From: lkcl <lkcl@web>
Date: Fri, 9 Jun 2023 18:28:40 +0000 (+0100)
Subject: (no commit message)
X-Git-Url: https://git.libre-soc.org/?a=commitdiff_plain;h=098079a9ea32ca293a9a1eedaadc5ee280bcc40f;p=libreriscv.git

---

diff --git a/openpower/sv/po9_encoding/discussion.mdwn b/openpower/sv/po9_encoding/discussion.mdwn
index d3f1d8df6..829888f91 100644
--- a/openpower/sv/po9_encoding/discussion.mdwn
+++ b/openpower/sv/po9_encoding/discussion.mdwn
@@ -11,7 +11,7 @@ a specific view to attaining high-speed massive-wide multi-issue decode
 is a high priority.  therefore especially length-detection must be kept
 brutally simple.
 
-# alternative 32-64 encoding (1) **superseded**
+# **superseded** alternative 32-64 encoding (1)
 
 conflict to resolve: EXT90x and EXT232. they are indistinguishable.
 
@@ -60,7 +60,7 @@ Instruction allocation restrictions:
 * but those types of space-saving instructions
   **also need to be Vectorizable*
 
-# alternative 32-64 encoding (2) **superseded**
+# **superseded** alternative 32-64 encoding (2)
 
 requires reducing SVP64Single to 23 bits. luckily there are 2 spare
 
@@ -106,27 +106,48 @@ Length detection:
 
 aim of this idea is to attempt simplification of area identification
 and length.  the 55-bit area is eliminated and may only be reintroduced
-by sacrificing parts of EXT200-231, bear in mind that EXT209 is already
+by sacrificing parts of EXT200-231, bear in mind that EXT240 is already
 unavailable (used by EXT900).
 
-
 ```
-|0-5| 6-29   |30 31|32|33-36|37 | 38-61 | 62-63 | Description      |
-|---|--------|-----|--|-----|---|-------|-------|------------------|
+|0-5| 6-29   |30 31|32 |33-37   | 38-61 | 62-63 | Description      |
+|---|--------|-----|---|--------|-------|-------|------------------|
 |PO9| xxxx   | 0 0 | (next new instruction)     |  32-bit EXT900   |
-|PO9| 0000   | 1 0 | 0 1000  1  |  xxxx |  00   | RESERVED         |
-|PO9| !ZERO  | 1 0 | 0 1000  1  |  xxxx |  00   | SSingle:EXT900   |
-|PO9| nnnn   | 1 0 | 0 1000  1  |  xxxx |  01   | SVP64:EXT900     |
-|PO9| xxxx   | 1 0 | 0 1000  1  |  xxxx |  10   | RESERVED         |
-|PO9| xxxx   | 1 0 | 0 1000  1  |  xxxx |  11   | RESERVED         |
-|PO9| !ZERO  | 1 0 | 0 | !PO9   |     xxxx      | SSingle:EXT200-231 |
-|PO9|  0000  | 1 0 | 0 | !PO9   |     xxxx      | Scalar EXT200-231  |
-|PO9|  nnnn  | 1 0 | 1 | !PO9   |     xxxx      | SVP64:EXT200-231   |
-|PO9|  0000  | 0 1 | Defined Word-instruction | 32-bit Unvec in 64b|
-|PO9| !ZERO  | 0 1 | Defined Word-instruction | SSingle:EXT000-063 |
-|PO9|  nnnn  | 1 1 | Defined Word-instruction | SVP64:EXT000-063   |
+|PO9| 0000   | 1 0 | 0 | 10001  |  //// |  00   | RESERVED         |
+|PO9| !ZERO  | 1 0 | 0 | 10001  |  xxxx |  00   | SSingle:EXT900   |
+|PO9| SVRM   | 1 0 | 0 | 10001  |  xxxx |  01   | SVP64:EXT900     |
+|PO9| ////   | 1 0 | 0 | 10001  |  //// |  10   | RESERVED         |
+|PO9| ////   | 1 0 | 0 | 10001  |  //// |  11   | RESERVED         |
+|PO9| !ZERO  | 1 0 | 0 | !PO9   |     xxxx      | SSingle:EXT232-263 |
+|PO9|  0000  | 1 0 | 0 | !PO9   |     xxxx      | Scalar EXT232-263  |
+|PO9|  SVRM  | 1 0 | 1 | !PO9   |     xxxx      | SVP64:EXT232-263   |
+|PO9|  0000  | 0 1 | Defined Word-instruction   | 32-bit Unvec in 64b|
+|PO9| !ZERO  | 0 1 | Defined Word-instruction   | SSingle:EXT000-063 |
+|PO9|  SVRM  | 1 1 | Defined Word-instruction   | SVP64:EXT000-063   |
 ```
 
+**Notes**:
+
+* with `setvl` `svindex` `svshape` and `svshape2` being Unvectorizable Management
+instructions requiring 6-bit `XO` and needing EXT1xx variants later, they are
+best placed in EXT0xx.  EXT900 is Vectorizable and extremely precious.  Further
+complication of Decode to create Vectorized EXT900 and Unvectorized EXT901
+is inadviseable (rejected in alternative 1)
+* EXT200-231 is intended to be partially-filled with LD/ST-Immediate-Postincrement
+  and it is strongly recommended that the exact same Primary Opcode be
+  used, matching the lower 5 bits. `lzbu` is EXT033, therefore `lzbup` is
+  recommended to be EXT233. This will greatly simplify Decode.
+* With no "overlapping" Encodings permitted on top of EXT232-263 (except
+  EXT240) Speculative Decode may begin early in Multi-Issue systems safe
+  that for the most part the 32-bit instruction is either going to be
+  EXT0xx or EXT2xx, regardless of whether it is SVP64-Prefixed.
+  LD/ST-Immediate-Postincrement discernment from
+  LD/ST-Immediate is delayed, but Decode of other EXT232-263 operations
+  are *also delayed* pending identification.
+  
+
+**Identification**:
+
 The following identification-criteria may be easily verified by taking
 a copy of the table above and striking out each line that fails each
 test.  Only EXT000-063 is not by a "process of elimination" (if ... return True)
@@ -140,13 +161,17 @@ test.  Only EXT000-063 is not by a "process of elimination" (if ... return True)
     else                     return 64
 ```
 
-**EXT200-231:**
+**EXT232-263:**
 
 ```
     if not PO9                return False
+    # eliminate EXT900
     if Word[30:31] == 0b00    return False
+    # eliminate EXT0xx
+    if Word[31] == 0b1        return False
+    # eliminate EXT900 and reserved areas
     if Word[33:37] != 0b10001 return False
-    # if Word[33:37] != XXXXX return False # 50-bit
+    # 
     return True
 ```