[Bug tree-optimization/29333] New: Generation of MAX_EXPRs and MIN_EXPRs missed by phiopt

[Bug tree-optimization/29333] New: Generation of MAX_EXPRs and MIN_EXPRs missed by phiopt

[roberto dot costa at st dot com]

There are cases in which phiopt doesn't recognize MAX_EXPRs or MIN_EXPRs
patterns.
In particular, source codes that look very similar at first sight may induce
phiopt to behave differently.

Let's consider the following two functions:

-----------------------------
int minmax_correct(int a)
{
        if (a > 32767) a = 32767;
        else if (a < -32768) a = -32768;

        return a;
}

int minmax_wrong(int a)
{
        if (a > 32767) a = 32767;
        if (a < -32768) a = -32768;

        return a;
}
-----------------------------

MIN_EXPRs and MAX_EXPRs are generated for the first function, but not for the
second.

Here is the contents of trace file minmax.c.042t.phicprop1:

-----------------------------
;; Function minmax_correct (minmax_correct)

minmax_correct (a)
{
<bb 2>:
  if (a_2 > 32767) goto <L3>; else goto <L1>;

<L1>:;
  if (a_2 < -32768) goto <L2>; else goto <L3>;

<L2>:;

  # a_1 = PHI <32767(2), a_2(3), -32768(4)>;
<L3>:;
  <retval> = a_1;
  return <retval>;

}

;; Function minmax_wrong (minmax_wrong)

Removing basic block 6
minmax_wrong (a)
{
<bb 2>:
  if (a_3 > 32767) goto <L6>; else goto <L1>;

<L1>:;
  if (a_3 < -32768) goto <L3>; else goto <L6>;

  # a_9 = PHI <a_3(3), 32767(2)>;
<L6>:;

  # a_2 = PHI <a_9(4), -32768(3)>;
<L3>:;
  <retval> = a_2;
  return <retval>;

}
-----------------------------

And here is minmax.c.043t.phiopt1:

-----------------------------
;; Function minmax_correct (minmax_correct)

Removing basic block 4
Removing basic block 3
Merging blocks 2 and 5
minmax_correct (a)
{
<bb 2>:
  a_7 = MAX_EXPR <-32768, a_2>;
  a_8 = MIN_EXPR <a_7, 32767>;
  <retval> = a_8;
  return <retval>;

}

;; Function minmax_wrong (minmax_wrong)

minmax_wrong (a)
{
<bb 2>:
  if (a_3 > 32767) goto <L6>; else goto <L1>;

<L1>:;
  if (a_3 < -32768) goto <L3>; else goto <L6>;

  # a_9 = PHI <a_3(3), 32767(2)>;
<L6>:;

  # a_2 = PHI <a_9(4), -32768(3)>;
<L3>:;
  <retval> = a_2;
  return <retval>;

}
-----------------------------


-- 
           Summary: Generation of MAX_EXPRs and MIN_EXPRs missed by phiopt
           Product: gcc
           Version: 4.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: roberto dot costa at st dot com


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=29333

[Bug tree-optimization/29333] Generation of MAX_EXPRs and MIN_EXPRs missed by phiopt

[roberto dot costa at st dot com]

------- Comment #1 from roberto dot costa at st dot com  2006-10-03 16:05 -------
Created an attachment (id=12378)
 --> (http://gcc.gnu.org/bugzilla/attachment.cgi?id=12378&action=view)
Source code of the example


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=29333

[Bug tree-optimization/29333] Generation of MAX_EXPRs and MIN_EXPRs missed by phiopt

[pinskia at gcc dot gnu dot org]

-- 

pinskia at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Severity|normal                      |enhancement
           Keywords|                            |missed-optimization


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=29333

[Bug tree-optimization/29333] Jump threading getting in the way of PHI-OPT

[pinskia at gcc dot gnu dot org]

------- Comment #2 from pinskia at gcc dot gnu dot org  2006-10-03 16:11 -------
Actually this is a case where jump threading gets in the way of PHI-OPT. 
Really PHI-OPT should be put before VRP and DOM, I don't know why it was not.


-- 

pinskia at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
     Ever Confirmed|0                           |1
   Last reconfirmed|0000-00-00 00:00:00         |2006-10-03 16:11:22
               date|                            |
            Summary|Generation of MAX_EXPRs and |Jump threading getting in
                   |MIN_EXPRs missed by phiopt  |the way of PHI-OPT


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=29333

[Bug tree-optimization/29333] Jump threading getting in the way of PHI-OPT

[roberto dot costa at st dot com]

------- Comment #3 from roberto dot costa at st dot com  2006-10-05 08:15 -------
I tested what happens if the first PHI-OPT pass is moved right before the first
VRP pass in gcc/passes.c

It looks like PHI-OPT should be run both before and after VRP and DOM.
The example reported shows that, when PHI-OPT is run after VRP and DOM, it
misses some MAX_EXPR and MIN_EXPR generation.
gcc/testsuite/gcc.dg/tree-ssa/phi-opt-5.c shows that, when PHI-OPT anticipates
VRP and DOM, it misses some MAX_EXPR and MIN_EXPR generation as well.

Should an additional PHI-OPT pass be added before VRP and DOM?
(In this case, a few testsuite examples should also be changed in order to
consider what happens after PHI-OPT2 instead of PHI-OPT1)


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=29333