[LLVMdev] No location info for artificial arguments: cannot step into class methods

[Oleg Ranevskyy]

> 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
>
>

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