:llvm.mail.list at
gmail.com>> wrote:
>
>     Hi David,
>
>     I got a problem related to the change made in r188651 - not
>     including file/line info for artificial arguments like "this"
and
>     "self".
>     Would you be able to take a look at it, please?
>
>
> Could you provide a simple test case? (& ideally: does this reproduce 
> on X86, a platform I've easy access to (& if it doesn't,
I'd be
> curious to understand why not...))
Sure. It's enough to have a simple class with a constructor and create 
its instance.
Compile it w/o fast ISel, e.g.: clang -g -O0 main.cpp 
--target=armv7l-unknown-linux-gnueabihf -mllvm -fast-isel=0

class MyClass
{
public:
     MyClass()
     {
     }
};
int main()
{
     MyClass obj;   // <-- can't step into MyClass::MyClass() here
     return 0;
}

It didn't reproduce on X86_64 though. I will try to find out
why.
>
>     I am compiling the code for ARM linux with fast ISel disabled:
>     -mllvm -fast-isel=0. The problem is that it's not possible to step
>     intothe constructor, destructor or a method of a class while
>     debugging. Clang generates correct .loc in function prologues, but
>     then,  due to r188651, inserts a debug info location with zeroed
>     line number (.loc 1 0 0) for "this", which misleads the
debugger.
>
>     Could you explain the reasoning behind this change, please?
>
>
> The reason for the change, if I recall correctly (perhaps I mentioned 
> in the commit message, I'm not sure) was to reduce debug info size for 
> the debug_info section - I don't believe the intent was to have any 
> impact on the line table.
>
> (artificial variables, like "this" don't have a location they
are
> declared on, so there was/is no need to have a DW_AT_decl_file/line in 
> the debug_info describing their declaration location)
I see, thanks.
The r188651 patch leads to inserting ".loc    1 0 0" into the 
constructor's code. Here is a part of the assembly I get for the MyClass 
constructor:

_ZN7MyClassC2Ev:                        @ @_ZN7MyClassC2Ev
.Lfunc_begin1:
     .loc    1 7 0                   @ main.cpp:7:0
     .fnstart
     .cfi_startproc
@ BB#0:                                 @ %entry
     .loc    1 0 0                   @ main.cpp:0:0 <-------------- loc 
with line == 0 confuses the debugger
     .pad    #4
     sub    sp, sp, #4
     ....

Fast ISel removes this ".loc    1 0 0", so the debugger successfully 
steps into the constructor due to the previous ".loc    1 7
0".
>
>
>     This hasn't been noticed before due to the fast ISel omitting
>     location info for arguments, so the correct location from the
>     prologue is used. It is probably yet another problem to handle.
>
>     Thanks in advance for any help.
>     Kind regards,
>     Oleg
>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL:
<http://lists.llvm.org/pipermail/llvm-dev/attachments/20150625/fa13a56b/attachment.html>