On Mon, 2009-08-31 at 09:49 +0100, José Fonseca wrote: 
> On Mon, 2009-08-31 at 01:14 -0700, Michel Dänzer wrote:
> 
> > BTW, unfortunately LLVM 2.5 can't seem to handle llvmpipe on Linux/PPC.
> > I'm attaching the full stderr output from glxgears, but the upshot is:
> > 
> > SplitVectorOperand Op #0: 0x10540ae8: f32 = select_cc 0x1052baf0, 
> > 0x1052c590, 0x1052c508, 0x10540a60, 0x1050c998
> > glxgears: 
> > /build/buildd/llvm-2.5/lib/CodeGen/SelectionDAG/LegalizeVectorTypes.cpp:861:
> >  bool llvm::DAGTypeLegalizer::SplitVectorOperand(llvm::SDNode*, unsigned 
> > int): Assertion `0 && "Do not know how to split this operator's operand!"' 
> > failed.
> > 
> > But this is most likely an LLVM issue, so don't worry about it unless
> > you have an idea for a workaround offhand.
> 
> Yes, this is a known LLVM limitation for all targets -- it can represent
> selection (like the C '?' ternary operator) of vectors but no machine
> target can actually support it yet. The temporary solution is to emit
> SIMD intrinsics for comparison directly. If you can somehow implement
> lp_bld_logic.c's lp_build_cmp() function in terms of altivec intrinsics
> then it should work.
> 
> Another last-resource solution is to manually break the vector
> comparison in element-wise comparisons, but performance will suck
> drastically, since we use these a lot for color saturation/clamping.
> 
> There are not many, but there are at least a couple other corner cases
> like this -- where vanilla LLVM IR isn't good enough. But I didn't spot
> any other in your log. 

Thanks for the pointers José! This is too involved for me right now, but
maybe I'll get back to it sometime.


-- 
Earthling Michel Dänzer           |                http://www.vmware.com
Libre software enthusiast         |          Debian, X and DRI developer

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