From: Luke Kenneth Casson Leighton <lkcl@lkcl.net>
Date: Wed, 15 Mar 2023 15:00:29 +0000 (+0000)
Subject: rewrite crternlogi and crbinlog to match new format, required to
X-Git-Tag: opf_rfc_ls001_v3~160
X-Git-Url: https://git.libre-soc.org/?a=commitdiff_plain;h=8e5326caf15e238494a20cb0937b0c41c0ea9d20;p=libreriscv.git

rewrite crternlogi and crbinlog to match new format, required to
reduce both instructions to 3-read 1-write.
https://bugs.libre-soc.org/show_bug.cgi?id=1023#c2
---

diff --git a/openpower/sv/bitmanip.mdwn b/openpower/sv/bitmanip.mdwn
index a4d68a0c6..15713958f 100644
--- a/openpower/sv/bitmanip.mdwn
+++ b/openpower/sv/bitmanip.mdwn
@@ -121,38 +121,47 @@ the second nh=1.*
 
 ## crternlogi
 
-another mode selection would be CRs not Ints. 
+another mode selection would be CRs not Ints.
 
-| 0.5|6.8 | 9.11|12.14|15.17|18.20|21.28 | 29.30|31|
-| -- | -- | --- | --- | --- |-----|----- | -----|--|
-| NN | BT | BA  | BB  | BC  |m0-2 | imm  |  01  |m3|
+CRB-Form:
+
+| 0.5|6.8 |9.10|11.13|14.15|16.18|19.25|26.30| 31|
+|----|----|----|-----|-----|-----|-----|-----|---|
+| NN | BF | msk|BFA  | msk | BFB | TLI | XO  |TLI|
 
-    mask = m0-3
     for i in range(4):
-        a,b,c = CRs[BA][i], CRs[BB][i], CRs[BC][i])
-        if mask[i] CRs[BT][i] = lut3(imm, a, b, c)
+        a,b,c = CRs[BF][i], CRs[BFA][i], CRs[BFB][i])
+        if msk[i] CRs[BF][i] = lut3(imm, a, b, c)
 
 This instruction is remarkably similar to the existing crops, `crand` etc.
 which have been noted to be a 4-bit (binary) LUT.  In effect `crternlogi`
-is the ternary LUT version of crops, having an 8-bit LUT.
+is the ternary LUT version of crops, having an 8-bit LUT.  However it
+is an overwrite instruction in order to save on register file ports,
+due to the mask requiring the contents of the BF to be both read and
+written.
 
 ## crbinlog
 
 With ternary (LUT3) dynamic instructions being very costly,
 and CR Fields being only 4 bit, a binary (LUT2) variant is better
 
-| 0.5|6.8 | 9.11|12.14|15.17|18.21|22...30  |31|
-| -- | -- | --- | --- | --- |-----| -------- |--|
-| NN | BT | BA  | BB  | BC  |m0-m3|000101110 |0 |
+CRB-Form:
+
+| 0.5|6.8 |9.10|11.13|14.15|16.18|19.25|26.30| 31|
+|----|----|----|-----|-----|-----|-----|-----|---|
+| NN | BF | msk|BFA  | msk | BFB | //  | XO  | //|
 
-    mask = m0..m3
     for i in range(4):
-        a,b = CRs[BA][i], CRs[BB][i])
-        if mask[i] CRs[BT][i] = lut2(CRs[BC], a, b)
+        a,b = CRs[BF][i], CRs[BF][i])
+        if msk[i] CRs[BF][i] = lut2(CRs[BFB], a, b)
 
 When SVP64 Vectorised any of the 4 operands may be Scalar or
-Vector, including `BC` meaning that multiple different dynamic
-lookups may be performed with a single instruction.
+Vector, including `BFB` meaning that multiple different dynamic
+lookups may be performed with a single instruction.  Note that
+this instruction is deliberately an overwrite in order to reduce
+the number of register file ports required: like `crternlogi`
+the contents of `BF` **must** be read due to the mask only
+writing back to non-masked-out bits of `BF`.
 
 *Programmer's note: just as with binlut and ternlogi, a pair
  of crbinlog instructions followed by a merging crternlogi may