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[libreriscv.git] / openpower / sv / vector_ops.mdwn

diff --git a/openpower/sv/vector_ops.mdwn b/openpower/sv/vector_ops.mdwn
index d9e401db7684cc6c9c0ef9f829cb48b774c49394..1a69d92070cc2980debbb5d6aa30e5ee0487c546 100644 (file)
--- a/openpower/sv/vector_ops.mdwn
+++ b/openpower/sv/vector_ops.mdwn
@@ -2,6 +2,8 @@
 
 # SV Vector Operations.
 
+TODO merge old standards page [[simple_v_extension/vector_ops/]]
+
 The core OpenPOWER ISA was designed as scalar: SV provides a level of abstraction to add variable-length element-independent parallelism. However, certain classes of instructions only make sense in a Vector context: AVX512 conflictd for example.  This section includes such examples.  Many of them are from the RISC-V Vector ISA (with thanks to the efforts of RVV's contributors)
 
 Notes:
@@ -264,6 +266,23 @@ used not just for carry lookahead, also a special type of predication mask opera
 * <https://i.stack.imgur.com/QSLKY.png>
 * <https://stackoverflow.com/questions/27971757/big-integer-addition-code>
   `((P|G)+G)^P`
+* <https://en.m.wikipedia.org/wiki/Carry-lookahead_adder>
+
+```
+     P = (A | B) & Ci
+     G = (A & B)
+```
+
+Stackoverflow algorithm `((P|G)+G)^P` works on the cumulated bits of P and G from associated vector units (P and G are integers here).  The result of the algorithm is the new carry-in which already includes ripple, one bit of carry per element.
+
+```
+    At each id, compute C[id] = A[id]+B[id]+0
+    Get G[id] = C[id] > radix -1
+    Get P[id] = C[id] == radix-1
+    Join all P[id] together, likewise G[id]
+    Compute newC = ((P|G)+G)^P
+    result[id] = (C[id] + newC[id]) % radix
+```   
 
 two versions: scalar int version and CR based version.