[Bug tree-optimization/15419] New: memcpy pessimization

[Bug tree-optimization/15419] New: memcpy pessimization

[falk at debian dot org]

gcc (GCC) 3.5.0 20040513 (experimental)

% cat test.c
typedef unsigned uint32_t;

uint32_t get_uint32(const void *p) {
    uint32_t w;
    memcpy(&w, p, sizeof (uint32_t));
    return w;
}

uint32_t get_uint32a(const void *p) {
    const struct { uint32_t w; } __attribute__((packed)) *wp = p;
    return wp->w;
}

% gcc -c -O3 test.c && objdump -d test.o

0000000000000000 <get_uint32>:
   0:   03 00 90 28     ldbu    t3,3(a0)
   4:   00 00 30 28     ldbu    t0,0(a0)
   8:   f0 ff de 23     lda     sp,-16(sp)
   c:   01 00 50 28     ldbu    t1,1(a0)
  10:   02 00 70 28     ldbu    t2,2(a0)
  14:   00 00 3e 38     stb     t0,0(sp)
  18:   03 00 9e 38     stb     t3,3(sp)
  1c:   01 00 5e 38     stb     t1,1(sp)
  20:   02 00 7e 38     stb     t2,2(sp)
  24:   00 00 1e a0     ldl     v0,0(sp)
  28:   10 00 de 23     lda     sp,16(sp)
  2c:   01 80 fa 6b     ret

0000000000000030 <get_uint32a>:
  30:   00 00 50 2c     ldq_u   t1,0(a0)
  34:   03 00 30 2c     ldq_u   t0,3(a0)
  38:   c2 04 50 48     extll   t1,a0,t1
  3c:   41 0d 30 48     extlh   t0,a0,t0
  40:   00 04 41 44     or      t1,t0,v0
  44:   00 00 e0 43     sextl   v0,v0
  48:   01 80 fa 6b     ret

The code for get_uint32 is really bad compared to get_uint32a, but it is
an important portable idiom to achieve the same.

-- 
           Summary: memcpy pessimization
           Product: gcc
           Version: 3.5.0
            Status: UNCONFIRMED
          Keywords: pessimizes-code
          Severity: enhancement
          Priority: P2
         Component: tree-optimization
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: falk at debian dot org
                CC: gcc-bugs at gcc dot gnu dot org
 GCC build triplet: alphaev68-unknown-linux-gnu
  GCC host triplet: alphaev68-unknown-linux-gnu
GCC target triplet: alphaev68-unknown-linux-gnu


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=15419

[Bug tree-optimization/15419] memcpy pessimization

[pinskia at gcc dot gnu dot org]

------- Additional Comments From pinskia at gcc dot gnu dot org  2004-05-13 14:10 -------
Confirmed, I think RTH was going to something with memcpy.

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
     Ever Confirmed|                            |1
   Last reconfirmed|0000-00-00 00:00:00         |2004-05-13 14:10:55
               date|                            |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=15419

[Bug tree-optimization/15419] memcpy pessimization

[rth at gcc dot gnu dot org]

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
         AssignedTo|unassigned at gcc dot gnu   |rth at gcc dot gnu dot org
                   |dot org                     |
             Status|NEW                         |ASSIGNED
   Last reconfirmed|2005-02-10 04:23:15         |2005-05-18 07:36:30
               date|                            |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=15419

[Bug tree-optimization/15419] memcpy pessimization

[rth at gcc dot gnu dot org]

------- Additional Comments From rth at gcc dot gnu dot org  2005-05-18 08:21 -------
Not alpha specific.  The tunings we have in the alpha memcpy expansion are 
correct for real copies between memory.  We want to prefer

        ldbu $1,3($17)
        ldbu $2,0($17)
        ldbu $3,1($17)
        ldbu $4,2($17)
        stb $1,3($16)
        stb $2,0($16)
        stb $3,1($16)
        stb $4,2($16)

over 

        ldq_u $5,0($17)
        ldq_u $2,3($17)
        ldq_u $3,3($16)
        ldq_u $4,0($16)
        extlh $2,$17,$2
        extll $5,$17,$5
        msklh $3,$16,$3
        mskll $4,$16,$4
        bis $5,$2,$1
        inslh $1,$16,$6
        insll $1,$16,$1
        bis $3,$6,$3
        bis $4,$1,$4
        stq_u $3,3($16)
        stq_u $4,0($16)

The trick is that we want to notice at the tree level that the memcpy is
really a killing store of a local variable, and rewrite it into the 
assignment form.  I'll give it some thought.

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
 GCC target triplet|alphaev68-*-linux-gnu       |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=15419

[Bug tree-optimization/15419] memcpy pessimization

[rth at gcc dot gnu dot org]

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
         AssignedTo|rth at gcc dot gnu dot org  |unassigned at gcc dot gnu
                   |                            |dot org
             Status|ASSIGNED                    |NEW


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=15419