[LLVMdev] PHI and Allocas

[Kenneth Uildriks]

On Thu, Oct 1, 2009 at 4:35 PM, Renato Golin <rengolin at systemcall.org> wrote:
> Hi,
>
> I'm incrementing my toy compiler to account for variable mutations
> around if/else blocks.
>
> Reading the Kaleidoscope tutorial, I've learnt that I can substitute
> the PHI node creation by allocas and retrieving the value later, if
> needed. The Kaleidoscope example seems to account for one value only,
> the result of:
>
> Value *ThenV = Then->Codegen();
> (...)
> Value *ElseV = Else->Codegen();
> (...)
> PN->addIncoming(ThenV, ThenBB);
> PN->addIncoming(ElseV, ElseBB);
>
> But both Then and Else are expressions, so the Value returned is in
> the form of a single variable.
>
> ExprAST *Then = ParseExpression();
> (...)
> ExprAST *Else = ParseExpression();
>
> In my toy language, I accept any number of statements inside a block.
>
> Ignoring nested if statements, should I keep track of *each* variable
> mutation inside the block, or is there some LLVM magic (as I'm getting
> used to)?
>
> A simple C example would be:
>
> int a, b, c, d;
> (...)
> if (a > 10) {
>  b = c = d = 10 * a;
> } else {
>  b = c = d = 10 / a;
> }
>
> As I'm always using alloca, the mem2reg pass seems to do the trick,
> but I'm not sure it would in all cases. The tutorial states that it
> works on all scalar values (I have no structs or arrays), so I believe
> I'm safe.
>
> Am I?

I hope so, because I've been doing the same thing.  My local variables
are always bound to alloca's, and my test makefile just calls "opt
-std-compile-opts" after my compiler runs; so far it's been giving me
pretty good code as a result.