[Bug target/47920] New: strange code generated for expression (a+7)/8

[Bug target/47920] New: strange code generated for expression (a+7)/8

[carrot at google dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=47920

           Summary: strange code generated for expression (a+7)/8
           Product: gcc
           Version: 4.6.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: carrot@google.com
            Target: arm-linux-androideabi


Compile the following code with options -mthumb -march=armv7-a -O2

int t08(int a)
{
  return (a+7)/8;
}

GCC 4.6 generates

t08:
    add    r3, r0, #14
    adds    r0, r0, #7
    it    mi
    movmi    r0, r3
    asrs    r0, r0, #3
    bx    lr


Why not simply generate

    adds    r0, r0, #7
    asrs    r0, r0, #3
    bx    lr

[Bug target/47920] strange code generated for expression (a+7)/8

[mikpe at it dot uu.se]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=47920

--- Comment #1 from Mikael Pettersson <mikpe at it dot uu.se> 2011-02-28 09:56:36 UTC ---
Presumably because arithmetic right-shift by 3 isn't the same as a division by
8 when (a+7) is negative.  Changing the types to unsigned gives the code you
want.

[Bug target/47920] strange code generated for expression (a+7)/8

[jakub at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=47920

Jakub Jelinek <jakub at gcc dot gnu.org> changed:

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

--- Comment #2 from Jakub Jelinek <jakub at gcc dot gnu.org> 2011-02-28 10:09:25 UTC ---
Yeah, exactly.  Try for say a = -20.

[Bug target/47920] strange code generated for expression (a+7)/8

[carrot at google dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=47920

--- Comment #3 from Carrot <carrot at google dot com> 2011-03-01 06:44:47 UTC ---
(In reply to comment #1)
> Presumably because arithmetic right-shift by 3 isn't the same as a division by
> 8 when (a+7) is negative.  Changing the types to unsigned gives the code you
> want.

You are right. Right shift and division are different for negative numbers.

Thumb1 uses a different code sequence since it doesn't support conditional
execution

t08:
    add    r0, r0, #7
    asr    r3, r0, #31
    lsr    r3, r3, #29
    add    r0, r3, r0
    asr    r0, r0, #3
    bx    lr

If we use the similar code sequence in thumb2 we can reduce one instruction
because the logical shift and the following add can be merged into one
instruction

t08:
    add    r0, r0, #7
    asr    r3, r0, #31
    add    r0, r0, r3, LSR #29
    asr    r0, r0, #3
    bx    lr

I wonder if this will be faster.

[Bug target/47920] strange code generated for expression (a+7)/8

[ibolton at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=47920

Ian Bolton <ibolton at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |WAITING
   Last reconfirmed|                            |2011.03.03 10:49:54
                 CC|                            |ibolton at gcc dot gnu.org
     Ever Confirmed|0                           |1

--- Comment #4 from Ian Bolton <ibolton at gcc dot gnu.org> 2011-03-03 10:49:54 UTC ---
I don't think "strange code" applies any more, thanks to input from Mikael and
Jakub.  I am wondering whether to reject this bug.  Any further comments?

[Bug target/47920] strange code generated for expression (a+7)/8

[rearnsha at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=47920

Richard Earnshaw <rearnsha at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|WAITING                     |RESOLVED
         Resolution|                            |INVALID