From: lkcl <lkcl@web>
Date: Mon, 22 May 2023 14:58:52 +0000 (+0100)
Subject: (no commit message)
X-Git-Url: https://git.libre-soc.org/?a=commitdiff_plain;h=874c693228f1f8d82ebad52e8889fb3e125d23d4;p=libreriscv.git

---

diff --git a/openpower/sv/svp64.mdwn b/openpower/sv/svp64.mdwn
index f4a44661e..1f42ecc14 100644
--- a/openpower/sv/svp64.mdwn
+++ b/openpower/sv/svp64.mdwn
@@ -731,7 +731,7 @@ on context after decoding of the Scalar suffix:
 | Field Name | Field bits | Description                            |
 |------------|------------|----------------------------------------|
 | ELWIDTH       | `4:5`      | Element Width                       |
-| ELWIDTH_SRC   | `6:7`      | Element Width for Source      |
+| ELWIDTH_SRC   | `6:7`      | Element Width for Source (or MASK_SRC in 2PM)    |
 | EXTRA         | `10:18`    | Register Extra encoding                |
 | MODE          | `19:23`    | changes Vector behaviour               |
 
@@ -894,6 +894,9 @@ Additionally Programmers should avoid using r3 r10 or r30
 as destination registers when these are also used as a Predicate
 Mask. Doing so is again UNDEFINED behaviour.
 
+Usually in 2P `MASK_SRC` is exclusively in the EXTRA area. However for
+LD/ST-Indexed a different Encoding is required, designated `2PM`.
+
 ### Integer Predication (MASKMODE=0)
 
 When the predicate mode bit is zero the 3 bits are interpreted as below.
@@ -916,7 +919,6 @@ following meaning:
 r10 and r30 are at the high end of temporary and unused registers,
 so as not to interfere with register allocation from ABIs.
 
-
 ### CR-based Predication (MASKMODE=1)
 
 When the predicate mode bit is one the 3 bits are interpreted as below.
@@ -986,6 +988,13 @@ some compromises have to be made.
 * `RM-2P-1S1D` Twin Predication (src=1, dest=1)
 * `RM-2P-2S1D` Twin Predication (src=2, dest=1) primarily for LDST (Indexed)
 * `RM-2P-1S2D` Twin Predication (src=1, dest=2) primarily for LDST Update
+* `RM-2PM-2S1D` Twin Predication (src=2, dest=1) primarily for LDST (Indexed)
+* `RM-2PM-1S2D` Twin Predication (src=1, dest=2) primarily for LDST Update
+
+The `2PM` designation uses bits 6 and 7 as well as the 9 EXTRA bits
+in order to extend two registers to
+EXTRA3, sacrificing destination elwidths in the process.
+`MASK_SRC` has a different encoding in `2PM`.
 
 ### RM-1P-3S1D
 
@@ -1100,6 +1109,24 @@ RM-2P-2S1D and the src spec for RA is also used for the same RA as a dest.
 Note that if ELWIDTH != ELWIDTH_SRC this may result in reduced performance
 or increased latency in some implementations due to lane-crossing.
 
+### RM-2PM-2S1D/1S2D/3S
+
+The primary purpose for this encoding is for Twin Predication on LOAD
+and STORE operations providing EXTRA3 for RT, RA and RS.
+see [[sv/ldst]] for detailed analysis.
+
+**RM-2PM-2S1D:**
+
+RT or RS requires EXTRA3, RA requires EXTRA3, but for RB EXTRA2 will
+suffice.  `MASK_SRC` may be read from the bits normally used for dest-elwidth.
+
+| Field Name | Field bits | Description                     |
+|------------|------------|----------------------------|
+| Rdest_EXTRA3 | `10:12`  | extends Rdest (R\*\_EXTRA2 Encoding)   |
+| Rsrc1_EXTRA3 | `13:15`  | extends Rsrc1 (R\*\_EXTRA2 Encoding)   |
+| Rsrc2_EXTRA2 | `16:17`  | extends Rsrc2 (R\*\_EXTRA2 Encoding)   |
+| MASK_SRC     | `6:7,18` | Execution Mask for Source     |
+
 ## R\*\_EXTRA2/3
 
 EXTRA is the means by which two things are achieved: