-fno-delayed-branch and the bne-instruction on MIPS

[Simon Kagstrom <simon.kagstrom@bth.se>]

Hello!

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, which
is documented as

     If supported for the target machine, attempt to reorder
     instructions to exploit instruction slots available after delayed
     branch instructions.

I use GCC 3.4.4 from the emdebian project,

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

-fno-delayed-branch seems to mostly work, i.e., I get code as

     10001d0:       27a40010        addiu   a0,sp,16
     10001d4:       0c400008        jal     1000020 <game_loop>
     10001d8:       00000000        nop

which is what I want. However, on conditional branches, it doesn't
work as expected:

     1000160:       8c670000        lw      a3,0(v1)
     1000164:       24020013        li      v0,19
     1000168:       0000000c        syscall
     100016c:       25080001        addiu   t0,t0,1
     1000170:       29020018        slti    v0,t0,24
     1000174:       1440fff9        bnez    v0,100015c <game_init+0x7c>
     1000178:       24630060        addiu   v1,v1,96

Here, I would have expected that the addiu would go before bnez, and
that a nop is inserted after the branch.


So the question is if this is the intended behavior, or if it simply
is a bug with the -fno-delayed-branch implementation? 

// Simon