-fno-delayed-branch and the bne-instruction on MIPS
Simon Kagstrom
simon.kagstrom@bth.se
Fri Jul 7 06:17:00 GMT 2006
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
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