ssa problem

[Ulrich Drepper]

The current mainline gcc has a problem with the conversion from SSA on
x86.  When the code appended below is compiled with

	gcc -O3 -fssa -march=i686 -o u u.c

the return value is 4 and not 3.  The problem is that the increment
performed by the va_arg() use is moved before the memory access.
Here's the part of the dump after conversion to SSA:

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
(note 47 51 15 [bb 1] NOTE_INSN_BASIC_BLOCK)

(note 15 47 17 ("u.c") 9)

(insn 17 15 19 (set (reg:SI 29)
        (reg:SI 31)) 41 {*movsi_1} (nil)
    (nil))

(insn 19 17 21 (parallel[ 
            (set (reg:SI 32)
                (plus:SI (reg:SI 31)
                    (const_int 4 [0x4])))
            (clobber (reg:CC 17 flags))
        ] ) 189 {*addsi_1} (nil)
    (nil))

(insn 21 19 22 (set (reg/v:SI 28)
        (mem:SI (reg:SI 29) 3)) 41 {*movsi_1} (nil)
    (nil))

(note 22 21 24 NOTE_INSN_LOOP_CONT)
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Note the use of res:SI 29.  Now after converting back from SSA:

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
(note 47 29 15 [bb 1] NOTE_INSN_BASIC_BLOCK)

(note 15 47 17 ("u.c") 9)

(insn 17 15 19 (set (reg:SI 31)
        (reg:SI 31)) 41 {*movsi_1} (nil)
    (nil))

(insn 19 17 21 (parallel[ 
            (set (reg:SI 31)
                (plus:SI (reg:SI 31)
                    (const_int 4 [0x4])))
            (clobber (reg:CC 17 flags))
        ] ) 189 {*addsi_1} (nil)
    (nil))

(insn 21 19 22 (set (reg/v:SI 28)
        (mem:SI (reg:SI 31) 3)) 41 {*movsi_1} (nil)
    (nil))

(note 22 21 24 NOTE_INSN_LOOP_CONT)
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The move in insn 17 is converted into a nop and insn 21 uses now
reg:SI 31 even though this register is modified in insn 19.

-- 
---------------.      drepper at gnu.org  ,-.   1325 Chesapeake Terrace
Ulrich Drepper  \    ,-------------------'   \  Sunnyvale, CA 94089 USA
Red Hat          `--' drepper at redhat.com   `------------------------

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
#include <stdarg.h>

int
bar (int a, va_list ap)
{
  int b;

  do
    b = va_arg (ap, int);
  while (b > 10);

  return a + b;
}

int
foo (int a, ...)
{
  va_list ap;

  va_start (ap, a);
  return bar (a, ap);
}

int
main ()
{
  return foo (1, 2, 3);
}