Add additional documentation to the extract-and-trunc dagcombine optimization.
llvm-svn: 149823
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@ -4985,9 +4985,14 @@ SDValue DAGCombiner::visitTRUNCATE(SDNode *N) {
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return N0.getOperand(0);
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}
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// Fold Extract-and-trunc into a narrow extract:
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// trunc(extract(x)) -> extract(bitcast(x))
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// We only run this optimization after type legalization (which often
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// Fold extract-and-trunc into a narrow extract. For example:
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// i64 x = EXTRACT_VECTOR_ELT(v2i64 val, i32 1)
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// i32 y = TRUNCATE(i64 x)
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// -- becomes --
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// v16i8 b = BITCAST (v2i64 val)
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// i8 x = EXTRACT_VECTOR_ELT(v16i8 b, i32 8)
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//
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// Note: We only run this optimization after type legalization (which often
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// creates this pattern) and before operation legalization after which
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// we need to be more careful about the vector instructions that we generate.
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if (N0.getOpcode() == ISD::EXTRACT_VECTOR_ELT &&
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