[LLVMdev] JIT allocates global data in function body memory

[Andrew Haley]

Dale Johannesen wrote:
> On Jun 30, 2009, at 11:18 AMPDT, Jeffrey Yasskin wrote:
> 
>> On Mon, Jun 29, 2009 at 5:50 PM, Dale Johannesen<dalej at apple.com>  
>> wrote:
>>> On Jun 29, 2009, at 5:41 PMPDT, Reid Kleckner wrote:
>>>
>>>> So I (think I) found a bug in the JIT:
>>>> http://llvm.org/bugs/show_bug.cgi?id=4483
>>>>
>>>> Basically, globals used by a function are allocated in the same  
>>>> buffer
>>>> as the first code that uses it.  However, when you free the machine
>>>> code, you also free the memory holding the global's data.  The  
>>>> address
>>>> is still in the GlobalValue map, so any other code using that global
>>>> will access freed memory, which will cause problems as soon as you
>>>> reallocate that memory for something else.
>>>>
>>>> I tracked down the commit that introduced the bug:
>>>> http://llvm.org/viewvc/llvm-project?view=rev&revision=54442
>>>>
>>>> It very nicely explains what it does, but not why it does it, which
>>>> I'd like to know before I change it.  I couldn't find the author
>>>> (johannes) on IRC so ssen told me to ask LLVMdev about this  
>>>> behavior.
>>> That's me (and I'm not on IRC because I like messages to be
>>> archived).  The reason everything needs to go in the same buffer is
>>> that we're JITting code on one machine, then sending it to another to
>>> be executed, and references from one buffer to another won't work in
>>> that environment.  So that model needs to continue to work.  If you
>>> want to generalize it so other models work as well, go ahead.
>> So, you're moving code across machines without running any relocations
>> on it? How can that work? Are you just assuming that everything winds
>> up at the same addresses? Or is everything PC-relative on your
>> platform, so all that matters is that globals and the code are in the
>> same relative positions?

I presume (hope, really) that we don't end up with code and data in the
same page.  From
Intel® 64 and IA-32 Architectures Optimization Reference Manual:
Assembly/Compiler Coding Rule 57. (H impact, L generality) Always put
code and data on separate pages.

Sorry, I guess you know this already.

Andrew.