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[PowerPC] Add future work for vector insert/extract to README_ALTIVEC.txt

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git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@235309 91177308-0d34-0410-b5e6-96231b3b80d8
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Bill Schmidt 2015-04-20 15:54:26 +00:00
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commit 54f902e367
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lib/Target/PowerPC/README_ALTIVEC.txt
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@ -313,3 +313,17 @@ Opportunies to use instructions from PPCInstrVSX.td during code gen
Related to this: we currently do not generate the lxvw4x instruction for either
v4f32 or v4i32, probably because adding a dag pattern to the recognizer requires
a single target type. This should probably be addressed in the PPCISelDAGToDAG logic.
//===----------------------------------------------------------------------===//
Currently EXTRACT_VECTOR_ELT and INSERT_VECTOR_ELT are type-legal only for v2f64 with VSX available. We should create custom lowering support for the other vector types. Without this support, we generate sequences with load-hit-store hazards.
v4f32 can be supported with VSX by shifting the correct element into big-endian lane 0, using xscvspdpn to produce a double-precision representation of the single-precision value in big-endian double-precision lane 0, and reinterpreting lane 0 as an FPR or vector-scalar register.
v2i64 can be supported with VSX and P8Vector in the same manner as v2f64, followed by a direct move to a GPR.
v4i32 can be supported with VSX and P8Vector by shifting the correct element into big-endian lane 1, using a direct move to a GPR, and sign-extending the 32-bit result to 64 bits.
v8i16 can be supported with VSX and P8Vector by shifting the correct element into big-endian lane 3, using a direct move to a GPR, and sign-extending the 16-bit result to 64 bits.
v16i8 can be supported with VSX and P8Vector by shifting the correct element into big-endian lane 7, using a direct move to a GPR, and sign-extending the 8-bit result to 64 bits.