[LLVMdev] Not allowed to reuse variables?

[Chris Lattner]

On Wed, 5 May 2004, Misha Brukman wrote:

> On Wed, May 05, 2004 at 03:35:46PM +0200, Anders Alexandersson wrote:
> > %tmpFunction = load int ()** %puts_kernelPTR
> > ...
> > %tmpFunction = load int ()** %puts_kernelPTR
> >
> > generates
> >
> >  Redefinition of value named 'tmpFunction' in the 'int () *' type plane!
> >
> > Is it not allowed to reuse variables? Is there some way to do it?
>
> If this is in a loop and you MUST use the same variable name, then what
> you need is a 'phi' node which merges multiple values across control
> flow edges into one variable. An example: if you compile this function

Note that if you're working on a front-end though, that you shouldn't
worry about creating PHI nodes at all.  Typically you should just stack
allocate any variables that can be modified, and use load/store
instructions to access them.  The LLVM optimizer will promote the stack
locations to SSA form (registers) for you.

Thus, if you have code like this:

  X = A + B;

You should generate code that looks like this:

  %X = alloca int
  ...

  %tmp1 = load int* %A
  %tmp2 = load int* %B
  %tmp3 = add int %tmp1, %tmp2
  store int %tmp3, int* %X

Note that this is really ugly, but is usually all eliminated by the
optimizers, and makes the front-end much easier to handle. :)

-Chris

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