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Subject: Re: [PATCH] Speed-up use-after-scope (re-writing to SSA) (version 2)
To: Jakub Jelinek <jakub@redhat.com>
References: <20161116130733.GT3541@tucnak.redhat.com> <469bf86a-e43c-c571-66e4-87db78b6fb11@suse.cz> <20161116162841.GX3541@tucnak.redhat.com> <c02f7364-44ed-406f-cee4-7b1fc8899f75@suse.cz> <20161221085200.GS21933@tucnak> <4ec48432-9df6-154a-1b13-065b9772cbbf@suse.cz> <20161222172140.GF21933@tucnak> <29d32a7c-a95d-ddb1-d64e-ae8f659d3a4b@suse.cz> <20170116142025.GO1867@tucnak> <7e7f795d-a7a7-584e-8c77-61ea01207c40@suse.cz> <20170117164721.GE1867@tucnak>
Cc: Richard Biener <richard.guenther@gmail.com>, GCC Patches <gcc-patches@gcc.gnu.org>
From: =?UTF-8?Q?Martin_Li=c5=a1ka?= <mliska@suse.cz>
Message-ID: <c371b68c-6d86-abe2-8eec-1438376b4c10@suse.cz>
Date: Wed, 18 Jan 2017 15:37:00 -0000
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Hello.

During bootstrap, I came to following test-case:

struct A
{
  int regno;
};
struct
{
  A base;
} typedef *df_ref;
int *a;
void
fn1 (int N)
{
  for (int i = 0; i < N; i++)
    {
      df_ref b;
      a[(b)->base.regno]++;
    }
}

As we expand all usages of an LHS of a ASAN_POISON to all uses, we propagate that to
a PHI node that originally contained ASAN_MARK (UNPOISON):

  <bb 4> [0.00%]:
  ASAN_MARK (UNPOISON, &b, 8);
  a.0_1 = a;
  b.1_2 = b;
  _3 = b.1_2->base.regno;
  _4 = (long unsigned int) _3;
  _5 = _4 * 4;
  _6 = a.0_1 + _5;
  _7 = *_6;
  _8 = _7 + 1;
  *_6 = _8;
  ASAN_MARK (POISON, &b, 8);
  i_17 = i_9 + 1;
  goto <bb 3>; [0.00%]

Is transformed to:

  <bb 3> [0.00%]:
  # i_9 = PHI <0(2), i_17(4)>
  # b_18 = PHI <b_19(D)(2), b_20(4)>
  if (i_9 >= N_13(D))
    goto <bb 5>; [0.00%]
  else
    goto <bb 4>; [0.00%]

  <bb 4> [0.00%]:
  a.0_1 = a;
  b.1_2 = b_18;
  _3 = b.1_2->base.regno;
  _4 = (long unsigned int) _3;
  _5 = _4 * 4;
  _6 = a.0_1 + _5;
  _7 = *_6;
  _8 = _7 + 1;
  *_6 = _8;
  b_20 = ASAN_POISON ();
  i_17 = i_9 + 1;
  goto <bb 3>; [0.00%]

Motivation for propagation over PHI nodes was:

cat use.c
int
main (int argc, char **argv)
{
  int *ptr = 0;

  if (argc == 1)
    {
      int my_char;
      ptr = &my_char;
    }

  if (ptr)
    return *ptr;

  return 0;
}

I'm thinking whether the selected approach is fundamentally wrong, our we'll have to stop the PHI propagation
and we still be able to catch some cases with -O2?

Thanks,
Martin