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From: "amker at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug tree-optimization/43174] Teaching SCEV about ADDR_EXPR causes regression
Date: Mon, 17 Aug 2015 09:25:00 -0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=43174

--- Comment #3 from amker at gcc dot gnu.org ---
Note for the three levels of loop example, GCC chooses one IV for both j and k
loops, thus generates pretty clean output on x86_64 with O2.  

For the simple example, now gcc can eliminate comparison iv with the address
candidate, and generates below codes:

  <bb 2>:
  ivtmp.18_14 = (unsigned long) &a;
  _10 = &a + 60516;
  _28 = (unsigned long) _10;
  goto <bb 7>;

  <bb 3>:

  <bb 4>:
  # s_18 = PHI <s_9(3), s_19(7)>
  # ivtmp.9_12 = PHI <ivtmp.9_1(3), ivtmp.9_4(7)>
  _22 = (void *) ivtmp.9_12;
  _8 = MEM[base: _22, offset: 0B];
  s_9 = _8 + s_18;
  ivtmp.9_1 = ivtmp.9_12 + 4;
  if (ivtmp.9_1 != _27)
    goto <bb 3>;
  else
    goto <bb 5>;

  <bb 5>:
  # s_17 = PHI <s_9(4)>
  ivtmp.18_15 = ivtmp.18_5 + 492;
  if (ivtmp.18_15 != _28)
    goto <bb 6>;
  else
    goto <bb 8>;

  <bb 6>:

  <bb 7>:
  # s_19 = PHI <s_17(6), 0(2)>
  # ivtmp.18_5 = PHI <ivtmp.18_15(6), ivtmp.18_14(2)>
  ivtmp.9_4 = ivtmp.18_5;
  _29 = ivtmp.18_5 + 492;
  _27 = _29;
  goto <bb 4>;

  <bb 8>:
  # s_16 = PHI <s_17(5)>
  return s_16;

With this, following gimple optimizers are able to CSE the opportunity of
"ivtmp.18_5 + 492".  As a result, optimal code is generated as in optimized
dump:


  <bb 2>:
  ivtmp.18_14 = (unsigned long) &a;
  _28 = (unsigned long) &MEM[(void *)&a + 60516B];
  goto <bb 5>;

  <bb 3>:
  # s_18 = PHI <s_9(3), s_19(5)>
  # ivtmp.9_12 = PHI <ivtmp.9_1(3), ivtmp.18_5(5)>
  _22 = (void *) ivtmp.9_12;
  _8 = MEM[base: _22, offset: 0B];
  s_9 = _8 + s_18;
  ivtmp.9_1 = ivtmp.9_12 + 4;
  if (ivtmp.9_1 != _29)
    goto <bb 3>;
  else
    goto <bb 4>;

  <bb 4>:
  if (_28 != _29)
    goto <bb 5>;
  else
    goto <bb 6>;

  <bb 5>:
  # s_19 = PHI <s_9(4), 0(2)>
  # ivtmp.18_5 = PHI <_29(4), ivtmp.18_14(2)>
  _29 = ivtmp.18_5 + 492;
  goto <bb 3>;

  <bb 6>:
  return s_9;

This in effect is the transformation you wanted in this PR, but I doubt if GCC
can do this if it can't eliminate the inner loop's comparison using ivtmp.9_1
at the first place.

On the other hand, for cases that we can use IV's final value, it maybe likely
for GCC to eliminate the comparison IV.