[kaffe] MIPS JIT3 and null pointers.

[Casey Marshall]

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>>>>> "Kevin" == Kevin D Kissell <kevink at mips.com> writes:

Kevin> Casey Marshall wrote:

>> 2) When it tries to do this when walking back from a `soft' C
>> method, it gets wrong values. In this case, it gets an address in
>> `mips_do_fixup_trampoline' for the previous pc (which I think
>> should be a jitted method instead), and a pointer to an instance of
>> NullPointerException for the previous fp (this should be an address
>> on the stack).  3) The value for the previous frame pointer is
>> invalid since it's way lower than the current frame pointer, so
>> __mipsGetNextFrame fills in pc and fp with zero to mark what it
>> thinks is the end of this stack.

Kevin> The register used as a "frame pointer" in the MIPS calling
Kevin> sequence/ABI, GPR 30, isn't generally used as a frame pointer
Kevin> in MIPS Linux.  It's treated as a ninth "static" register ("s8"
Kevin> in the assembler idiom).

Would this be the case for functions in the Kaffe runtime too?  When
they're compiled with -fno-omit-frame-pointer?

Kevin> Frame pointers aren't strictly speaking necessary if one's
Kevin> compiler and debugger know how to keep track of stack frames
Kevin> on-the-fly, which has been the case with gcc/gdb for a long
Kevin> time now.  If the Kaffe JIT needs to use one, I could
Kevin> understand that, but it should not assume that it's been passed
Kevin> a valid one if invoked from C.  If it wants to create and use
Kevin> one itself, it can use r30, provided that it respects the
Kevin> static character of the register, and saves/restores the value
Kevin> provided at entry to a subroutine.

It looks like the prologue and epilogue do this.

But this makes it sound as though __mipsGetNextFrame might just not
work in general.

- -- 
Casey Marshall || rsdio at metastatic.org
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