[Bug c/36135] New: GCC creates suboptimal ASM : suboptimal Adressing-Modes used

[Bug c/36135] New: GCC creates suboptimal ASM : suboptimal Adressing-Modes used

[gunnar at greyhound-data dot com]

+++ This bug was initially created as a clone of Bug #36133 +++

Hello,

The ASM code created by GCC 4.2.1 for 68k/Coldfire platform is not optimal.
Comparing ASM output created by GCC 2.9 with GCC 4.2,
the generated code got partially much worse with GCC 4.


One problem that was visible a lot was that GCC uses suboptimal addressing
modes.

Please see the below example for details.
In line 14 to line 2E this code was created:
14:       2290            movel %a0@,%a1@
16:       2368 0004 0004  movel %a0@(4),%a1@(4)
1c:       2368 0008 0008  movel %a0@(8),%a1@(8)
22:       2368 000c 000c  movel %a0@(12),%a1@(12)
28:       d3fc 0000 0010  addal #16,%a1
2e:       d1fc 0000 0010  addal #16,%a0

Much shorter and more efficient would have been this:
14:       20d9            movel %a1@+,%a0@+
16:       20d9            movel %a1@+,%a0@+
18:       20d9            movel %a1@+,%a0@+
1a:       20d9            movel %a1@+,%a0@+


Example: C-source
Code:
void * copy_32x4a(void *destparam, const void *srcparam, size_t size)
{
        int *dest = destparam;
        const int *src = srcparam;
        int size32;
        size32 = size / 16;
        for (; size32; size32--) {
                *dest++ = *src++;
                *dest++ = *src++;
                *dest++ = *src++;
                *dest++ = *src++;
        }
}

Compile option: m68k-linux-gnu-gcc -mcpu=54455 -msoft-float -o example -Os
-fomit-frame-pointer example.c

Code generated by GCC 4.2:
04:       202f 000c       movel %sp@(12),%d0
08:       226f 0004       moveal %sp@(4),%a1
0c:       206f 0008       moveal %sp@(8),%a0
10:       e888            lsrl #4,%d0
12:       6022            bras 36
14:       2290            movel %a0@,%a1@
16:       2368 0004 0004  movel %a0@(4),%a1@(4)
1c:       2368 0008 0008  movel %a0@(8),%a1@(8)
22:       2368 000c 000c  movel %a0@(12),%a1@(12)
28:       d3fc 0000 0010  addal #16,%a1
2e:       d1fc 0000 0010  addal #16,%a0
34:       5380            subql #1,%d0
36:       4a80            tstl %d0
38:       66da            bnes 14
3a:       4e75            rts    

For comparison here is code that you would expect:
04:       202f 000c       movel %sp@(12),%d0
08:       226f 0004       moveal %sp@(4),%a1
0c:       206f 0008       moveal %sp@(8),%a0
10:       e888            lsrl #4,%d0
12:       6022            beq 20
14:       20d9            movel %a1@+,%a0@+
16:       20d9            movel %a1@+,%a0@+
18:       20d9            movel %a1@+,%a0@+
1a:       20d9            movel %a1@+,%a0@+
1c:       5380            subql #1,%d0
1e:       66da            bnes 14
20:       4e75            rts 

Compiler used:
m68k-linux-gnu-gcc -v
Using built-in specs.
Target: m68k-linux-gnu
Configured with: /scratch/shinwell/cf-fall-linux-lite/src/gcc-4.2/configure
--build=i686-pc-linux-gnu --host=i686-pc-linux-gnu --target=m68k-linux-gnu
--enable-threads --disable-libmudflap --disable-libssp --disable-libgomp
--disable-libstdcxx-pch --with-arch=cf --with-gnu-as --with-gnu-ld
--enable-languages=c,c++ --enable-shared --enable-symvers=gnu
--enable-__cxa_atexit --with-pkgversion=Sourcery G++ Lite 4.2-47
--with-bugurl=https://support.codesourcery.com/GNUToolchain/ --disable-nls
--prefix=/opt/freescale/usr/local/gcc-4.2.47-eglibc-2.5.47/m68k-linux
--with-sysroot=/opt/freescale/usr/local/gcc-4.2.47-eglibc-2.5.47/m68k-linux/m68k-linux-gnu/libc
--with-build-sysroot=/scratch/shinwell/cf-fall-linux-lite/install/m68k-linux-gnu/libc
--enable-poison-system-directories
--with-build-time-tools=/scratch/shinwell/cf-fall-linux-lite/install/m68k-linux-gnu/bin
--with-build-time-tools=/scratch/shinwell/cf-fall-linux-lite/install/m68k-linux-gnu/bin
Thread model: posix
gcc version 4.2.1 (Sourcery G++ Lite 4.2-47)


I hope that this report help you to improve the quality of GCC.

Kind regards

Gunnar von Boehn
--
P.S. I put the noticed issues in individual tickets for easier tracking. I hope
that this is helpful to you.


-- 
           Summary: GCC creates suboptimal ASM : suboptimal Adressing-Modes
                    used
           Product: gcc
           Version: 4.2.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: gunnar at greyhound-data dot com
 GCC build triplet: i686-pc-linux-gnu
  GCC host triplet: i686-pc-linux-gnu
GCC target triplet: m68k-linux-gnu


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=36135

[Bug target/36135] GCC creates suboptimal ASM : suboptimal Adressing-Modes used

[rguenth at gcc dot gnu dot org]

------- Comment #1 from rguenth at gcc dot gnu dot org  2008-05-07 19:33 -------
It would have been nice to check at least gcc 4.3 (or better current trunk).


