RFA: Improve jump threading #2 of N

[Jeff Law]

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On 04/21/11 11:26, Steven Bosscher wrote:
>> Bootstrapped and regression tested on x86_64-unknown-linux-gnu.  OK for
>> trunk?
> 
> Would this also fix PR18046?
Not right now.

If we look at VRP2 (and this only affects VRP's jump threading) we have:

SSA form after inserting ASSERT_EXPRs
bar ()
{
  int prephitmp.5;
  int pretmp.4;
  int i.0;

  # BLOCK 2 freq:10000
  # PRED: ENTRY [100.0%]  (fallthru,exec)
  # VUSE <.MEM_5(D)>
  i.0_1 = i;
  switch (i.0_1) <default: <L2>, case 0: <L0>>
  # SUCC: 4 [71.0%]  (exec) 3 [29.0%]  (exec)

  # BLOCK 3 freq:2900
  # PRED: 2 [29.0%]  (exec)
<L0>:
  i.0_2 = ASSERT_EXPR <i.0_1, i.0_1 == 0>;
  # .MEM_6 = VDEF <.MEM_5(D)>
  foo ();
  # VUSE <.MEM_6>
  pretmp.4_8 = i;
  # SUCC: 4 [100.0%]  (fallthru,exec)

  # BLOCK 4 freq:10000
  # PRED: 2 [71.0%]  (exec) 3 [100.0%]  (fallthru,exec)
  # .MEM_3 = PHI <.MEM_5(D)(2), .MEM_6(3)>
  # prephitmp.5_9 = PHI <i.0_1(2), pretmp.4_8(3)>
<L2>:
  switch (prephitmp.5_9) <default: <L6>, case 0: <L4>>
  # SUCC: 6 [61.0%]  (exec) 5 [39.0%]  (exec)

  # BLOCK 5 freq:3898
  # PRED: 4 [39.0%]  (exec)
<L4>:
  # .MEM_7 = VDEF <.MEM_3>
  foo ();
  # SUCC: 6 [100.0%]  (fallthru,exec)

  # BLOCK 6 freq:10000
  # PRED: 4 [61.0%]  (exec) 5 [100.0%]  (fallthru,exec)
  # .MEM_4 = PHI <.MEM_3(4), .MEM_7(5)>
<L6>:
  # VUSE <.MEM_4>
  return;
  # SUCC: EXIT [100.0%]

}


Note the lack of range information for i.0_1 for the default case of the
first switch.  That's going to be a prerequisite for threading through
the second switch which its reached via the default path from the first
switch.

jeff
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