[cfe-dev] [RFC][OpenMP][CUDA] Unified Offloading Support in Clang Driver
Hal Finkel via cfe-dev
cfe-dev at lists.llvm.org
Mon Mar 7 17:11:38 PST 2016
----- Original Message -----
> From: "C Bergström via cfe-dev" <cfe-dev at lists.llvm.org>
> To: "Justin Lebar" <jlebar at google.com>
> Cc: "Alexey Bataev" <a.bataev at hotmail.com>, "C Bergström via cfe-dev" <cfe-dev at lists.llvm.org>, "Samuel F Antao"
> <sfantao at us.ibm.com>, "John McCall" <rjmccall at gmail.com>
> Sent: Monday, March 7, 2016 6:46:57 PM
> Subject: Re: [cfe-dev] [RFC][OpenMP][CUDA] Unified Offloading Support in Clang Driver
> #1 OMP allows multiple code generation, but doesn't *require* it. It
> wouldn't be invalid code if you only generated for a single target at
> a time - which imho isn't that unreasonable. Why?! It's unlikely that
> a user is going to try to build a single binary that runs across
> different supercomputers. It's not like what ANL is getting will be a
> mix (today/near future) of PHI+GPU. It will be a PHI shop.. ORNL is a
> GPU shop. The only burden is the user building the source twice (not
> that hard and what they do today anyway)
I agree, but supercomputers are not the only relevant platforms. Lot's of people have GPUs, and OpenMP offloading can also be used on various kinds of heterogeneous systems. I see no reason to design, at the driver level, for only a single target device type unless doing more is a significant implementation burden.
> #2 This proposed tarball hack/wrapper thingie is just yuck design
> imho. I think there are better and more clean long term solutions
> #3 re: "ARM ISA and Thumb via a runtime switch. As a result, objdump
> has a very difficult time figuring out how to disassemble code that
> uses both ARM and Thumb."
> My proposed solution of prefixing/name mangling the symbol to include
> "target" or optimization level solves this. It's almost exactly as
> what glibc (I spent 15 minutes looking for this doc I've seen before,
> but couldn't find it.. if really needed I'll go dig in the glibc
> sources for examples - the "doc" I'm looking for could be on the
> loader side though)
> In the meantime there's also this
> "For x86_64 vector functions names are created based on #2.6. Vector
> Function Name Mangling from Vector ABI"
> Which explicitly handles this case and explicitly mentions OMP.
> "Vector Function ABI provides ABI for vector functions generated by
> compiler supporting SIMD constructs of OpenMP 4.0 ."
> it may also be worthwhile looking at existing solutions more closely
> "The target attribute is used to specify that a function is to be
> compiled with different target options than specified on the command
> line. This can be used for instance to have functions compiled with a
> different ISA"
> Side note on ARM - Their semi-unified ISA is actually the "right way"
> to go. It's imho a good thing to have the vector or gpu instructions
> unified as a direct extension to the scalar stuff. I won't go into
> level details why, but in the end that design would win over one
> unified memory is possible, but separate load/store is required for
> left side to talk to right side. (iow ARM thumb is imho not the
> problem.. it's objdump - anyway objdump can't handle nvidia saas/ptx
> blobs so it's probably more a "nice to have" instead of absolute
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