[Bug target/41004] New: missed merge of basic blocks

[Bug target/41004] New: missed merge of basic blocks

[carrot at google dot com]

Compile the attached source code with options -Os -march=armv5te -mthumb
Gcc generates following code snippet:

      ...
       cmp     r0, r2
       bne     .L5
       b       .L15                            <--- A
.L9:
       ldr     r3, [r1]
       cmp     r3, #0
       beq     .L7
       str     r0, [r1, #8]
       b       .L8
.L7:
       str     r3, [r1, #8]
.L8:
       ldr     r1, [r1, #4]
       b       .L12                          <---- C
.L15:
       mov     r0, #1                       <--- B
.L12:
       cmp     r1, r2                       <---- D
       bne     .L9
       ...

inst A jump to B then fall through to D
inst C jump to D

there is no other instructions jump to instruction B, so we can put inst B just
before A, then A jump to D, and C can be removed.

There are two possible functions can potentially do this optimization. They are
merge_blocks_move and try_forward_edges. 

Function try_forward_edges can only redirect a series of forwarder blocks. It
can't move the target blocks before the forwarder blocks.

In function merge_blocks_move only when both block b and c aren't forwarder
blocks then can they be merged. In this case block A is a forwarder block, so
they are not merged.


-- 
           Summary: missed merge of basic blocks
           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=41004

[Bug target/41004] missed merge of basic blocks

[carrot at google dot com]

------- Comment #1 from carrot at google dot com  2009-08-08 00:10 -------
Created an attachment (id=18326)
 --> (http://gcc.gnu.org/bugzilla/attachment.cgi?id=18326&action=view)
test case


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=41004

[Bug target/41004] missed merge of basic blocks

[steven at gcc dot gnu dot org]

------- Comment #2 from steven at gcc dot gnu dot org  2009-08-08 09:07 -------
Why does the basic block reordering pass also not handle this?


-- 

steven 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         |2009-08-08 09:07:04
               date|                            |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=41004

[Bug middle-end/41004] missed merge of basic blocks

[steven at gcc dot gnu dot org]

-- 

steven at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
         AssignedTo|unassigned at gcc dot gnu   |steven at gcc dot gnu dot
                   |dot org                     |org
             Status|NEW                         |ASSIGNED
   Last reconfirmed|2009-08-08 09:07:04         |2009-08-08 10:06:54
               date|                            |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=41004

[Bug middle-end/41004] missed merge of basic blocks

[rguenth at gcc dot gnu dot org]

------- Comment #3 from rguenth at gcc dot gnu dot org  2009-08-08 10:36 -------
I would suggest to closely look at SSA expansion as well.  There is I think
still the regression from pre-SSA expand that it doesn't expand PHI nodes
optimally at -Os (pre-SSA expand split common PHI args into extra blocks to
minimize copy instructions which is something that can be done as a tree
level transform).  Like

BB5:
 # a_1 = PHI <a_2(2), a_2(3), b_3(4)>

to

BB6:
 # tmp_4 = PHI <a_2(2), a_2(3)>

BB5:
 # a_1 = PHI <tmp_4(6), b_3(4)>

which we usually don't want and in fact undo during mergephi.  But it needs
one less copy if a_2 and tmp_4 can be coalesced together.


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=41004

[Bug middle-end/41004] missed merge of basic blocks

[carrot at google dot com]

------- Comment #4 from carrot at google dot com  2009-08-19 21:55 -------
(In reply to comment #2)
> Why does the basic block reordering pass also not handle this?
> 

Basic block reordering is disabled with options -Os. 

The basic block reordering algorithm is for performance only, it usually
increases code size. So it won't be called when do optimization for size. But
for this specific case, the extra branch can be removed when I compile it with
-O2.


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=41004

[Bug middle-end/41004] missed merge of basic blocks

[rth at gcc dot gnu dot org]

------- Comment #5 from rth at gcc dot gnu dot org  2009-08-19 23:06 -------
Basic block reordering *should* be altered to work with -Os.
It's definitely required to clean up silly jump sequences that
exception handling creates.

There's no reason, in principal, that the ordering algorithm
can't be adjusted to minimize the number of branches, or the
total size of all of the branch instructions (though that could
require some clever changes to the shorten_branches code).


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=41004

[Bug middle-end/41004] missed merge of basic blocks

[steven at gcc dot gnu dot org]

------- Comment #6 from steven at gcc dot gnu dot org  2010-01-11 09:35 -------
Re. comment #5 -- rth, any suggestions what an algorithm would look like to
"minimize the number of branches, or the total size of all of the branch
instructions"?  And what do you mean with "some clever changes to the
shorten_branches code"?  Did you mean the calculation of insn sizes, or
avoiding padding, or something else completely?  Hope you can help a bit...


-- 

steven at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |rth at gcc dot gnu dot org


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=41004