[Bug tree-optimization/31261] New: Missed tree optimizations: (8 - (x & 7)) & 7

[Bug tree-optimization/31261] New: Missed tree optimizations: (8 - (x & 7)) & 7

[jakub at gcc dot gnu dot org]

In 4.2 with -O2 -m32 -fomit-frame-pointer on x86_64:
unsigned int foo (unsigned int x)
{
  return (8 - (x & 7)) & 7;
}
results in andl $7, reg; negl reg; andl $7, reg.
On 4.3 apparently some RTL optimization catches this, but it is still a missed
tree optimization, fold should be able to fold:
(cst - (x & cstmask)) & cstmask2 as
(cst & cstmask2) + (-x & cstmask2) if x is unsigned or if -INT_MIN wraps to
INT_MIN, both cstmask and cstmask2 are constants z^2-1 for some z and
(cstmask & cstmask2) == cstmask2.
BTW, even for
(8 + (x & 7)) & 7
the optimized dump contains:
(x & 7) + 8 & 7
for both 4.2/4.3 (no idea why 8 & 7 hasn't been simplified as 0).


-- 
           Summary: Missed tree optimizations: (8 - (x & 7)) & 7
           Product: gcc
           Version: 4.3.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: jakub at gcc dot gnu dot org


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=31261

[Bug tree-optimization/31261] Missed tree optimizations: (8 - (x & 7)) & 7

[rguenth at gcc dot gnu dot org]

------- Comment #1 from rguenth at gcc dot gnu dot org  2007-03-19 11:17 -------
(x & 7) + 8 & 7 is actually ((x & 7) + 8) & 7


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=31261

[Bug tree-optimization/31261] Missed tree optimizations: (8 - (x & 7)) & 7

[rguenth at gcc dot gnu dot org]

------- Comment #2 from rguenth at gcc dot gnu dot org  2007-03-19 11:19 -------
You would need to enhance associate_trees () or the reassoc pass to fix this.


-- 

rguenth at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Severity|normal                      |enhancement
             Status|UNCONFIRMED                 |NEW
     Ever Confirmed|0                           |1
           Keywords|                            |missed-optimization, TREE
   Last reconfirmed|0000-00-00 00:00:00         |2007-03-19 11:19:48
               date|                            |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=31261

[Bug tree-optimization/31261] Missed tree optimizations: (8 - (x & 7)) & 7

[jakub at gcc dot gnu dot org]

------- Comment #3 from jakub at gcc dot gnu dot org  2007-03-21 15:05 -------
Created an attachment (id=13244)
 --> (http://gcc.gnu.org/bugzilla/attachment.cgi?id=13244&action=view)
gcc43-pr31261.patch

Patch I'm testing ATM.


-- 

jakub at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
         AssignedTo|unassigned at gcc dot gnu   |jakub at gcc dot gnu dot org
                   |dot org                     |
             Status|NEW                         |ASSIGNED


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=31261

[Bug tree-optimization/31261] Missed tree optimizations: (8 - (x & 7)) & 7

[trt at acm dot org]

------- Comment #4 from trt at acm dot org  2007-03-21 18:28 -------
I think this could be generalized to more operators, e.g. 

     (y | (x & 7)) & 7
        ^ (bitwise or, xor, multiply, ...)

This optimization could be for "e & M" where e contains a subexpression of the
form "t & N" which can (sometimes) be simplified to "t".  I suppose that would
require walking the tree.


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=31261