[Bug c/36693] New: missed optimization for pointer access with offset on powerpc

[Bug c/36693] New: missed optimization for pointer access with offset on powerpc

[rbuergel at web dot de]

the following two equivalent functions are compiled into different asm-code.
The bad thing is, that the more readable function (get_and_increment2) creates
worse code. It is bigger and slower. This is because it uses one register more
than the more optimized version get_and_increment1.


struct IntPtr
{
        int* m_ReadPtr;
};

int get_and_increment1(struct IntPtr* i)
{
        return *(i->m_ReadPtr++);
}

int get_and_increment2(struct IntPtr* i)
{
        i->m_ReadPtr++;
        return *(i->m_ReadPtr - 1);
}


00000000 <get_and_increment1>:
   0:   81 23 00 00     lwz     r9,0(r3)
   4:   80 09 00 00     lwz     r0,0(r9)
   8:   39 29 00 04     addi    r9,r9,4
   c:   91 23 00 00     stw     r9,0(r3)
  10:   7c 03 03 78     mr      r3,r0
  14:   4e 80 00 20     blr

00000018 <get_and_increment2>:
  18:   81 23 00 00     lwz     r9,0(r3)
  1c:   39 29 00 04     addi    r9,r9,4
  20:   91 23 00 00     stw     r9,0(r3)
  24:   80 69 ff fc     lwz     r3,-4(r9)
  28:   4e 80 00 20     blr


-- 
           Summary: missed optimization for pointer access with offset on
                    powerpc
           Product: gcc
           Version: 4.2.3
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: rbuergel at web dot de
  GCC host triplet: x86_64-pc-linux-gnu
GCC target triplet: powerpc-linux-uclibc


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=36693

[Bug rtl-optimization/36693] missed optimization for pointer access with offset on powerpc

[pinskia at gcc dot gnu dot org]

------- Comment #1 from pinskia at gcc dot gnu dot org  2008-08-11 01:31 -------
Hmm, this works on the trunk with -O2 -mtune=cell which means this is a
scheduling issue:
[apinski@dhcp-10-98-10-216 ~]$ ~/gcc-mainline/bin/gcc -O2 -o - -S t.c
-mtune=cell
        .file   "t.c"
        .section        ".text"
        .align 2
        .p2align 3,,7
        .globl get_and_increment1
        .type   get_and_increment1, @function
get_and_increment1:
        lwz 9,0(3)
        addi 0,9,4
        stw 0,0(3)
        lwz 3,0(9)
        blr
        .size   get_and_increment1,.-get_and_increment1
        .align 2
        .p2align 3,,7
        .globl get_and_increment2
        .type   get_and_increment2, @function
get_and_increment2:
        lwz 9,0(3)
        addi 0,9,4
        stw 0,0(3)
        lwz 3,0(9)
        blr
        .size   get_and_increment2,.-get_and_increment2
        .ident  "GCC: (GNU) 4.4.0 20080810 (experimental) [trunk revision
138922]"
        .section        .note.GNU-stack,"",@progbits


-- 

pinskia at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
          Component|tree-optimization           |rtl-optimization
   GCC host triplet|x86_64-pc-linux-gnu         |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=36693