Bug in Jitter
oliva at dcc.unicamp.br
Tue Jul 7 15:27:32 PDT 1998
Godmar Back <gback at cs.utah.edu> writes:
>> > I suspect the proper fix would be to ensure that all live
>> > registers are written to memory whenever an exception, including a
>> > null pointer exception, may occur.
> Tim wrote about it over a year ago:
> This is a JIT related bug. Essentially the value of 'i' is in a
> register when the exception occurs (SEGV in this case) and is lost by
> the time the relevant handler is found.
AFAIK, the only case that may raise this kind of exception is
NullPointerException. Any other case would be the result of calling a
soft_* function or a method, and this would have spilled all registers
before the call. Thus, I see two alternatives:
1) #define CHECK_NULL(a,b,c) sync_registers()
unless it is already defined in some platform-specific
This would ensure that any registers are saved in the stack before
a NullPointerException may be thrown, with a very small performance
impact. Only registers that are dirty and would be overwritten in
the near future will be unnecessarily spilled; others may be
written to the stack frame earlier and marked as clean, so they
won't be spilled any more.
sync_registers() does not exist on the intrp, so it may be defined
to nothing in this case. Furthermore, on x86, a patch that ensures
that sync_registers() does not remove a floating-point value from
the top of the FP stack must be installed.
2) have the SIGSEGV/SIGBUS signal catcher save registers where they
belong. But I'm not sure this is doable, anyway.
I strongly favor suggestion 1.
mailto:oliva at dcc.unicamp.br mailto:aoliva at acm.org
Universidade Estadual de Campinas, SP, Brasil
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