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

[David Blaikie]

On Thu, Dec 6, 2012 at 12:33 AM, Karthik Bhat <karthikthecool at gmail.com> wrote:
> 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.

I haven't looked at this carefully yet (& I'm not the authority on ABI
issues - not sure who pays most attention to this in the backend, John
McCall deals with it mostly in Clang proper but I've CC'd him here in
case things rings any bells for him) but a simple way you could
provide strong motivation for this change is if you can demonstrate
that this is also a correctness issue: If clang/llvm are really
incorrectly implementing the ABI this should cause interoperability
issues if clang is used to compile a caller and gcc a callee to the
same function (or the other way around).

If you can come up with a simple example that demonstrates that it
should be a fairly unquestionable change. (test cases along with the
change all in a patch file help too)

Thanks,
- David

>
> 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
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