[Bug optimization/13629] New: [tree-ssa][Regression] dominator optimizations can pessimize code

[Bug optimization/13629] New: [tree-ssa][Regression] dominator optimizations can pessimize code

[steven at gcc dot gnu dot org]

If an expression dominates another one on a path, the redundant 
expression is always eliminated.  This can pessimize the code 
if the original expression was dead code: 
 
int *data; 
void partial_dead (int i, int j) 
{ 
  int k = i * j; 
  if (i & j) 
    data[0] = i * j; 
} 
 
--> 
 
;; Function partial_dead (partial_dead) 
 
partial_dead (i, j) 
{ 
  int k; 
 
<bb 0>: 
  k = i * j; 
  if ((i & j) != 0) goto <L0>; else goto <L1>; 
 
<L0>:; 
  *data = k; 
 
<L1>:; 
  return; 
} 
 
This is, in fact, a regression: 
 
Resulting assembly with tree-ssa: 
partial_dead: 
        movl    4(%esp), %edx   # i 
        movl    8(%esp), %eax   # j 
        movl    %edx, %ecx      # k = i 
        imull   %eax, %ecx      # k = i * j 
        testl   %eax, %edx      # if (j & i) 
        je      .L1             #   goto <L1> 
        movl    data, %eax 
        movl    %ecx, (%eax)    # *data = k 
.L1: 
        ret 
 
Resulting assembly with 3.3.1 (SuSE Linux): 
partial_dead: 
        movl    4(%esp), %edx   #  i 
        movl    8(%esp), %eax   #  j 
        testl   %eax, %edx      #  if (j &  i) 
        je      .L1		#    goto <L1> 
        imull   %eax, %edx 
        movl    data, %eax 
        movl    %edx, (%eax)    #  *data = i * j 
.L1: 
        ret

-- 
           Summary: [tree-ssa][Regression] dominator optimizations can
                    pessimize code
           Product: gcc
           Version: tree-ssa
            Status: UNCONFIRMED
          Keywords: pessimizes-code
          Severity: normal
          Priority: P2
         Component: optimization
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: steven at gcc dot gnu dot org
                CC: dnovillo at redhat dot com,gcc-bugs at gcc dot gnu dot
                    org,law at redhat dot com


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=13629

[Bug optimization/13629] [tree-ssa][Regression] dominator optimizations can pessimize code

[steven at gcc dot gnu dot org]

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|---                         |tree-ssa


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=13629

[Bug optimization/13629] [tree-ssa][Regression] dominator optimizations can pessimize code

[pinskia at gcc dot gnu dot org]

------- Additional Comments From pinskia at gcc dot gnu dot org  2004-01-09 17:15 -------
Confirmed.

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
     Ever Confirmed|                            |1
   Last reconfirmed|0000-00-00 00:00:00         |2004-01-09 17:15:40
               date|                            |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=13629

[Bug optimization/13629] [tree-ssa][Regression] dominator optimizations can pessimize code

[steven at gcc dot gnu dot org]

------- Additional Comments From steven at gcc dot gnu dot org  2004-01-15 23:52 -------
Running a DCE pass before DOM1 cures the problem: 
 
Resulting assembly with tree-ssa + patch.DCE_before_DOM1: 
partial_dead: 
        movl    4(%esp), %edx 
        movl    8(%esp), %eax 
        testl   %eax, %edx 
        je      .L1 
        imull   %eax, %edx 
        movl    data, %eax 
        movl    %edx, (%eax) 
.L1: 
        ret 
 

-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=13629

[Bug optimization/13629] [tree-ssa][Regression] dominator optimizations can pessimize code

[law at redhat dot com]

------- Additional Comments From law at redhat dot com  2004-01-16 02:37 -------
Subject: Re:  [tree-ssa][Regression] dominator 
 optimizations can pessimize code

In message <20040115235224.12798.qmail@sources.redhat.com>, "steven at gcc dot 
gnu dot org" writes:
 >
 >------- Additional Comments From steven at gcc dot gnu dot org  2004-01-15 23
 >:52 -------
 >Running a DCE pass before DOM1 cures the problem: 
 > 
 >Resulting assembly with tree-ssa + patch.DCE_before_DOM1: 
 >partial_dead: 
 >        movl    4(%esp), %edx 
 >        movl    8(%esp), %eax 
 >        testl   %eax, %edx 
 >        je      .L1 
 >        imull   %eax, %edx 
 >        movl    data, %eax 
 >        movl    %edx, (%eax) 
 >.L1: 
 >        ret 
I mentioned this as a short term plan to Diego yesterday, so consider it
on my todo list (though it looks like you're already doing some of the
evaluation of how that impacts the compiler which will definitely make
my life easier :-)

Long term we may want to handle this with partial dead code elimination
since real code often has partial dead code which we would like to be
able to move to eliminate it on paths where it's not needed.

jeff



-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=13629

[Bug optimization/13629] [tree-ssa][Regression] dominator optimizations can pessimize code

[steven at gcc dot gnu dot org]

------- Additional Comments From steven at gcc dot gnu dot org  2004-01-17 15:15 -------
the case i showed is fixed.  we need more advanced optimizations 
such as partial dead code elimination in most other similar cases, 
but there's no point in keeping this bug report open just for that. 
So closed. 

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


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=13629