Re: lvalues and rvalues

[David Malcolm <dmalcolm@redhat.com>]

On Sun, 2015-06-14 at 11:08 +0100, Dibyendu Majumdar wrote:
> I am still trying to get the lvalue / rvalue concepts so that I know
> exactly what to use when.
> 
> I have struct with a pointer member.
> 
> struct X {
>  struct Y *base;
> };
> 
> Now this pointer base can be updated while the function is running so
> I need the value of the pointer refreshed (loaded) whenever it can
> potentially change. In LLVM I emit explicit load instructions which
> are then optimized so that any redundant loads are deleted. What would
> be the equivalent method here? Should I just use a rvalue reference?

Just use an gcc_jit_rvalue *: it represents an expression tree.

> Will this be automatically refreshed?

Yes.  libgccjit will generate code for the expression tree.

In theory the generated code will re-evaluate the full expression tree
each time, but some of that code may get optimized away if you've got
optimizations enabled, but only when the optimizer can "know" that it is
safe.  Typically if you have a pointer chain to something that's visible
outside of local scope then the optimizer can't know whether or not
something else is writing to the ptr, so the generated code has to
re-read it each time.

> Should I use a local variable
> and assign the value of the base pointer to it explicitly (i.e.
> similar to explicit load) ?

That's worth doing if you know more than the optimizer can about when
the value can change (e.g. if you're calling some function, and know
that the value can't be changed by it, whereas the optimizer has to
assume that it might touch it).

Local variables are much more "optimizable" than accesses to memory,
especially memory that might get touched in a function call.  GCC
internally uses an SSA representation and can do a lot of
simplifications to uses of local vars.

It may be worth reading:
https://gcc.gnu.org/onlinedocs/jit/intro/tutorial04.html#behind-the-curtain-how-does-our-code-get-optimized

and playing with:

gcc_jit_context_set_bool_option (state.ctxt,
                                 GCC_JIT_BOOL_OPTION_DUMP_EVERYTHING,
                                 1);
gcc_jit_context_set_bool_option (state.ctxt,
                                 GCC_JIT_BOOL_OPTION_KEEP_INTERMEDIATES,
                                 1);

to see what happens to the code you supply.

> Thanks and Regards

Hope the above makes sense
Dave