Re: JIT compiling with LLVM v9.0

On Wed, Jan 24, 2018 at 11:02 PM, Andres Freund <andres(at)anarazel(dot)de> wrote:
> Not entirely sure what you mean. You mean why I don't inline
> slot_getsomeattrs() etc and instead generate code manually? The reason
> is that the generated code is a *lot* smarter due to knowing the
> specific tupledesc.

I would like to see if we can get a combination of JIT and LTO to work
together to specialize generic code at runtime.

Let's say you have a function f(int x, int y, int z). You want to be
able to specialize it on y at runtime, so that a loop gets unrolled in
the common case where y is small.

1. At build time, create bitcode for the generic implementation of f().
2. At run time, load the generic bitcode into a module (let's call it
the "generic module")
3. At run time, create a new module (let's call it the "bind module")
that only does the following things:
a. declares a global variable bind_y, and initialize it to the value 3
b. declares a wrapper function f_wrapper(int x, int z), and all the
function does is call f(x, bind_y, z)
4. Link the generic module and the bind module together (let's call
the result the "linked module")
5. Optimize the linked module

After sorting out a few details about symbols and inlining, what will
happen is that the generic f() will be inlined into f_wrapper, and it
will see that bind_y is a constant, and then unroll a "for" loop over
y.

I experimented a bit before and it works for basic cases, but I'm not
sure if it's as good as your hand-generated LLVM.

If we can make this work, it would be a big win for
readability/maintainability. The hand-generated LLVM is limited to the
bind module, which is very simple, and doesn't need to be changed when
the implementation of f() changes.

Regards,
Jeff Davis