[Bug c++/13511] Problem with compiler optimization -o2

["osoentgen at lucent dot com" <gcc-bugzilla at gcc dot gnu dot org>]

------- Additional Comments From osoentgen at lucent dot com  2004-01-05 12:43 -------
Subject: RE:  Problem with compiler optimization -o2

Andrew,

Now with commented assembler code.
'*Frameptr >> 8' leads to missing pointer increment:

void test ()
{
        char *Frameptr = (char *) buffer;
        char *CEmsgArray = (char *)buffer2;
  100000:       3d 40 00 10     lis     r10,16                  Prepare
CEmsgArray address
  100004:       39 4a 00 98     addi    r10,r10,152             CEmsgArray
addr has been loaded into R10


        Frameptr++;
        {  CEmsgArray [12 ] = (char)(  *Frameptr  >> 8 );  CEmsgArray [13 ]
= (char)(  *Frameptr ); } ;
  100008:       38 00 00 00     li      r0,0                    Shift by 8
is always zero
  10000c:       3d 60 00 10     lis     r11,16                  Prepare
Frameptr address
  100010:       98 0a 00 0c     stb     r0,12(r10)
CEmsgArray[12] is being assigned (OK)
  100014:       39 6b 00 34     addi    r11,r11,52              Frameptr
addr has been loaded into R11
  100018:       88 0b 00 00     lbz     r0,0(r11)               *Frameptr
  10001c:       98 0a 00 0d     stb     r0,13(r10)
CEmsgArray[13] is being assigned (WRONG!!!)
        CEmsgArray[10]=(char) (*Frameptr);
  100020:       89 2b 00 00     lbz     r9,0(r11)               *Frameptr
  100024:       99 2a 00 0a     stb     r9,10(r10)
CEmsgArray[10] is being assigned (WRONG!!!)
        CEmsgArray[11]=(char) (*Frameptr);
  100028:       88 0b 00 00     lbz     r0,0(r11)               *Frameptr
  10002c:       98 0a 00 0b     stb     r0,11(r10)
CEmsgArray[11] is being assigned (WRONG!!!)
  100030:       4e 80 00 20     blr

=================

Example of good case (shift of 7):

void test ()
{
        char *Frameptr = (char *) buffer;
  100000:       3d 40 00 10     lis     r10,16                  Prepare
Frameptr address
  100004:       39 4a 00 38     addi    r10,r10,56              Frameptr
addr has been loaded into R10
        char *CEmsgArray = (char *)buffer2;


        Frameptr++;
        {  CEmsgArray [12 ] = (char)(  *Frameptr  >> 7 );  CEmsgArray [13 ]
= (char)(  *Frameptr ); } ;
  100008:       8c 0a 00 01     lbzu    r0,1(r10)               Frameptr++
  10000c:       3d 60 00 10     lis     r11,16                  Prepare
CEmsgArray address
  100010:       39 6b 00 9c     addi    r11,r11,156             CEmsgArray
addr has been loaded into R11
  100014:       54 00 c9 fe     rlwinm  r0,r0,25,7,31           >> 7
  100018:       98 0b 00 0c     stb     r0,12(r11)
CEmsgArray[12] is being assigned (OK)
  10001c:       89 2a 00 00     lbz     r9,0(r10)               *Frameptr
  100020:       99 2b 00 0d     stb     r9,13(r11)
CEmsgArray[13] is being assigned (OK)
        CEmsgArray[10]=(char) (*Frameptr);
  100024:       88 0a 00 00     lbz     r0,0(r10)               *Frameptr
  100028:       98 0b 00 0a     stb     r0,10(r11)
CEmsgArray[10] is being assigned (OK)
        CEmsgArray[11]=(char) (*Frameptr);
  10002c:       89 2a 00 00     lbz     r9,0(r10)               *Frameptr
  100030:       99 2b 00 0b     stb     r9,11(r11)
CEmsgArray[11] is being assigned (OK)
  100034:       4e 80 00 20     blr


Regards,
 Olaf

-----Original Message-----
From: pinskia at gcc dot gnu dot org [mailto:gcc-bugzilla@gcc.gnu.org]
Sent: Dienstag, 30. Dezember 2003 16:36
To: osoentgen@lucent.com
Subject: [Bug c++/13511] Problem with compiler optimization -o2



------- Additional Comments From pinskia at gcc dot gnu dot org  2003-12-30
15:35 -------
The relocations which are 0 in your output will show that the problem is
that you are not reading 
the asm right:
-O1:
        lis r9,ha16(_buffer2)
        la r9,lo16(_buffer2)(r9)
        lis r2,ha16(_buffer+1)
        lbz r2,lo16(_buffer+1)(r2) <--- see the plus one
        extsb r0,r2
        srawi r0,r0,8
        stb r0,12(r9)
        stb r2,13(r9)
        stb r2,10(r9)
        stb r2,11(r9)
        blr
-O2 (the same on the mainline).

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|                            |INVALID
            Summary|Problem with compiler       |Problem with compiler
                   |optimization -o2            |optimization -o2


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=13511

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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=13511