[Bug c/36770] New: PowerPC generated PTR code ineffiency

[Bug c/36770] New: PowerPC generated PTR code ineffiency

[gunnar at greyhound-data dot com]

GCC fails to generate efficient code for basic pointer operations.

Please have a look at this example:
***
test.c:
register int * src asm("r15");

int test( ){
  src[1]=src[0];
  src++;
}

main(){
}

***

compile the above with gcc -S -O3 test.c

shows us the following ASM output:

test:
        mr 9,15
        addi 15,15,4
        lwz 0,0(9)
        stw 0,4(9)
        blr

compile with gcc -S -Os test.c
Gives this output
test:
        mr 9,15
        addi 15,15,4
        lwz 0,0(9)
        stw 0,4(9)
        blr


As you can see both -O3 and -Os produce the same output.
The generated output is far from optimal.

GCC generates for the simple pointer operation this code:
        mr 9,15
        addi 15,15,4
        lwz 0,0(9)
        stw 0,4(9)

But GCC should rather generate this:
        lwz 0,0(15)
        stwu 0,4(15)


Two of the four instructions are unneeded.
We've here code with literally thousands of unneeded instructions generated
like this.


I very much hope that this information is helpful to you and that you can fix
this.

Many thanks in advance

Gunnar von Boehn


-- 
           Summary: PowerPC generated PTR code ineffiency
           Product: gcc
           Version: 4.3.0
            Status: UNCONFIRMED
          Severity: major
          Priority: P3
         Component: c
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: gunnar at greyhound-data dot com
 GCC build triplet: powerpc64-unknown-linux-gnu
  GCC host triplet: powerpc64-unknown-linux-gnu
GCC target triplet: powerpc64-unknown-linux-gnu


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=36770

[Bug middle-end/36770] PowerPC generated PTR code ineffiency

[pinskia at gcc dot gnu dot org]

------- Comment #1 from pinskia at gcc dot gnu dot org  2008-07-09 18:22 -------
forward-propagate is causing some of the issues as shown by:
int *test(int *a ){
  a[1]=a[0];
  a++;
  return a;
}
But using the register extension is causing the rest :).  I would recommend
against using them in real code really, they are not that useful and cause too
many issues in general.


-- 

pinskia at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |pinskia at gcc dot gnu dot
                   |                            |org, bonzini at gnu dot org
           Severity|major                       |normal
             Status|UNCONFIRMED                 |NEW
          Component|c                           |middle-end
     Ever Confirmed|0                           |1
   Last reconfirmed|0000-00-00 00:00:00         |2008-07-09 18:22:19
               date|                            |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=36770

[Bug middle-end/36770] PowerPC generated PTR code inefficiency

[gunnar at greyhound-data dot com]

------- Comment #2 from gunnar at greyhound-data dot com  2008-07-10 09:18 -------
(In reply to comment #1)
> forward-propagate is causing some of the issues as shown by:
> int *test2(int *a ){
>   a[1]=a[0];
>   a++;
>   return a;
> }

Your example creates the following ASM code:
test2:
        mr 9,3
        addi 3,3,4
        lwz 0,0(9)
        stw 0,4(9)
        blr

Correct would be:
test2:
        lwz 0,0(3)
        stwu 0,4(3)
        blr

Is you can see the created bad code is just the same.
This is independent of the register pinning.

Can I understand you comment a verification that the forward propagation is
broken in GCC/PPC?


Kind regards

Gunnar von Boehn


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=36770

[Bug middle-end/36770] PowerPC missed autoincrement opportunity

[bonzini at gnu dot org]

------- Comment #3 from bonzini at gnu dot org  2008-07-11 11:56 -------
Yes, the code produced shows that something (probably fwprop, I trust Andrew
though I'd like to see dumps) is turning the GIMPLE code

  temp = a[0];
  a[1] = temp;
  temp++;

into something harder to optimize.  It might be also a pass-ordering problem.


-- 

bonzini at gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |missed-optimization
            Summary|PowerPC generated PTR code  |PowerPC missed autoincrement
                   |inefficiency                |opportunity


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=36770

[Bug middle-end/36770] PowerPC missed autoincrement opportunity

[bonzini at gnu dot org]

------- Comment #4 from bonzini at gnu dot org  2008-07-18 13:41 -------
auto-inc-dec should be taught about transforming

   a <- b + c
   ...
   *(b + c)

into

   a <- b
   ...
   *(a += c) pre


-- 

bonzini at gnu dot org changed:

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


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=36770

[Bug middle-end/36770] PowerPC missed autoincrement opportunity

[bonzini at gnu dot org]

------- Comment #5 from bonzini at gnu dot org  2008-07-18 14:13 -------
Hmm, even that wouldn't restore the optimization.  The problem here is that
there is another access via b, like

   a <- b + c
   *b
   *(b + c)

The bad placement of the first assignment (bad because doing it the other way
round would have lower register pressure) in turn happens as soon as
gimplification.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=36770

[Bug middle-end/36770] PowerPC missed autoincrement opportunity

[steven at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=36770

Steven Bosscher <steven at gcc dot gnu.org> changed:

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

--- Comment #6 from Steven Bosscher <steven at gcc dot gnu.org> ---
(In reply to Gunnar von Boehn from comment #2)
> Correct would be:
> test2:
>         lwz 0,0(3)
>         stwu 0,4(3)
>         blr
> 
> Is you can see the created bad code is just the same.
> This is independent of the register pinning.

At least with gcc 4.7.1 and gcc 4.9.0 (r206195) the register pinning
makes all the difference.

$ cat t.c
register int * src asm("r15");

int test1( ){
    src[1]=src[0];
    src++;
}

int *test2(int *a ){
    a[1]=a[0];
    a++;
    return a;
}

$ ./cc1 -quiet -O2 t.c
$ cat t.s
...
.L.test1:
        lwz 10,0(15)
        mr 9,15
        addi 15,15,4
        stw 10,4(9)
        blr
...
.L.test2:
        lwz 9,0(3)
        stwu 9,4(3)
        blr


This is basically the same as bug 44281.