[Bug target/41481] missed optimization in cse

[Bug target/41481] missed optimization in cse

[carrot at google dot com]

------- Comment #1 from carrot at google dot com  2009-09-27 09:13 -------
Created an attachment (id=18662)
 --> (http://gcc.gnu.org/bugzilla/attachment.cgi?id=18662&action=view)
test case


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=41481

[Bug target/41481] New: missed optimization in cse

[carrot at google dot com]

Compile following code with options -Os -march=armv5te -mthumb,

class A
{
 public:
  int ah;
  unsigned field : 2;
};

void foo(A* p)
{
  p->ah = 1;
  p->field = 1;
}

We can get:

        mov     r3, #1             // A
        str     r3, [r0]
        ldrb    r3, [r0, #4]
        mov     r2, #3
        bic     r3, r3, r2
        mov     r2, #1             // B
        orr     r3, r3, r2
        strb    r3, [r0, #4]
        @ sp needed for prologue
        bx      lr

Both instruction A and B load a constant 1 into register. We can load 1 into r1
in instruction A and use r1 when constant 1 is required. So instruction B can
be removed.

cse pass doesn't find this opportunity is because it needs all expressions to
be of the same mode. But in rtl level the first 1 is in mode SI and the second
1 is in mode QI. Arm doesn't has any physical register of QI mode, so all of
them are put into 32 bit physical register and causes redundant load of
constant 1.


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           Summary: missed optimization in cse
           Product: gcc
           Version: 4.5.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: carrot at google dot com
 GCC build triplet: i686-linux
  GCC host triplet: i686-linux
GCC target triplet: arm-eabi


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=41481

[Bug target/41481] missed optimization in cse

[ramana at gcc dot gnu dot org]

------- Comment #2 from ramana at gcc dot gnu dot org  2009-10-06 07:20 -------
confirmed - FYI, the problem doesn't arise with !-mthumb.


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ramana 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         |2009-10-06 07:20:28
               date|                            |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=41481

[Bug target/41481] missed optimization in cse

[rearnsha at gcc dot gnu dot org]

-- 

rearnsha at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Severity|normal                      |enhancement


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=41481