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

To: "Thiemo Seufer" <>
Subject: Re: unwind_stack() and an exception at the last instruction (after the epilogue)
From: "Dmitry Adamushko" <>
Date: Wed, 13 Dec 2006 13:45:55 +0100
Cc: "Ralf Baechle" <>,
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Was this example generated by a real world compiler? (Which one?)

[adamushkad@cplx219]/>mips-linux-uclibc-gcc -v
Reading specs from
Configured with:
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.


Best regards,
Dmitry Adamushko

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