[Bug tree-optimization/27809] New: inefficient gimplification of globals

[Bug tree-optimization/27809] New: inefficient gimplification of globals

[dann at godzilla dot ics dot uci dot edu]

This code:

int G;
int lll (int a) { bar (G, G, G, G); }

is gimplified like this:

lll (a)
{
  int G.2;

  G.2 = G;
  G.2 = G;
  G.2 = G;
  G.2 = G;
  bar (G.2, G.2, G.2, G.2);
}

Creating that many identical expressions is wastefull...


-- 
           Summary: inefficient gimplification of globals
           Product: gcc
           Version: 4.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: dann at godzilla dot ics dot uci dot edu


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=27809

[Bug tree-optimization/27809] inefficient gimplification of globals

[pinskia at gcc dot gnu dot org]

------- Comment #1 from pinskia at gcc dot gnu dot org  2006-05-29 22:04 -------
Hmm, it should have produced G.3, G.n, at least I would have thought.


-- 

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         |2006-05-29 22:04:22
               date|                            |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=27809

[Bug tree-optimization/27809] inefficient gimplification of globals

[rakdver at gcc dot gnu dot org]

------- Comment #2 from rakdver at gcc dot gnu dot org  2006-06-13 14:09 -------
(In reply to comment #1)
> Hmm, it should have produced G.3, G.n, at least I would have thought.
> 

No, we intentionally use the same variable for the lexically identical
expressions, see internal_get_tmp_var/lookup_tmp_var.  Original intention was
to make PRE and other redundancy elimination optimization passes more efficient
(this was essential especially for the old SSAPRE pass that used lexical
equality of expressions to check for redundancies).  These reasons are no
longer relevant, but keeping the code saves a significant amount of memory and
compile time (I tried removing the code a few months ago, but since it slows
down compilation by some 1-2%, I never bothered with posting the patch).


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=27809

[Bug tree-optimization/27809] inefficient gimplification of globals

[dann at godzilla dot ics dot uci dot edu]

------- Comment #3 from dann at godzilla dot ics dot uci dot edu  2006-06-13 14:22 -------
(In reply to comment #2)
> (In reply to comment #1)
> > Hmm, it should have produced G.3, G.n, at least I would have thought.
> > 
> 
> No, we intentionally use the same variable for the lexically identical
> expressions, see internal_get_tmp_var/lookup_tmp_var.  Original intention was
> to make PRE and other redundancy elimination optimization passes more efficient
> (this was essential especially for the old SSAPRE pass that used lexical
> equality of expressions to check for redundancies).  These reasons are no
> longer relevant, but keeping the code saves a significant amount of memory and
> compile time (I tried removing the code a few months ago, but since it slows
> down compilation by some 1-2%, I never bothered with posting the patch).

Using the same variable is surely good, wouldn't it be even better to not
create the redundant "G.2 = G" assignments in this PR?


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=27809

[Bug tree-optimization/27809] inefficient gimplification of globals

[dberlin at gcc dot gnu dot org]

------- Comment #4 from dberlin at gcc dot gnu dot org  2006-06-13 14:26 -------
gimplification is almost certainly the wrong place to be doing the kind of
dataflow we'd need to determine where we could insert load/save pairs of
globals.
Really.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=27809

[Bug tree-optimization/27809] inefficient gimplification of globals

[rguenth at gcc dot gnu dot org]

------- Comment #5 from rguenth at gcc dot gnu dot org  2008-12-07 12:08 -------
I agree.


-- 

rguenth at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
         Resolution|                            |INVALID


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=27809