[LLVMdev] bug in the JIT global variable emitter

Dale Johannesen dalej at apple.com
Mon Oct 13 14:57:19 PDT 2008

On Oct 13, 2008, at 2:42 PMPDT, Nuno Lopes wrote:

> Hi,
> Today I found a nice bug in the JIT global variable emitter.
> The problem may lead to an assert() failure when doing the following:
> 1) compile some function
> 2) emit a global variable
> 3) compile another function. an assert() may trigger in the JIT memory
> manager
> This happens because the JIT global variable emitter is using the
> MachineCodeEmitter::allocate() function, which uses memory allocated  
> by the
> JIT memory manager (which should be used for functions only).

No, this was a deliberate change, 54442.  We have a situation where a  
wants to emit JIT code on one machine, then send it off to another  
machine to
execute.  Putting statically allocated data in the same buffer as code  
is the
easiest approach to make this work, although there may be others.

Is this the known situation that the JIT memory manager doesn't handle  
buffer overflow
well?  You could get into problems with that when it contained only  
code, although
it obviously gets hit more often now.

> The assert()
> is triggered because the global variable is dumped to the header of  
> the free
> blocks list (provided that ThisAllocated becomes 1).
> My proposed patch:
> http://web.ist.utl.pt/nuno.lopes/llvm_jit_global_emitter.txt  
> (similar to
> what other JIT variable emitters do).
> Please comment.
> Thanks,
> Nuno
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