From: lkcl <lkcl@web>
Date: Mon, 13 Jun 2022 13:36:00 +0000 (+0100)
Subject: (no commit message)
X-Git-Tag: opf_rfc_ls005_v1~1816
X-Git-Url: https://git.libre-soc.org/?a=commitdiff_plain;h=6563cf644bb65c9347f4d561c8cba805a1ec1c00;p=libreriscv.git

---

diff --git a/openpower/sv/mv.swizzle.mdwn b/openpower/sv/mv.swizzle.mdwn
index d41944e1d..1e2ce0f7e 100644
--- a/openpower/sv/mv.swizzle.mdwn
+++ b/openpower/sv/mv.swizzle.mdwn
@@ -47,7 +47,7 @@ to each:
 the options for each Swizzle are:
 
 * 0b000 to indicate "skip".  this is equivalent to predicate masking
-* 0b001 is not needed (reserved)
+* 0b001 subvector length end marker (length=4 if not present)
 * 0b010 to indicate "constant 0"
 * 0b011 to indicate "constant 1" (or 1.0)
 * 0b1NN index 0 thru 3 to copy from subelement in pos XYZW
@@ -150,19 +150,21 @@ Swizzle Pseudocode:
     swiz[0] = imm[0:3]   # X
     swiz[1] = imm[3:6]   # Y
     swiz[2] = imm[6:9]   # Z
-    swiz[3] = imm[9:12]  # W, maybe
-    # determine Subvector lengths
-    subvl = MAX(SRC_SUBVL, SUBVL)
-    if subvl == 4:
-        src_subvl = 4
-    else:
-        src_subvl = swiz[3]
-    dest_subvl = subvl
+    swiz[3] = imm[9:12]  # W
+    # determine implied subvector length from Swizzle
+    dst_subvl = 4
+    for i in range(4):
+        if swiz[i] == 0b001: dst_subvl = i+1
+    # source subvector length is maximum of both RM fields
+    src_subvl = MAX(SRC_SUBVL, SUBVL)
 ```
 
 What is going on here is that the option is provided to have different
 source and destination subvector lengths, by exploiting redundancy in
-the Swizzle Immediate.
+the Swizzle Immediate.  With the Swizzles marking what goes into
+each destination position, the marker "0b001" may be used to indicate
+the end. If no marker is present then the destination subvector length
+may be assimed to be 4.
 
 # RM Mode Concept: