[Bug target/11825] New: [ARM] Redundant move in some cases (dead code before return)

[Bug target/11825] New: [ARM] Redundant move in some cases (dead code before return)

[alga at rgai dot hu]

PLEASE REPLY TO gcc-bugzilla@gcc.gnu.org ONLY, *NOT* gcc-bugs@gcc.gnu.org.

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=11825

           Summary: [ARM] Redundant move in some cases (dead code before
                    return)
           Product: gcc
           Version: 3.4
            Status: UNCONFIRMED
          Severity: minor
          Priority: P3
         Component: target
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: alga at rgai dot hu
                CC: gcc-bugs at gcc dot gnu dot org
 GCC build triplet: i686-pc-linux-gnu
  GCC host triplet: i686-pc-linux-gnu
GCC target triplet: arm-unknown-elf

In some cases before the function return GCC generates a redundant mov defining
r0 just before another instruction defines r0 again. The first (dead) mov is
unnecessary (see the example).

--- c example ---
int foo (int a, int x)
{
  long long b,c;
  b = a << 16;
  c = x << 8;
  b += c;
  return (int)(b >> 32);
}

--- arm code ---
foo:
  mov	r1, r1, asl #8
  mov	r2, r1, asr #31
  mov	r0, r0, asl #16
  stmfd	sp!, {r4, lr}
  adds	r3, r1, r0
  adc	r4, r2, r0, asr #31
  mov	r1, r4
  mov	r0, r3  <- OLD
  mov	r0, r1
  ldmfd	sp!, {r4, pc}

--- possible solution ---
foo:
  mov	r1, r1, asl #8
  mov	r2, r1, asr #31
  mov	r0, r0, asl #16
  stmfd	sp!, {r4, lr}
  adds	r3, r1, r0
  adc	r4, r2, r0, asr #31
  mov	r1, r4
  mov	r0, r1
  ldmfd	sp!, {r4, pc}

[Bug optimization/11825] Redundant move in some cases (dead code before return) for long long on 32bit targets

[pinskia at physics dot uc dot edu]

PLEASE REPLY TO gcc-bugzilla@gcc.gnu.org ONLY, *NOT* gcc-bugs@gcc.gnu.org.

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=11825


pinskia at physics dot uc dot edu changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Severity|minor                       |enhancement
             Status|UNCONFIRMED                 |NEW
          Component|target                      |optimization
     Ever Confirmed|                            |1
 GCC target triplet|arm-unknown-elf             |arm-*-elf, powerpc-*-*
           Keywords|                            |pessimizes-code
   Last reconfirmed|0000-00-00 00:00:00         |2003-08-06 12:37:31
               date|                            |
            Summary|[ARM] Redundant move in some|Redundant move in some cases
                   |cases (dead code before     |(dead code before return)
                   |return)                     |for long long on 32bit
                   |                            |targets


------- Additional Comments From pinskia at physics dot uc dot edu  2003-08-06 12:37 -------
A fix might to add to combine:
(set b (ashiftrt:DI a (const_int 32))
(set c (subreg:SI b 4))
To (set c (subreg:SI a 8))

(Aka b = a >> 32; c = (int)b; To c = (upper part of long long)b;  where b and a are long 
long's).
This also happens on powerpc-*-*.  I can confirm this on the mainline (20030806).

[Bug optimization/11825] Redundant move in some cases (dead code before return) for long long on 32bit targets

[pinskia at physics dot uc dot edu]

PLEASE REPLY TO gcc-bugzilla@gcc.gnu.org ONLY, *NOT* gcc-bugs@gcc.gnu.org.

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=11825


pinskia at physics dot uc dot edu changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
 GCC target triplet|arm-*-elf, powerpc-*-*      |arm-*


------- Additional Comments From pinskia at physics dot uc dot edu  2003-08-06 15:11 -------
I looked at powerpc's rtl and it does not match arm's rtl.  Arm's problem is that ashifrt is a 
libcall and that is where the problem is.

[Bug optimization/11825] Redundant move in some cases (dead code before return) for long long on 32bit targets

[dhazeghi at yahoo dot com]

PLEASE REPLY TO gcc-bugzilla@gcc.gnu.org ONLY, *NOT* gcc-bugs@gcc.gnu.org.

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=11825


dhazeghi at yahoo dot com changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
  GCC build triplet|i686-pc-linux-gnu           |
   GCC host triplet|i686-pc-linux-gnu           |
   Target Milestone|3.4                         |---