Basic blocks, jump targets and locals

[David Malcolm]

On Thu, 2015-06-11 at 21:26 +0100, Dibyendu Majumdar wrote:
> Hi,
> 
> In LLVM the IRBuilder has the concept of a current block where
> instructions are inserted. Reading the libgccjit docs I think that
> there is no notion of current block - i.e. this needs to be maintained
> by the caller - is that correct?

Correct.

> Can one create several blocks in advance - and set a block that is not
> yet inserted into the function as the target for a jump? 

Yes.  See e.g.:
https://gcc.gnu.org/onlinedocs/jit/intro/tutorial04.html#populating-the-function
which does two passes:
(i) iterates through all of the bytecodes in a function, creating
gcc_jit_block instances for each one
(ii) iterates again, populating the gcc_jit_block instances with
statements

> The scenario
> is this: when compiling Lua bytecode I already know all the jump
> targets. So I pre-create all the basic blocks that relate to the jump
> targets up front - and as each jump is encountered I can easily set
> the jump target to the right block. As and when I cross the bytecode
> that is a jump target the block gets added to the function.

(nods)

> I hope above is clear - if not I can explain by an example.
> 
> Can I do something similar with libgccjit?

Hopefully the example above makes it clear.

> Finally - how are local variable related to blocks. In LLVM one has to
> create the local vars in the entry block of the function. Is it the
> same with libgccjit?

Local variables in libgccjit are local to a *function*; there's no
association between them and blocks.  Blocks in libgccjit relate to
control flow, not to scope.

Note that any initialization of a local variable has to be done
explicitly, by adding an assignment statement.  Typically that
assignment statement would be added to the initial block, as in the
example above (so I suppose that's an association of sorts between
locals and the entry block).

Hope this is clear
Dave