Re: unwind_stack() and an exception at the last instruction (after the epilogue)

["Dmitry Adamushko" <dmitry.adamushko@gmail.com>]

> Was this example generated by a real world compiler? (Which one?)

[adamushkad@cplx219]/>mips-linux-uclibc-gcc -v
Reading specs from
/vobs/linux/tools/mips/gcc-3.4.2/bin/../lib/gcc/mips-linux-uclibc/3.4.2/specs
Configured with:
/vobs/linux/tools/buildroot/toolchain_build_mips/gcc-3.4.2/configure
--prefix=/vobs/linux/tools/buildroot/build_mips/staging_dir
--build=i386-pc-linux-gnu
--host=i386-pc-linux-gnu
--target=mips-linux-uclibc
--enable-languages=c,c++
--enable-shared
--disable-__cxa_atexit
--enable-target-optspace
-with-gnu-ld
--disable-nls
--enable-multilib
Thread model: posix

gcc version 3.4.2


> I'm inclined to claim the example is broken WRT ABI rules since it
> doesn't enclose the whole user code in the prologue/epilogue bracket.
>

It's o32. So it explicitly requires that when the prologue and
epilogue have been used in the function, all the user code must be
place in between, right?

In this light, the unlike(ofs == 0) in unwind_stack() aims at catching
cases when <sp> is wrong (if addiu sp,sp,OFFSET is normally the very
first instruction)

ok, here is an example from kernel/sched.o (the same compiler).

00000098 <enqueue_task>:
      98:       8c820018        lw      v0,24(a0)   <----- before the prologue
      9c:       27bdfff8        addiu   sp,sp,-8
      a0:       afbe0000        sw      s8,0(sp)
      a4:       000210c0        sll     v0,v0,0x3
      a8:       00a21021        addu    v0,a1,v0
      ac:       24420018        addiu   v0,v0,24
      b0:       8c460004        lw      a2,4(v0)
      b4:       24830020        addiu   v1,a0,32
      b8:       ac430004        sw      v1,4(v0)
      bc:       ac820020        sw      v0,32(a0)
      c0:       ac660004        sw      a2,4(v1)
      c4:       acc30000        sw      v1,0(a2)
      c8:       8c860018        lw      a2,24(a0)
      cc:       24a70004        addiu   a3,a1,4
      d0:       03a0f021        move    s8,sp
      d4:       00061142        srl     v0,a2,0x5
      d8:       00021080        sll     v0,v0,0x2
      dc:       00e23821        addu    a3,a3,v0
      e0:       8ce30000        lw      v1,0(a3)
      e4:       30c6001f        andi    a2,a2,0x1f
      e8:       24020001        li      v0,1
      ec:       00c21004        sllv    v0,v0,a2
      f0:       00621825        or      v1,v1,v0
      f4:       ace30000        sw      v1,0(a3)
      f8:       8ca20000        lw      v0,0(a1)
      fc:       03c0e821        move    sp,s8
     100:       8fbe0000        lw      s8,0(sp)
     104:       24420001        addiu   v0,v0,1
     108:       27bd0008        addiu   sp,sp,8
     10c:       aca20000        sw      v0,0(a1)
     110:       03e00008        jr      ra
     114:       ac850028        sw      a1,40(a0)   <------------
after the epilogue


As I can see, normally this compiler places "addiu   sp,sp,FRAME_SIZE"
at the branch delay slot of "jr ra" but e.g. enqueue_task() (example
above) and request_task() are exceptions. btw, the very first
instruction is also placed before the epilogue.

Are there any configure options that might have caused such a
behaviour [hmmm... e.g. gcc was configured with --ignore-abi-rulles :]
? Although, I don't think this would be an option-dependent case.


> Thiemo
>


-- 
Best regards,
Dmitry Adamushko