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diff --git a/openpower/sv/biginteger/analysis.mdwn b/openpower/sv/biginteger/analysis.mdwn
index f745748c7d904f2adae3c0a5d037d9ecae9316d2..52833d4bffbe75ff72da5c3f96c3515245d6dd93 100644 (file)
--- a/openpower/sv/biginteger/analysis.mdwn
+++ b/openpower/sv/biginteger/analysis.mdwn
@@ -100,6 +100,31 @@ to people unfamiliar with Cray-style Vectors: if VL is not
 permitted to exceed 1 (because MAXVL is set to 1) then the above
 actually becomes a Scalar Big-Int add algorithm.
 
+# Vector Shift
+
+Like add and subtract, strictly speaking these need no new instructions.
+Keeping the shift amount within the range of the element (64 bit)
+a Vector bit-shift may be synthesised from a pair of shift operations
+and an OR, all of which are standard Scalar Power ISA instructions
+that when Vectorised are exactly what is needed.
+
+```
+void biglsh(unsigned s, unsigned vn[], unsigned const v[], int n)
+{
+    for (int i = n - 1; i > 0; i--)
+        vn[i] = (v[i] << s) | ((unsigned long long)v[i - 1] >> (32 - s));
+    vn[0] = v[0] << s;
+}
+```
+
+The reason why three instructions are needed instead of one in the
+case of big-add is because multiple bits chain through to the
+next element, where for add it is a single bit (carry-in, carry-out).
+For multiply and divide as shown later it is worthwhile to use
+one scalar register effectively as a full 64-bit carry/chain
+but in the case of shift, an OR may glue things together, easily,
+and in parallel.
+
 # Vector Multiply
 
 Long-multiply, assuming an O(N^2) algorithm, is performed by summing