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Add additional documentation to the extract-and-trunc dagcombine optimization.

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git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@149823 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Nadav Rotem 2012-02-05 11:39:23 +00:00
parent d52d9cd038
commit cc870a8f93
1 changed files with 8 additions and 3 deletions
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11
lib/CodeGen/SelectionDAG/DAGCombiner.cpp
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@ -4985,9 +4985,14 @@ SDValue DAGCombiner::visitTRUNCATE(SDNode *N) {
return N0.getOperand(0);
}
// Fold Extract-and-trunc into a narrow extract:
// trunc(extract(x)) -> extract(bitcast(x))
// We only run this optimization after type legalization (which often
// Fold extract-and-trunc into a narrow extract. For example:
// i64 x = EXTRACT_VECTOR_ELT(v2i64 val, i32 1)
// i32 y = TRUNCATE(i64 x)
// -- becomes --
// v16i8 b = BITCAST (v2i64 val)
// i8 x = EXTRACT_VECTOR_ELT(v16i8 b, i32 8)
//
// Note: We only run this optimization after type legalization (which often
// creates this pattern) and before operation legalization after which
// we need to be more careful about the vector instructions that we generate.
if (N0.getOpcode() == ISD::EXTRACT_VECTOR_ELT &&