[LLVMdev] Value of structure passed byval to a recurse function not initialized when accessed through GDB

Karthik Bhat karthikthecool at gmail.com
Thu Dec 6 00:33:39 PST 2012

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.


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
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