[Bug middle-end/41396] New: missed space optimization related to basic block reorder

[Bug middle-end/41396] New: missed space optimization related to basic block reorder

[carrot at google dot com]

Compile the attached source code with options -march=armv5te -mthumb -Os, I got

        push    {r4, lr}
        ldr     r4, [r0, #8]
        ldr     r3, [r0, #4]
        b       .L2
.L7:
        ldr     r2, [r3, #8]
        ldr     r1, [r2]
        ldr     r2, [r3]
        add     r2, r1, r2
        ldr     r1, [r3, #4]
        ldr     r1, [r1]
        sub     r2, r2, r1
        ldr     r1, [r3, #12]
        cmp     r1, #1
        beq     .L4
        cmp     r1, #2
        bne     .L3
        b       .L12       // C
.L4:                       // ---------BEGIN BLOCK B
        ldr     r1, [r0]
        neg     r1, r1
        cmp     r2, r1
        bge     .L3
        b       .L9        // ----------END BLOCK B
.L12:                      // ---------------BEGIN BLOCK A-------
        ldr     r1, [r0]
        cmp     r2, r1
        bgt     .L9
.L3:
        add     r3, r3, #16
.L2:
        cmp     r3, r4
        bcc     .L7
        mov     r0, #0
        b       .L6      // -----------------END BLOCK A---------
.L9:
        mov     r0, #1
.L6:
        @ sp needed for prologue
        pop     {r4, pc}


If we change the order of block A and block B, we can remove 2 branch
instructions, inst C and another inst at the end of block B.

Need new basic block reorder algorithm for code size optimization?


-- 
           Summary: missed space optimization related to basic block reorder
           Product: gcc
           Version: 4.5.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: middle-end
        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=41396

[Bug middle-end/41396] missed space optimization related to basic block reorder

[carrot at google dot com]

------- Comment #1 from carrot at google dot com  2009-09-18 07:57 -------
Created an attachment (id=18602)
 --> (http://gcc.gnu.org/bugzilla/attachment.cgi?id=18602&action=view)
test case


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=41396

[Bug middle-end/41396] missed space optimization related to basic block reorder

[pinskia at gcc dot gnu dot org]

------- Comment #2 from pinskia at gcc dot gnu dot org  2009-09-18 07:57 -------
yes, currently at -Os basic block reordering is disabled :).


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=41396

[Bug middle-end/41396] missed space optimization related to basic block reorder

[rguenth at gcc dot gnu dot org]

------- Comment #3 from rguenth at gcc dot gnu dot org  2009-09-18 08:44 -------
So, confirmed.


-- 

rguenth 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         |2009-09-18 08:44:20
               date|                            |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=41396

[Bug middle-end/41396] missed space optimization related to basic block reorder

[pinskia at gcc dot gnu dot org]

-- 

pinskia at gcc dot gnu dot org changed:

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


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=41396