From: lkcl <lkcl@web>
Date: Sun, 15 May 2022 19:41:36 +0000 (+0100)
Subject: (no commit message)
X-Git-Tag: opf_rfc_ls005_v1~2222
X-Git-Url: https://git.libre-soc.org/?a=commitdiff_plain;h=43402fdf11b969453cd6c2a5de03648f2fea0e06;p=libreriscv.git

---

diff --git a/openpower/sv/bitmanip.mdwn b/openpower/sv/bitmanip.mdwn
index faeebfe7c..8588e6e91 100644
--- a/openpower/sv/bitmanip.mdwn
+++ b/openpower/sv/bitmanip.mdwn
@@ -97,7 +97,7 @@ TODO: convert all instructions to use RT and not RS
 | NN | RT | RA  | RB   | im0-4   | im5-7  00 |1 | grevlog |
 | NN |    |     |      |         | -----  01 |m3| crternlog |
 | NN | RT | RA  | RB   | RC      | mode  010 |Rc| bitmask\* |
-| NN |    |     |      |         | 00    011 |  | rsvd |
+| NN | RT | RA  | RB   | RC      | 00    011 |nh| binlut |
 | NN |    |     |      |         | 01    011 |0 | svshape |
 | NN |    |     |      |         | 01    011 |1 | svremap |
 | NN |    |     |      |         | 10    011 |Rc| svstep |
@@ -153,7 +153,7 @@ double check that instructions didn't need 3 inputs.
 | NN | RT | RA  | RB  | 1  | 11    | 1110 110 |Rc| clmulh    | X-Form  |
 | NN |    |     |     |    |       | --11 110 |Rc| rsvd      |         |
 
-# ternlog bitops
+# binary and ternary bitops
 
 Similar to FPGA LUTs: for every bit perform a lookup into a table using an 8bit immediate, or in another register.
 
@@ -172,34 +172,28 @@ Like the x86 AVX512F [vpternlogd/vpternlogq](https://www.felixcloutier.com/x86/v
     for i in range(64): 
         RT[i] = lut3(imm, RB[i], RA[i], RT[i]) 
 
-## ternlogv
-
-also, another possible variant involving swizzle-like selection
-and masking, this only requires 3 64 bit registers (RA, RS, RB) and
-only 16 LUT3s.
-
-Note however that unless XLEN matches sz, this instruction
-is a Read-Modify-Write: RS must be read as a second operand
-and all unmodified bits preserved.  SVP64 may provide limited
-alternative destination for RS from RS-as-source, but again
-all unmodified bits must still be copied.
-
-| 0.5|6.10|11.15|16.20|21.28 | 29.30 |31|
-| -- | -- | --- | --- | ---- | ----- |--|
-| NN | RS | RA  | RB  |idx0-3|  01   |sz|
-
-    SZ = (1+sz) * 8 # 8 or 16
-    raoff = MIN(XLEN, idx0 * SZ)
-    rboff = MIN(XLEN, idx1 * SZ)
-    rcoff = MIN(XLEN, idx2 * SZ)
-    rsoff = MIN(XLEN, idx3 * SZ)
-    imm = RB[0:8]
-    for i in range(MIN(XLEN, SZ)):
-        ra = RA[raoff:+i]
-        rb = RA[rboff+i]
-        rc = RA[rcoff+i]
-        res = lut3(imm, ra, rb, rc)
-        RS[rsoff+i] = res
+## binlut
+
+Binary lookup is a dynamic LUT2 version of ternlogi. Firstly, the
+lookup table is 4 bits wide not 8 bits, and secondly the lookup
+table comes from a register not an immediate.
+
+| 0.5|6.10|11.15|16.20| 21..25|26..30|31|
+| -- | -- | --- | --- | ----- | ---- |--|
+| NN | RT | RA  | RB  | RC    |00011 |nh|
+
+    lut2(imm, a, b):
+        idx = b << 1 | a
+        return imm[idx] # idx by LSB0 order
+
+    imm = (RC>>(nh*4))&0b1111
+    for i in range(64): 
+        RT[i] = lut2(imm, RB[i], RA[i]) 
+
+*Programmer's note: a dynamic ternary lookup may be synthesised from
+a pair of `binlut` instructions followed by a `ternlogi` to select which
+to merge. Use `nh` to select which nibble to use as the lookup table
+from the RC source register (`nh=1` nibble high)*
 
 ## ternlogcr