[LLVMdev] Explicitly Freeing Allocas

[Olivier Meurant]

In the TargetData class (available from you ExecutionEngine), you have some
informations available (such as StructLayout...).


On Thu, Jun 18, 2009 at 5:08 PM, Nyx <mcheva at cs.mcgill.ca> wrote:

>
> That sounds rather cumbersome, is there no simpler way to get the actual
> size
> of a struct?
>
>
> John Criswell wrote:
> >
> > Nyx wrote:
> >> Hello,
> >>
> >> I would just like to ask if it's possible to explicitly free allocas.
> >> This
> >> is because I need to call functions that take structs of different sizes
> >> as
> >> input, (possibly inside of loops) and I would rather avoid a stack
> >> overflow.
> >>
> > No, it's not legal to free memory returned by alloca.  Such memory is
> > allocated on the stack; the code generator usually converts this to an
> > adjustment of the stack pointer on function entry whenever possible.
> >> If this is not possible, an alternate solution would be for me to
> >> allocate
> >> an array of bytes larger than all the struct types I may be using, and
> >> cast
> >> that pointer to the right type each type. In that case, I would like to
> >> know
> >> how I can find out what the size of a struct is. I've heard of people
> >> talking about using "the right target data", but I have no idea how to
> >> actually do that, and I find the doxygen info doesn't make it any
> >> clearer.
> >>
> > This sounds like your best option.  The TargetData pass is a pass that
> > informs other passes of  machine specific information and can, for
> > example, give you the number of bytes used to store a type.
> >
> > Read the document on Writing an LLVM Pass
> > (http://llvm.org/docs/WritingAnLLVMPass.html) to learn how to have your
> > pass declare the TargetData pass as a prerequisite and to get a pointer
> > to it when your
> > runOnFunction/runOnModule/runOnBasicBlock/runOn<whatever> method is
> > called.  Then use the doxygen documentation to find the correct method
> > of the TargetData pass
> > (http://llvm.org/doxygen/classllvm_1_1TargetData.html) to use to get the
> > size of the type that you allocated via alloca.
> >
> > -- John T.
> >
> >> Thank you for your help,
> >>
> >> - Max
> >>
> >
> > _______________________________________________
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> > LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
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> >
> >
>
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