[Bug target/44575] [4.5 Regression] __builtin_va_arg overwrites into adjacent stack location

[eraman at google dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=44575

--- Comment #7 from Easwaran Raman <eraman at google dot com> 2010-09-30 00:21:17 UTC ---
This is a variation of the same problem where __builtin_va_arg overwrites into
adjacent stack location [Not sure if I should reopen this bug or file a new
one]:

$ cat vararg.cc

#include <stdarg.h>
#include <stdlib.h>
struct S933 { struct{struct{}b[6];union{}c[7];}a;char d;char e; };

struct S933 arg;
void check933va (int z, ...) {
  char c;
  va_list ap;
  __builtin_va_start(ap,z);
  c = 'a';
  arg = __builtin_va_arg(ap,struct S933);
  if (c != 'a')
    abort();

}
int main() {
  struct S933 s933;
  check933va (1, s933);
}

$ ./trunk-g++  -O0  vararg.cc && ./a.out
Aborted

./trunk-g++ is GNU C++  version 4.6.0 20100924 (experimental)
(x86_64-unknown-linux-gnu)

The relevant portion of the gimple is below:
  D.2773_4 = ap.reg_save_area;
  D.2774_5 = ap.gp_offset;
  D.2775_6 = (long unsigned int) D.2774_5;
  int_addr.1_7 = D.2773_4 + D.2775_6;
  addr.0_8 = &va_arg_tmp.3;
  D.2777_9 = addr.0_8 + 8;
  D.2778_10 = MEM[(long unsigned int *)int_addr.1_7];
  *D.2777_9 = D.2778_10;    <--- Bad move

The move to address D.2777_9 is the problem

For this struct type, construct_container returns the following:

(parallel:BLK [
        (expr_list:REG_DEP_TRUE (reg:DI 0 ax)
            (const_int 8 [0x8]))
    ])

The destination of the move is at offset 8 (INTVAL (XEXP (slot, 1))) of the
temporary created. The size of the temp (sizeof(S933)) is 15 bytes and the move
is in DI mode. I think the problem is the check  if (prev_size + cur_size >
size) doesn't really check if the destination is overwritten.