[LLVMdev] variable sized structs in LLVM

[Reid Spencer]

Its certainly possible to generate .ll files but its probably about the
same amount of work to use the LLVM API and there are significant speed
and validity benefits to doing so.

Here's some ideas on how to handle this situation for the variable sized
structs you need for PyPy:

1. Don't support the feature in your first release (not sure if this is
   viable or not).

2. Make some worst case assumptions about size and alignment (8 bytes)
   and allocate that size, possibly over-allocating memory.

3. Figure out the exact maximum size of the struct so you can declare
   the type correctly.

4. Implement variable sized fields as separately allocated arrays so
   that your example becomes:
    %struct.array = type { int, int, int* }
   The third field is allocated according to the actual size needed.
   This is a bit more code generation but should be fairly efficient.

Reid.

On Tue, 2005-06-21 at 00:40 +0200, Carl Friedrich Bolz wrote:
> Hi Reid,
> 
> Reid Spencer wrote:
> > Yes, for that you need TargetData:
> > 
> > http://illuvium.net/docs/doxygen/classllvm_1_1TargetData.html
> 
> I feared that that would be the answer :-). We're not using the LLVM-API 
> at the moment. We're just generating a .ll file 'by hand', e.g. via 
> simple string operations. I guess we get what we deserve :-).
> 
> Thanks for your help,
> 
> Carl Friedrich
> 
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