[LLVMdev] global alignment

[Baozeng]

Hello all,
I found that the alignment for stack value has no limitation, but for
global value, it has a limitation.

Here is an example:

#include <stdio.h>
#include <stdlib.h>

char x[4000] __attribute__((aligned(4096)));

int
main (int argc, char ** argv) {
  char y[4000] __attribute__((aligned(4096)));
  printf("x is %p\n", x);
  printf("y is %p\n", y);

  return 0;
}

After compiled with clang, the result is:
x is 0x804b000
y is 0xbf9d8000

They are both aligned to be 4096 as we expected.

Then we change the example as the following:

char x[4000] __attribute__((aligned(8192)));

int
main (int argc, char ** argv) {
  char y[4000] __attribute__((aligned(8192)));
  printf("x is %p\n", x);
  printf("y is %p\n", y);

  return 0;
}

The result is :
x is 0x804d000
y is 0xbffd2000

We can see that the stack value y is aligned to be 8192, but the global
value x is not!

My target OS is 32-bit Linux. Anyone can explain this? or is this a bug of
clang?




-- 
     Best Regards,
                                                                 Baozeng
Ding

OSTG,NFS,ISCAS
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