From: lkcl <lkcl@web>
Date: Wed, 16 Dec 2020 00:37:33 +0000 (+0000)
Subject: (no commit message)
X-Git-Tag: convert-csv-opcode-to-binary~1306^2~5
X-Git-Url: https://git.libre-soc.org/?a=commitdiff_plain;h=2f85b95c5559f902ab2f54bbb8b7a022a728faa7;p=libreriscv.git

---

diff --git a/openpower/sv/svp_rewrite/svp64.mdwn b/openpower/sv/svp_rewrite/svp64.mdwn
index 8c6e8603e..12a825647 100644
--- a/openpower/sv/svp_rewrite/svp64.mdwn
+++ b/openpower/sv/svp_rewrite/svp64.mdwn
@@ -30,7 +30,7 @@ defined in the Prefix Fields section.
 | MASK                         | `0:3`      | Execution Mask |
 | TBD                          | `4:23`     | TBD            |
 
-## MASK Encoding
+## Predicate MASK Encoding
 
 One bit indicates the mode: CR or Int predication.   The two types may not be mixed.
 
@@ -98,6 +98,28 @@ CR based predication.  TODO: select alternate CR for twin predication? see [[dis
 | SVP64_9             | `9`        | `1`            | Indicates this is a SVP64 instruction      |
 | `RM[2:23]`          | `10:31`    |                | Bits 2 through 23 of the Remapped Encoding |
 
+# Twin Predication
+
+This is a novel concept that allows predication to be applied to a single source and a single dest register.  The following types of traditional Vector operations may be encoded with it, *without requiring explicit opcodes to do so*
+
+* VSPLAT
+* VEXTRACT
+* VINSERT
+* VREDUCE
+* VEXPAND
+* VCOMPRESS
+
+Those patterns (and more) may be applied to:
+
+* mv (the usual way that V\* operations are created)
+* exts\* sign-extension
+* rwlinm and other RS-RA shift operations
+* LD and ST (treating AGEN as one source)
+* FP fclass, fsgn, fneg, fabs, fcvt, frecip, fsqrt etc.
+* Condition Register ops mfcr, mtcr and other similar
+
+This is a huge list that creates extremely powerful combinations, particularly given that one of the predicate options is (1<<r3)
+
 # Register Naming
 
 SV Registers are numbered using the notation `SV[F]R<N>_<M>` where `<N>` is a decimal integer and `<M>` is a binary integer. Two integers are used to enable future register expansions to add more registers by appending more LSB bits to `<M>`.