[LLVMdev] [RFC] Proposal to make LLVM-IR endian agnostic

[Villmow, Micah]

> -----Original Message-----
> From: llvmdev-bounces at cs.uiuc.edu [mailto:llvmdev-bounces at cs.uiuc.edu]
> On Behalf Of Török Edwin
> Sent: Monday, October 03, 2011 2:00 PM
> To: llvmdev at cs.uiuc.edu
> Subject: Re: [LLVMdev] [RFC] Proposal to make LLVM-IR endian agnostic
> 
> On 10/03/2011 09:36 PM, Villmow, Micah wrote:
> > One of the projects I am working on with others is to make LLVM-IR
> endian agnostic.
> >
> >
> >
> > So, I am sending out this proposal for feedback to the LLVM
> community. I've attached
> >
> > pretty version of the proposal in PDF format and pasted a 80-column
> safe text version
> >
> > below.
> >
> >
> >
> > A second smaller set could be:
> >
> > declare <type> @llvm.portable.load.<type>(<type>* ptr, i32 alignment,
> >
> > i1 host, i1 littleEndian, i1 atomic, i1 volatile,
> >
> > i1 nontemporal, i1 singlethread)
> >
> >
> >
> > declare void @llvm.portable.store.<type>(<type> data, <type>* ptr,
> >
> > i32 alignment, i1 host, i1 littleEndian, i1 atomic, i1 volatile,
> >
> > i1 nontemporal, i1 singlethread)
> 
> FWIW here is another way to do it (which is approximately what ClamAV
> does currently) by introducing just one intrinsic:
> declare i1 @llvm.is_bigendian()
[Villmow, Micah] I think the big difference in our requirements is that we can have both big endian(host) and little endian(device), or vice versa, accesses to the same pointer. So a global is_bigendian intrinsic would not work for what we are attempting to accomplish.
> 
> The advantage is that you can implement htonl() and ntohl() like
> functionality without using a temporary memory location.
> Actually I think having the 2 intrinsics you suggest and the
> is_bigendian() intrinsic would be optimal:
> you can use your 2 intrinsics for initial codegen, and mem2reg can
> transform it to is_bigendian().
> 
> For load/store:
> <type> %val = load <type>* %ptr
> <type> %sval = bswap.i<type> %val
> %result = <type> select @llvm.is_bigendian(), %val, %sval
> 
> For htonl():
> <type> %sval = bswap.i<type> %val
> %result = <type> select @llvm.is_bigendian(), %val, %sval
> 
> (store is similar, byteswap before the store)
> 
> At bytecode JIT time / assembly emission time @llvm.is_bigendian() is a
> known constant, and constant propagation is
> used to throw away the unwanted code path, so it becomes either:
> 
> <type> %result = load <type>* %ptr
> 
> or
> 
> <type> %val = load <type>* %ptr
> <type> %result = bswap.i<type> %val
> 
> Best regards,
> --Edwin
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