From: lkcl <lkcl@web>
Date: Wed, 26 Apr 2023 06:55:01 +0000 (+0100)
Subject: (no commit message)
X-Git-Url: https://git.libre-soc.org/?a=commitdiff_plain;h=bbb35dd4882235a18d2a1efc80eeda0e75d7ae7d;p=libreriscv.git

---

diff --git a/openpower/sv/vector_ops/discussion.mdwn b/openpower/sv/vector_ops/discussion.mdwn
index 6ff8b9f79..04763fc18 100644
--- a/openpower/sv/vector_ops/discussion.mdwn
+++ b/openpower/sv/vector_ops/discussion.mdwn
@@ -259,3 +259,60 @@ mask-out effect of resetting the count back to zero.
 However close examination shows that the above may actually
 be `sv.addi/mr/sm=EQ/dz r0.v, r0.v, 1` 
 
+# cprop
+
+* <https://www.geeksforgeeks.org/carry-look-ahead-adder/>
+* <https://media.geeksforgeeks.org/wp-content/uploads/digital_Logic6.png>
+* <https://electronics.stackexchange.com/questions/20085/whats-the-difference-with-carry-look-ahead-generator-block-carry-look-ahead-ge>
+* <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>
+
+From QLSKY.png:
+
+```
+    x0 = nand(CIn, P0)
+    C0 = nand(x0, ~G0)
+
+    x1 = nand(CIn, P0, P1)
+    y1 = nand(G0, P1)
+    C1 = nand(x1, y1, ~G1)
+
+    x2 = nand(CIn, P0, P1, P2)
+    y2 = nand(G0, P1, P2)
+    z2 = nand(G1, P2)
+    C1 = nand(x2, y2, z2, ~G2)
+
+    # Gen*
+    x3 = nand(G0, P1, P2, P3)
+    y3 = nand(G1, P2, P3)
+    z3 = nand(G2, P3)
+    G* = nand(x3, y3, z3, ~G3)
+```
+
+```
+     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.
+
+scalar int version acts as a scalar carry-propagate, reading XER.CA as input, P and G as regs, and taking a radix argument.  the end bits go into XER.CA and CR0.ge
+
+vector version takes CR0.so as carry in, stores in CR0.so and CR.ge end bits.
+
+if zero (no propagation) then CR0.eq is zero
+
+CR based version, TODO.