-- 

rguenth at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Severity|normal                      |enhancement
           Keywords|                            |missed-optimization


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=36135

[Bug target/36135] GCC creates suboptimal ASM : suboptimal Adressing-Modes used

[gunnar at greyhound-data dot com]

------- Comment #2 from gunnar at greyhound-data dot com  2008-05-28 16:23 -------
(In reply to comment #1)
> It would have been nice to check at least gcc 4.3 (or better current trunk).
> 

I have verified this for you with the most current GCC source.
Verified with gcc version 4.4.0 20080523 (experimental) (GCC) 

The problem that GCC uses bad addressing modes is still persistent.

Code generated by GCC 4.4 
copy_32x4:
        link.w %fp,#-12
        movem.l #3076,(%sp)
        move.l 16(%fp),%d2
        lsr.l #4,%d2
        move.l 8(%fp),%a3
        move.l 12(%fp),%a2
        jra .L6
.L7:
        move.l (%a2),%a1
        subq.l #1,%d2
        move.l 4(%a2),%d0
        move.l 8(%a2),%d1
        move.l 12(%a2),%a0
        add.l #16,%a2
        move.l %a1,(%a3)
        move.l %d0,4(%a3)
        move.l %d1,8(%a3)
        move.l %a0,12(%a3)
        add.l #16,%a3
.L6:
        tst.l %d2
        jne .L7
        movem.l (%sp),#3076
        unlk %fp
        rts


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=36135

[Bug target/36135] GCC creates suboptimal ASM : suboptimal Adressing-Modes used

[gunnar at greyhound-data dot com]

------- Comment #3 from gunnar at greyhound-data dot com  2008-06-12 14:34 -------
Andreas,

What is your opinion to this?

GCC 2.9 used to combine the move with increment in the combine step to
something like this:
***
(insn 32 30 33 (set (reg/v:SI 32)
        (mem:SI (post_inc:SI (reg/v:SI 34)) 0)) 42 {movsi+1} (nil)
    (expr_list:REG_INC (reg/v:SI 34)
        (nil)))
***


So problem is that now GCC seems not to be able to do this anymore by itself
With GCC 4.4 the output is:
**
(insn 34 33 35 4 example2.c:11 (set (reg/v:SI 54 [ value ])
        (mem:SI (reg/v/f:SI 52 [ src ]) [2 S4 A16])) 37 {*movsi_cf} (nil))

(insn 35 34 36 4 example2.c:12 (set (reg/v:SI 53 [ value2 ])
        (mem:SI (plus:SI (reg/v/f:SI 52 [ src ])
                (const_int 4 [0x4])) [2 S4 A16])) 37 {*movsi_cf} (nil))

(insn 36 35 38 4 example2.c:5 (set (reg/v/f:SI 52 [ src ])
        (plus:SI (reg/v/f:SI 52 [ src ])
            (const_int 8 [0x8]))) 133 {*addsi3_5200} (nil))

(insn 38 36 40 4 example2.c:10 (set (reg/v:SI 50 [ size.21 ])
        (plus:SI (reg/v:SI 50 [ size.21 ])
            (const_int -1 [0xffffffff]))) 133 {*addsi3_5200} (nil))
***

Any ideas about this?


Kind regards

Gunnar von Boehn


-- 

gunnar at greyhound-data dot com changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |schwab at suse dot de


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=36135

[Bug target/36135] GCC creates suboptimal ASM : suboptimal Adressing-Modes used

[pinskia at gcc dot gnu dot org]

------- Comment #4 from pinskia at gcc dot gnu dot org  2008-06-12 14:38 -------
This comes down to IV-OPTs not understanding {post,pre}_{dec,inc} at all. 
There is another bug about this somewhere I think for arm.  PowerPC has the
same issue too ...


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=36135

[Bug target/36135] GCC creates suboptimal ASM : suboptimal Adressing-Modes used

[gunnar at greyhound-data dot com]

------- Comment #5 from gunnar at greyhound-data dot com  2008-06-13 09:31 -------
(In reply to comment #4)
> This comes down to IV-OPTs not understanding {post,pre}_{dec,inc} at all. 
> There is another bug about this somewhere I think for arm.  PowerPC has the
> same issue too ...
> 

If this effects so many platforms this sounds like an important issue to me.
Maybe someone should increase the priority and severity of the issue in this
case?

Andrew, do you plan to fix this issue?

Cheers
Gunnar


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=36135

[Bug target/36135] GCC creates suboptimal ASM : suboptimal Adressing-Modes used

[gunnar at greyhound-data dot com]

------- Comment #6 from gunnar at greyhound-data dot com  2008-06-13 13:34 -------
(In reply to comment #4)
> This comes down to IV-OPTs not understanding {post,pre}_{dec,inc} at all. 
> There is another bug about this somewhere I think for arm.  PowerPC has the
> same issue too ...
> 

Hi Andrew,

I want to make clear that the 68K backend used to be able to do this
optimization in the GCC 2.9 times. Later with 3.4 or 4.x this optmization did
not work anymore and the code became worth.
Does this make sense in your opinion?


Cheers


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=36135

[Bug target/36135] GCC creates suboptimal ASM : suboptimal Adressing-Modes used

[pinskia at gcc dot gnu dot org]

------- Comment #7 from pinskia at gcc dot gnu dot org  2008-06-13 14:34 -------
>Andrew, do you plan to fix this issue?

Personally no.  Mostly because IV-opts is hard to understand.

Also it is not the m68k back-end doing the optimization rather loop.c did it.


See PR 31849.

*** This bug has been marked as a duplicate of 31849 ***


-- 

pinskia at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|                            |DUPLICATE


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=36135