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* [Bug target/41481]  New: missed optimization in cse
@ 2009-09-27  9:13 carrot at google dot com
  2009-09-27  9:13 ` [Bug target/41481] " carrot at google dot com
                   ` (2 more replies)
  0 siblings, 3 replies; 4+ messages in thread
From: carrot at google dot com @ 2009-09-27  9:13 UTC (permalink / raw)
  To: gcc-bugs

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.


-- 
           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


^ permalink raw reply	[flat|nested] 4+ messages in thread
end of thread, other threads:[~2009-12-12  1:05 UTC | newest]

Thread overview: 4+ messages (download: mbox.gz / follow: Atom feed)
-- links below jump to the message on this page --
2009-09-27  9:13 [Bug target/41481] New: missed optimization in cse carrot at google dot com
2009-09-27  9:13 ` [Bug target/41481] " carrot at google dot com
2009-10-06  7:20 ` ramana at gcc dot gnu dot org
2009-12-12  1:05 ` rearnsha at gcc dot gnu dot org

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