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

[Ian Lance Taylor <iant@google.com>]

Simon Kagstrom <simon.kagstrom@bth.se> writes:

> 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? 

This looks like a bug to me.  You may want to open a bug report; see
    http://gcc.gnu.org/bugs.html

I suspect the bug is that -fdelayed-branch is failing to pass -O0 to
the assembler.  Try compiling with -Wa,-O0.

Ian