[Bug rtl-optimization/21182] New: gcc can use registers but uses stack instead

[Bug rtl-optimization/21182] New: gcc can use registers but uses stack instead

[vda at port dot imtp dot ilyichevsk dot odessa dot ua]

in this long but relatively simple function gcc
can store all frequently used local variables in registers,
but it fails to do so.

gcc can be forced to do this optimization by asm("reg") modifiers.
Resulting code is ~1k smaller.

# gcc -v
Reading specs from
/.share/usr/app/gcc-3.4.3/bin/../lib/gcc/i386-pc-linux-gnu/3.4.3/specs
Configured with: ../gcc-3.4.3/configure --prefix=/usr/app/gcc-3.4.3
--exec-prefix=/usr/app/gcc-3.4.3 --bindir=/usr/bin --sbindir=/usr/sbin
--libexecdir=/usr/app/gcc-3.4.3/libexec --datadir=/usr/app/gcc-3.4.3/share
--sysconfdir=/etc --sharedstatedir=/usr/app/gcc-3.4.3/var/com
--localstatedir=/usr/app/gcc-3.4.3/var --libdir=/usr/lib
--includedir=/usr/include --infodir=/usr/info --mandir=/usr/man
--with-slibdir=/usr/app/gcc-3.4.3/lib --with-local-prefix=/usr/local
--with-gxx-include-dir=/usr/app/gcc-3.4.3/include/g++-v3
--enable-languages=c,c++ --with-system-zlib --disable-nls --enable-threads=posix
i386-pc-linux-gnu
Thread model: posix
gcc version 3.4.3

-- 
           Summary: gcc can use registers but uses stack instead
           Product: gcc
           Version: 3.4.3
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: rtl-optimization
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: vda at port dot imtp dot ilyichevsk dot odessa dot ua
                CC: gcc-bugs at gcc dot gnu dot org
 GCC build triplet: i386-pc-linux-gnu
  GCC host triplet: i386-pc-linux-gnu
GCC target triplet: i386-pc-linux-gnu


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=21182

[Bug rtl-optimization/21182] gcc can use registers but uses stack instead

[vda at port dot imtp dot ilyichevsk dot odessa dot ua]

------- Additional Comments From vda at port dot imtp dot ilyichevsk dot odessa dot ua  2005-04-23 22:32 -------
Created an attachment (id=8719)
 --> (http://gcc.gnu.org/bugzilla/attachment.cgi?id=8719&action=view)
testcase. change #if 0 into #if 1 and compare resulting asm


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=21182

[Bug rtl-optimization/21182] gcc can use registers but uses stack instead

[pinskia at gcc dot gnu dot org]

------- Additional Comments From pinskia at gcc dot gnu dot org  2005-04-23 22:39 -------
Hmm, on the mainline, we get for wc -l:
 1613 t.s
 1459 t1.s

t1 is the normal #if 0.

Note I used "-O2 -fomit-frame-pointer".

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |missed-optimization, ra


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=21182

[Bug rtl-optimization/21182] gcc can use registers but uses stack instead

[vda at port dot imtp dot ilyichevsk dot odessa dot ua]

------- Additional Comments From vda at port dot imtp dot ilyichevsk dot odessa dot ua  2005-04-23 22:49 -------
Aha!
I found out that gcc will use registers with -O3, but not with -O2.

# gcc -O3 serpent.c -S -o serpent-O3.s
# gcc -O2 serpent.c -S -o serpent-O2.s
# ls -l
-rw-r--r--  1 root root 27975 Apr 24 01:47 serpent-O2.s
-rw-r--r--  1 root root 21566 Apr 24 01:47 serpent-O3.s
# wc -l serpent-O2.s serpent-O3.s
 1558 serpent-O2.s
 1265 serpent-O3.s
 2823 total

I don't have 4.0.0 here yet...

