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[mips][msa] Added MSA.txt to describe instruction selection quirks.

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This file contains notes about the instruction selection for MSA. For example,
it notes that ilvl.d is cannot be selected because ilvev.d covers the same
cases and is selected instead of ilvl.d.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@191507 91177308-0d34-0410-b5e6-96231b3b80d8
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Daniel Sanders 2013-09-27 10:42:22 +00:00
parent f1e7cba627
commit 9c0f5ed594
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lib/Target/Mips/MSA.txt Normal file
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Code Generation Notes for MSA
=============================
Intrinsics are lowered to SelectionDAG nodes where possible in order to enable
optimisation, reduce the size of the ISel matcher, and reduce repetition in
the implementation. In a small number of cases, this can cause different
(semantically equivalent) instructions to be used in place of the requested
instruction, even when no optimisation has taken place.
Instructions
============
This section describes any quirks of instruction selection for MSA. For
example, two instructions might be equally valid for some given IR and one is
chosen in preference to the other.
vshf.w:
It is not possible to emit vshf.w when the shuffle description is
constant since shf.w covers exactly the same cases. shf.w is used
instead. It is also impossible for the shuffle description to be
unknown at compile-time due to the definition of shufflevector in
LLVM IR.
ilvl.d, pckev.d:
It is not possible to emit ilvl.d, or pckev.d since ilvev.d covers the
same shuffle. ilvev.d will be emitted instead.
ilvr.d, ilvod.d, pckod.d:
It is not possible to emit ilvr.d, or pckod.d since ilvod.d covers the
same shuffle. ilvod.d will be emitted instead.