[LLVMdev] Issues with IPO optimization passes and JIT

[Owen Anderson]

The general issue is that IPO can do very little safely unless the  
entire program is already present.  As Nicholas mentioned, the  
Internalize pass is used to mark that this has happened, allowing the  
IPO passes to make assumptions that would not be safe otherwise.  If  
you're generating code a function at a time, you should not be running  
internalize, in which case running IPO passes is probably also a waste.

--Owen

On Jan 23, 2008, at 12:55 AM, Samit Basu wrote:

> Hello,
>
> I am working on an LLVM-based JIT for a dynamically typed language
> (freemat.sf.net), and would like to commend the LLVM team on an
> awesome piece of work.  One issue I ran into that I was hoping for
> some clarification.  Nominally, I had started out by performing code
> generation from the AST into a function that was added to the current
> module.  I then ran a PassManager on that module to optimize the
> function -- this included several IPO passes (the entire list from -O3
> for opt, in fact).  I ran the optimizer after each function was
> generated.   I found that when I tried to execute functions via the
> JIT, I got unexpected and incorrect results -- for example, calling
> Invoke on a function would execute the wrong function.  The only
> solution was to replace the PassManager with a FunctionPassManager
> (following the example in the tutorial), and keep only basic block and
> function-level optimization steps.  Is this expected behavior?  Is it
> documented?  Should it be considered a bug?
>
> Thanks in advance for any clarification.
> Samit
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