-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=21182

[Bug rtl-optimization/21182] gcc can use registers but uses stack instead

[vda at port dot imtp dot ilyichevsk dot odessa dot ua]

------- Additional Comments From vda at port dot imtp dot ilyichevsk dot odessa dot ua  2005-04-23 22:54 -------
These are -O2 and -O3 code comparison.
-O3 code have all modified variables in registers
and thus is smaller and most likely faster.

serpent_encrypt:
        pushl   %ebp
        movl    %esp, %ebp
        pushl   %edi
        pushl   %esi
        pushl   %ebx
        subl    $256, %esp
        movl    8(%ebp), %edx
        movl    16(%ebp), %eax
        movl    12(%eax), %ebx
        movl    12(%edx), %ecx
        xorl    %ebx, %ecx
        movl    (%edx), %edi
        movl    %ecx, -20(%ebp)
        xorl    (%eax), %edi
        movl    8(%edx), %ecx
        movl    4(%edx), %ebx
        movl    -20(%ebp), %esi
        xorl    8(%eax), %ecx
        orl     %edi, -20(%ebp)
        xorl    4(%eax), %ebx
        xorl    %ebx, -20(%ebp)
        xorl    %esi, %edi
        xorl    %ecx, %esi
        andl    %edi, %ebx
        xorl    %edi, %ecx
        notl    %esi
        xorl    -20(%ebp), %edi
        movl    %edx, -16(%ebp)

serpent_encrypt:
        pushl   %ebp
        movl    %esp, %ebp
        pushl   %edi
        pushl   %esi
        pushl   %ebx
        pushl   %edx
        movl    8(%ebp), %edi
        movl    16(%ebp), %ecx
        movl    12(%edi), %eax
        xorl    12(%ecx), %eax
        movl    8(%edi), %esi
        movl    4(%edi), %edx
        movl    (%edi), %ebx
        xorl    8(%ecx), %esi
        xorl    4(%ecx), %edx
        xorl    (%ecx), %ebx
        movl    %eax, %ecx
        orl     %ebx, %ecx
        xorl    %eax, %ebx
        xorl    %esi, %eax
        xorl    %edx, %ecx
        notl    %eax
        andl    %ebx, %edx
        xorl    %eax, %edx
        xorl    %ebx, %esi
        xorl    %ecx, %ebx
        orl     %ebx, %eax
        xorl    %esi, %ebx
        andl    %edx, %esi
        xorl    %esi, %eax
        notl    %edx



-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=21182

[Bug rtl-optimization/21182] gcc can use registers but uses stack instead

[vda at port dot imtp dot ilyichevsk dot odessa dot ua]

------- Additional Comments From vda at port dot imtp dot ilyichevsk dot odessa dot ua  2005-04-24 13:05 -------
With 4.0.0: gcc -O2 gives the same result as gcc -O3,
which is better than gcc 3.4.3 -O2 but worse than 3.4.3 -O3.
For example:

        movl    %edx, -20(%ebp)
        orl     %ecx, %edi
        movl    %ebx, %esi
        xorl    %ecx, %esi
        andl    %eax, %ebx
        xorl    %edi, %ebx
        movl    %eax, %ecx
        notl    %ecx
        xorl    %ebx, %ecx
        orl     %edi, %eax
        xorl    %eax, %esi
        rorl    $19, %esi
        rorl    $29, -20(%ebp)
        xorl    %esi, %ebx
        xorl    -20(%ebp), %ecx
        xorl    -20(%ebp), %ebx
        rorl    $31, %ebx
        leal    0(,%esi,8), %edx

1) Why %edx was stored in -20(%ebp), there is no %edx usage
in the following insns. %edx value could stay in register
and we can continue to work on its value in register.
2) rorl $31, %ebx == roll $1, %ebx, but 1 bit roll insn is
smaller.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=21182

[Bug rtl-optimization/21182] gcc can use registers but uses stack instead

[steven at gcc dot gnu dot org]

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
     Ever Confirmed|                            |1
   Last reconfirmed|0000-00-00 00:00:00         |2005-05-07 15:23:12
               date|                            |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=21182