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

Karthik Bhat karthikthecool at gmail.com
Tue Dec 4 21:19:39 PST 2012

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.


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
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