llvm dev - Dec 2012 - [LLVMdev] Value of structure passed byval to a recurse function not initialized when accessed through GDB

Hi All,

I was debugging a clang binary when i found this problem. The
following code is complied with clang.

typedef struct s
{
  short s;
} SVAL;	


void recurse (SVAL a, int depth)
{
  a.s = --depth;
  if (depth == 0)
    return;
  else
   recurse(a,depth);
}

int main ()
{
  SVAL s; s.s = 5;
  recurse (s, 5);
  return 0;
}

When i try to access value of a.s in function recurse through gdb(i.e
gdb > p a.s) it gives me an uninitialized value.
The problem occurs only when we have a function call within function
to which we have passed a structure.

Could someone guide me were can i look to fix this issue.

I have started with LowerFormalArguments in X86ISelLowering.cpp file.

Thanks
Karthik

Karthik,
    At what point within recurse() are you asking gdb to display the value of
argument a?
    What I'm wondering about is if the debug information gdb is using to get
at a might not be correct at the particular point you are checking a,
particularly if that is before the prolog has completed execution.
    The way debug information for arguments pushed on the stack is represented
in DWARF requires some effort to get right at every instruction boundary, and
the fact that what's being passed is a structure, but one that is less than
an integer in size, may be exposing a bug in DWARF generation, not code
generation.
    Just a thought.
Richard

On Dec 4, 2012, at 5:34 AM, Karthik Bhat <karthikthecool at gmail.com>
wrote:
> Hi All,
> 
> I was debugging a clang binary when i found this problem. The
> following code is complied with clang.
> 
> typedef struct s
> {
>  short s;
> } SVAL;	
> 
> 
> void recurse (SVAL a, int depth)
> {
>  a.s = --depth;
>  if (depth == 0)
>    return;
>  else
>   recurse(a,depth);
> }
> 
> int main ()
> {
>  SVAL s; s.s = 5;
>  recurse (s, 5);
>  return 0;
> }
> 
> When i try to access value of a.s in function recurse through gdb(i.e
> gdb > p a.s) it gives me an uninitialized value.
> The problem occurs only when we have a function call within function
> to which we have passed a structure.
> 
> Could someone guide me were can i look to fix this issue.
> 
> I have started with LowerFormalArguments in X86ISelLowering.cpp file.
> 
> Thanks
> Karthik
> _______________________________________________
> LLVM Developers mailing list
> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>

This seems to be another case of PR13303 - since GDB can't figure out
where to break for this function based on the debug info (you'll
notice when you "break recurse" that it's not breaking on a line
or
source file, just an address) it's breaking at the very start, before
the prologue

I'm about to commit a fix to this.

On Tue, Dec 4, 2012 at 5:34 AM, Karthik Bhat <karthikthecool at gmail.com>
wrote:
> Hi All,
>
> I was debugging a clang binary when i found this problem. The
> following code is complied with clang.
>
> typedef struct s
> {
>   short s;
> } SVAL;
>
>
> void recurse (SVAL a, int depth)
> {
>   a.s = --depth;
>   if (depth == 0)
>     return;
>   else
>    recurse(a,depth);
> }
>
> int main ()
> {
>   SVAL s; s.s = 5;
>   recurse (s, 5);
>   return 0;
> }
>
> When i try to access value of a.s in function recurse through gdb(i.e
> gdb > p a.s) it gives me an uninitialized value.
> The problem occurs only when we have a function call within function
> to which we have passed a structure.
>
> Could someone guide me were can i look to fix this issue.
>
> I have started with LowerFormalArguments in X86ISelLowering.cpp file.
>
> Thanks
> Karthik
> _______________________________________________
> LLVM Developers mailing list
> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev

Hi Relph,
I'm trying to print the value of 'a' while executing  a.s = --depth;
I
have used break line number instead of break function so that the
initial prologue part gets executed.
The problem seems to be happening when parameters are pushed into
stack and we call a function recursively.

For example in the code when we have a int s; inside the struct
instead of short s; gdb is able to print the structure value properly.
The difference is when we have a short inside struct we are treating
it as byval parameter object inside
X86TargetLowering::LowerMemArgument were as when we have an "int" it
doesn't treat it as byval.

