Turns out LLVM has insertvalue & extractvalue for struct in registers. This is
faster than pointers because null checks are now subject to more optimizations.
Given that the list of supported types is hardcoded in
LLVMContext::Data::toNativeType, this method is redundant because
LLVMPreparedFunction can create a ColumnVector itself.
Not actually implemented, though. It does print out some jit-compiled stuff,
but that's about it. For example, this query:
select number from system.numbers where something(cast(number as Float64)) == 4
results in this on server's stderr:
define double @"something(CAST(number, 'Float64'))"(void**, i8*, void*) {
"something(CAST(number, 'Float64'))":
ret double 1.234500e+04
}
(and an exception, because that's what the non-jitted method does.)
As one may notice, this function neither reads the input (first argument;
tuple of arrays) nor writes the output (third argument; array), instead
returning some general nonsense.
In addition, `#if USE_EMBEDDED_COMPILER` doesn't work for some reason,
including LLVM headers requires -Wno-unused-parameter, this probably only
works on LLVM 5.0 due to rampant API instability, and I'm definitely
no expert on CMake. In short, there's still a long way to go.
