[cfe-dev] Memory allocation in current function stack using __builtin_alloca

[Rahul Jain]

Hi,

Please consider the following code:

#include<stdio.h>

int main()
{
  void *sp1 = __builtin_alloca(0);
  void *sp2 = __builtin_alloca(0);

  printf("sp1 points at %p\n", sp1);
  printf("sp2 points at %p\n", sp2);

}

gcc version- gcc version 4.7.3 (Ubuntu/Linaro 4.7.3-1ubuntu1)
On compiling it with gcc the output is:

sp1 points at 0xbffbe9d8
sp2 points at 0xbffbe9d8

clang version- clang version 3.4 (trunk 186087)
On compiling it with clang the output is:
sp1 points at 0xbf87d30f
sp2 points at 0xbf87d30e

Please if someone could explain this one byte difference in clang?
Does clang actually allocate memory even when  __builtin_alloca(0)?

Would be really helpful if someone could throw some light on how stack
allocation and alignment takes place in clang?

Thanks,
Rahul
A Clang developer (beginner)
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