[Bug target/49903] New: AVR: Redundant comparisons in binary-seach switch/case expansion

[Bug target/49903] New: AVR: Redundant comparisons in binary-seach switch/case expansion

[gjl at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49903

           Summary: AVR: Redundant comparisons in binary-seach switch/case
                    expansion
           Product: gcc
           Version: 4.7.0
            Status: UNCONFIRMED
          Keywords: missed-optimization
          Severity: normal
          Priority: P3
         Component: target
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: gjl@gcc.gnu.org
                CC: eric.weddington@atmel.com
            Target: avr


Created attachment 24864
  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=24864
cswitch.c: C test case

avr-gcc emits redundant comparisons like these:

cpi r24,lo8(55)     ;  22    *cmpqi/3    [length = 1]
breq .L10     ;  23    branch    [length = 1]
cpi r24,lo8(56)     ;  24    *cmpqi/3    [length = 1]
brge .L19     ;  25    branch    [length = 1]

which could just as well be

cpi r24,lo8(55)     ;  22    *cmpqi/3    [length = 1]
breq .L10     ;  23    branch    [length = 1]
brge .L19     ;  25    branch    [length = 1]

or these:

cpi r24,lo8(100)     ;  61    *cmpqi/3    [length = 1]
breq .L16     ;  62    branch    [length = 1]
cpi r24,lo8(100)     ;  63    *cmpqi/3    [length = 1]
brlt .L15     ;  64    branch    [length = 1]

which could just as well be

cpi r24,lo8(100)     ;  61    *cmpqi/3    [length = 1]
breq .L16     ;  62    branch    [length = 1]
brlt .L15     ;  64    branch    [length = 1]

This concernes signed/unsigned comparisons for QI/HI/SI

-- command line ----------------------------------------------

avr-gcc cswitch.c -dp -S -Os

-- configure -------------------------------------------------

../../gcc.gnu.org/trunk/configure --target=avr
--prefix=/home/john/gnu/install/gcc-4.7 --disable-nls --disable-shared
--enable-languages=c,c++ --with-dwarf2 --disable-lto

-- GCC  -------------------------------------------------

as of 4.7.0 trunk, r176818 from 2011-07-27

[Bug target/49903] [avr] Redundant comparisons in binary-search switch/case expansion

[eric.weddington at atmel dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49903

Eric Weddington <eric.weddington at atmel dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
            Summary|AVR: Redundant comparisons  |[avr] Redundant comparisons
                   |in binary-seach switch/case |in binary-search
                   |expansion                   |switch/case expansion
           Severity|normal                      |enhancement

--- Comment #1 from Eric Weddington <eric.weddington at atmel dot com> 2011-07-29 15:07:52 UTC ---
Johann,
Would some type of peephole optimization help out on this?

[Bug target/49903] [avr] Redundant comparisons in binary-search switch/case expansion

[gjl at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49903

--- Comment #2 from Georg-Johann Lay <gjl at gcc dot gnu.org> 2011-07-29 16:11:12 UTC ---
(In reply to comment #1)
> Johann,
> Would some type of peephole optimization help out on this?

RTL peephole won't work because they operate just inside basic blocks.

Don't know the retionale behind that limitation, appears that it's just for
technical and implementational reasons.  RTL peep need not to be confused by
BBs, just code_labels etc. would bother.

Text peepholes match but it might be tedious and error prone to work out
instructions lengths or the correct instructions if jump offsets don't match.

Maybe a way is in avr_reorg, but it would be quite tedious to work out all
QI/HI/SI signed and unsigned combinations.  And it must ensure that no other
transformation takes place.  There's bunch of special treatment in md and c and
cc0 handling.  E.g. I don't quite understand why jumps clobber cc0; btw not
clobbering cc0 will not fix this PR.

[Bug target/49903] [avr] Redundant comparisons in binary-search switch/case expansion

[rth at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49903

Richard Henderson <rth at gcc dot gnu.org> changed:

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

--- Comment #3 from Richard Henderson <rth at gcc dot gnu.org> 2011-08-02 00:11:34 UTC ---
It would certainly be quite a lot of work, but this sort of thing *is*
handled by pass_compare_elim_after_reload.  That requires exposing the
flags register as a CCmode quantity.

C.f. the mn10300 port.

[Bug target/49903] [avr] Redundant comparisons in binary-search switch/case expansion

[gjl at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49903

Georg-Johann Lay <gjl at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |patch
   Target Milestone|---                         |4.7.0

[Bug target/49903] [avr] Redundant comparisons in binary-search switch/case expansion

[gjl at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49903

Georg-Johann Lay <gjl at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |ASSIGNED
   Last reconfirmed|                            |2011-08-11
         AssignedTo|unassigned at gcc dot       |gjl at gcc dot gnu.org
                   |gnu.org                     |
     Ever Confirmed|0                           |1

--- Comment #4 from Georg-Johann Lay <gjl at gcc dot gnu.org> 2011-08-11 17:43:38 UTC ---
Created attachment 24984
  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=24984
opt-cswitch.diff

See http://gcc.gnu.org/ml/gcc-patches/2011-08/msg01029.html

[Bug target/49903] [avr] Redundant comparisons in binary-search switch/case expansion

[hp at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49903

Hans-Peter Nilsson <hp at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |hp at gcc dot gnu.org

--- Comment #5 from Hans-Peter Nilsson <hp at gcc dot gnu.org> 2011-08-12 22:54:27 UTC ---
I'd guess you're just looking at a NOTICE_UPDATE_CC bug in the AVR port.

[Bug target/49903] [avr] Redundant comparisons in binary-search switch/case expansion

[gjl at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49903

--- Comment #6 from Georg-Johann Lay <gjl at gcc dot gnu.org> 2011-08-14 09:10:20 UTC ---
Author: gjl
Date: Sun Aug 14 09:10:13 2011
New Revision: 177744

URL: http://gcc.gnu.org/viewcvs?root=gcc&view=rev&rev=177744
Log:

    * PR target/49903
    * config/avr/avr.md (UNSPEC_IDENTITY): New c_enum.
    (branch_unspec): New insn.
    (branch): Beauty farm.
    * config/avr/avr.c (compare_condition): Use JUMP_P.  Test SET_SRC
    to be IF_THEN_ELSE.
    (avr_compare_pattern, avr_reorg_remove_redundant_compare):
    New static functions.
    (avr_reorg): Use them.  Use next_real_insn instead of NEXT_INSN.
    Use CONST_INT_P.  Beauty.


Modified:
    trunk/gcc/ChangeLog
    trunk/gcc/config/avr/avr.c
    trunk/gcc/config/avr/avr.md

[Bug target/49903] [avr] Redundant comparisons in binary-search switch/case expansion

[gjl at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49903

Georg-Johann Lay <gjl at gcc dot gnu.org> changed:

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

--- Comment #7 from Georg-Johann Lay <gjl at gcc dot gnu.org> 2011-08-14 09:11:26 UTC ---
Closed with this patch.