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

[oota-llvm.git] / lib / Target / Mips / MSA.txt

diff --git a/lib/Target/Mips/MSA.txt 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.