[Bug c/28325] New: -fno-delayed-branch does not seem to work with the MIPS branch instructions

["ska at bth dot se" <gcc-bugzilla at gcc dot gnu dot org>]

I'm working on a binary translator that translates MIPS binaries into Java
bytecode (http://spel.bth.se/index.php/Cibyl) with the goal of "recompiling" C
programs to run on J2ME.  To simplify things, I use compile programs for a
subset of the MIPS1 instruction set, and ideally I would like to get rid of
delayed instructions.

So, I used the -fno-delayed-branch instruction when compiling. The GCC version
is

mips-linux-gcc (GCC) 3.4.4 20050314 (prerelease) (Debian 3.4.3-13)

(an emdebian.org binary), and the command line i used is

mips-linux-gcc -Wall -mips1 -fno-delayed-branch -O2 -fno-pic -mno-abicalls
-I/home/ska/projects/private/games/mophun/cibyl/trunk//include  -O2 -c main.c
-o main.o

For the code below,

#define _syscall1(type,name,atype,a) \
type name(atype a) \
{ \
        register unsigned long __a0 asm("$4") = (unsigned long) a; \
        register unsigned long __v0 asm("$2"); \
        \
        __asm__ volatile ( \
        ".set\tnoreorder\n\t" \
        "li\t$2, %2\t\t\t# " #name "\n\t" \
        "syscall\n\t" \
        ".set\treorder" \
        : "=&r" (__v0) \
        : "r" (__a0), "i" (__NR_##name) \
        ); \
        \
        return (type) __v0; \
}
#define __NR_exit 0
static inline _syscall1(void,exit , int, code );

int tst(int *argv)
{
  int x;

  for (x = 0; x < 24; x++)
    exit(argv[x]);

  return 0;
}

I get instructions in the delay slots of bgez and jr:

00000000 <tst>:
   0:   00801821        move    v1,a0
   4:   24050017        li      a1,23
   8:   8c640000        lw      a0,0(v1)
   c:   24020000        li      v0,0
  10:   0000000c        syscall
  14:   24a5ffff        addiu   a1,a1,-1
  18:   04a1fffb        bgez    a1,8 <tst+0x8>
  1c:   24630004        addiu   v1,v1,4
  20:   03e00008        jr      ra
  24:   00001021        move    v0,zero
        ...

Which is not what I had expected. If i replace the "syscall" exit with a
function call instead, I get the expected behavior:

00000000 <exit3>:
   0:   08000000        j       0 <exit3>
   4:   00000000        nop

00000008 <tst>:
   8:   27bdffe0        addiu   sp,sp,-32
   c:   afb10014        sw      s1,20(sp)
  10:   afb00010        sw      s0,16(sp)
  14:   afbf0018        sw      ra,24(sp)
  18:   00808021        move    s0,a0
  1c:   24110017        li      s1,23
  20:   8e040000        lw      a0,0(s0)
  24:   2631ffff        addiu   s1,s1,-1
  28:   0c000000        jal     0 <exit3>
  2c:   00000000        nop
  30:   26100004        addiu   s0,s0,4
  34:   0621fffa        bgez    s1,20 <tst+0x18>
  38:   00000000        nop
  3c:   8fbf0018        lw      ra,24(sp)
  40:   8fb10014        lw      s1,20(sp)
  44:   8fb00010        lw      s0,16(sp)
  48:   00001021        move    v0,zero
  4c:   27bd0020        addiu   sp,sp,32
  50:   03e00008        jr      ra
  54:   00000000        nop
        ...


-- 
           Summary: -fno-delayed-branch does not seem to work with the MIPS
                    branch instructions
           Product: gcc
           Version: 3.4.3
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: ska at bth dot se


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=28325