[LLVMdev] Intrinsic support

[Duncan Sands]

Hi Kenneth,

>>> I have no idea how to make use of llvm.invariant to help
>>> devirtualization. If no use or other use case for them can be found,
>>> maybe they should be removed.
>>
>> the dragonegg plugin uses llvm.invariant to help the code generators
>> optimize the code it produces for builtin_init_trampoline.  It writes
>> some values into memory which is then constant forever after.  Because
>> pointers to this memory are passed into all kinds of functions, the
>> optimizers don't understand that the values are constant without help,
>> thus the use of llvm.invariant.  It doesn't work very well, but it is
>> still better than not doing it at all.
>
> I suppose the LLVM trampoline intrinsics aren't well supported?  Or do
> they just not fit your use case?

the LLVM trampoline intrinsic works fine, but doesn't work quite the same as
the GCC builtins: GCC has two trampoline builtins that LLVM combines into one
intrinsic.  In llvm-gcc this "impedance mismatch" was overcome by tweaking the
nested function lowering code, but in dragonegg I don't have the luxury of
modifying GCC's code, so I had to instead emit a bunch of ugly fixup code when
lowering the GCC intrinsics.  The optimizers find it hard to clean up the
resulting mess without help.

Ciao,

Duncan.