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

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

diff --git a/lib/Target/Mips/MSA.txt b/lib/Target/Mips/MSA.txt
new file mode 100644 (file)
index 0000000..398c7af
--- /dev/null
+++ b/lib/Target/Mips/MSA.txt
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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.