Thanks



On Tue, Dec 4, 2012 at 9:26 PM, Relph, Richard <Richard.Relph at amd.com>
wrote:
> Karthik,
>     At what point within recurse() are you asking gdb to display the value
of argument a?
>     What I'm wondering about is if the debug information gdb is using
to get at a might not be correct at the particular point you are checking a,
particularly if that is before the prolog has completed execution.
>     The way debug information for arguments pushed on the stack is
represented in DWARF requires some effort to get right at every instruction
boundary, and the fact that what's being passed is a structure, but one that
is less than an integer in size, may be exposing a bug in DWARF generation, not
code generation.
>     Just a thought.
> Richard
>
> On Dec 4, 2012, at 5:34 AM, Karthik Bhat <karthikthecool at
gmail.com> wrote:
>
>> Hi All,
>>
>> I was debugging a clang binary when i found this problem. The
>> following code is complied with clang.
>>
>> typedef struct s
>> {
>>  short s;
>> } SVAL;
>>
>>
>> void recurse (SVAL a, int depth)
>> {
>>  a.s = --depth;
>>  if (depth == 0)
>>    return;
>>  else
>>   recurse(a,depth);
>> }
>>
>> int main ()
>> {
>>  SVAL s; s.s = 5;
>>  recurse (s, 5);
>>  return 0;
>> }
>>
>> When i try to access value of a.s in function recurse through gdb(i.e
>> gdb > p a.s) it gives me an uninitialized value.
>> The problem occurs only when we have a function call within function
>> to which we have passed a structure.
>>
>> Could someone guide me were can i look to fix this issue.
>>
>> I have started with LowerFormalArguments in X86ISelLowering.cpp file.
>>
>> Thanks
>> Karthik
>> _______________________________________________
>> LLVM Developers mailing list
>> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
>> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>>
>
>

Hi David,

I think it might not be exactly PR13303 which might be causing the
corruption of struct when accessed through GDB.
This seems to be an ABI problem in clang.
The problem seems to be that when we have pass by value of struct
(having indirect arguments) stack is not aligned properly.

I tried realigning the stack for indirect arguments in(TargetInfo.cpp) -

ABIArgInfo X86_32ABIInfo::getIndirectResult(QualType Ty, bool ByVal)

.....
 if (StackAlign == 0)
        return ABIArgInfo::getIndirect(4, /*ByVal=*/true,
                                   /*Realign=*/true);      // Do a
realign of stack.

...


This seems to have fixed the issue. Also in case we have a large
structure - e.g. -

typedef struct s
{
  long s;
  long i;
  long l;
  long s1;
  long i1;
  long l1;
} SVAL;	

in the above mentioned code the same issue(corruption of member
variables when accessed through GDB) was observed which has got fixed
after this change.

Need input if this change is correct.

Thanks

On Wed, Dec 5, 2012 at 1:45 AM, David Blaikie <dblaikie at gmail.com>
wrote:
> This seems to be another case of PR13303 - since GDB can't figure out
> where to break for this function based on the debug info (you'll
> notice when you "break recurse" that it's not breaking on a
line or
> source file, just an address) it's breaking at the very start, before
> the prologue
>
> I'm about to commit a fix to this.
>
> On Tue, Dec 4, 2012 at 5:34 AM, Karthik Bhat <karthikthecool at
gmail.com> wrote:
>> Hi All,
>>
>> I was debugging a clang binary when i found this problem. The
>> following code is complied with clang.
>>
>> typedef struct s
>> {
>>   short s;
>> } SVAL;
>>
>>
>> void recurse (SVAL a, int depth)
>> {
>>   a.s = --depth;
>>   if (depth == 0)
>>     return;
>>   else
>>    recurse(a,depth);
>> }
>>
>> int main ()
>> {
>>   SVAL s; s.s = 5;
>>   recurse (s, 5);
>>   return 0;
>> }
>>
>> When i try to access value of a.s in function recurse through gdb(i.e
>> gdb > p a.s) it gives me an uninitialized value.
>> The problem occurs only when we have a function call within function
>> to which we have passed a structure.
>>
>> Could someone guide me were can i look to fix this issue.
>>
>> I have started with LowerFormalArguments in X86ISelLowering.cpp file.
>>
>> Thanks
>> Karthik
>> _______________________________________________
>> LLVM Developers mailing list
>> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
>> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev