[llvm-commits] [llvm] r150060 - in /llvm/trunk: include/llvm/IntrinsicsX86.td lib/Target/X86/X86ISelLowering.cpp lib/Target/X86/X86InstrSSE.td

[David A. Greene]

Duncan Sands <baldrick at free.fr> writes:

>> Intrinsics are used by users and we want to respect that.
>>
>> The builtins are useful for running the C backend to get something
>> compilable by gcc.  This is very helpful for debugging.
>
> once auto-upgrade support for these intrinsics goes in, won't everything be
> fine?  Old bitcode using the intrinsics will be turned into generic LLVM IR
> (shuffles).  The C backend should turn this into valid C, though indeed not
> into the GCC builtins anymore. 

My experience with the C backend says otherwise.  I've run across lots
of cases where shuffles could not be handled.  Remember, gcc is not the
only vectorizer out there.

That's part of the reason I started to write a new C backend.  Though
that's stalled at the moment, it's still on my TODO list.

>  Frontends can pretty easily just generate the generic IR rather than
> the intrinsics (at least I hope it's easy, since I'm going to have to
> do this in my front-end).

I don't know of any way to generate generic IR and guarantee that the
instruction the user selected through an intrinsic will be the
instruction selected by LLVM's isel.  There are too many transformation
steps in-between.

                             -Dave