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[libreriscv.git] / openpower / sv / twin_butterfly.mdwn

diff --git a/openpower/sv/twin_butterfly.mdwn b/openpower/sv/twin_butterfly.mdwn
index 1333ed77b7e1ed056329cae96075dc558158ee69..21cb5f25131193f63ed3ceb679c3292be74e7a4a 100644 (file)
--- a/openpower/sv/twin_butterfly.mdwn
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@@ -1 +1,74 @@
-TODO
+* <https://bugs.libre-soc.org/show_bug.cgi?id=1074>
+* <https://libre-soc.org/openpower/sv/biginteger/> for format and
+  information about implicit RS/FRS
+* <https://git.libre-soc.org/?p=openpower-isa.git;a=blob;f=src/openpower/decoder/isa/test_caller_svp64_dct.py;hb=HEAD>
+* [[openpower/isa/svfparith]]
+
+# [DRAFT] Twin Butterfly DCT Instruction(s)
+
+The goal is to implement instructions that calculate the expression:
+
+```
+fdct_round_shift((a +/- b) * c)
+```
+
+For the single-coefficient butterfly instruction, and:
+
+```
+ fdct_round_shift(a * c1  +/- b * c2)
+```
+
+For the double-coefficient butterfly instruction.
+
+`fdct_round_shift` is defined as `ROUND_POWER_OF_TWO(x, 14)`
+
+```
+#define ROUND_POWER_OF_TWO(value, n) (((value) + (1 << ((n)-1))) >> (n))
+```
+
+These instructions are at the core of **ALL** FDCT calculations in many major video codecs, including -but not limited to- VP8/VP9, AV1, etc.
+Arm includes special instructions to optimize these operations, although they are limited in precision: `vqrdmulhq_s16`/`vqrdmulhq_s32`.
+
+The suggestion is to have a single instruction to calculate both values `((a + b) * c) >> N`, and `((a - b) * c) >> N`.
+The instruction will run in accumulate mode, so in order to calculate the 2-coeff version one would just have to call the same instruction with different order a, b and a different constant c.
+
+# [DRAFT] Integer Butterfly Multiply Add/Sub FFT/DCT
+
+DCTI-Form
+
+* maddsubrs  RT,RA,RB,SH
+
+Pseudo-code:
+
+```
+    sum <- (RT) + (RA)      # RT = a, RA = b
+    diff <- (RT) - (RA)
+    prod1 <- MUL(RB, sum)   # RB = c
+    prod2 <- MUL(RB, diff)  # TODO: Pick high half?
+    res1 <- ROTL64(prod1, XLEN-SH)
+    res2 <- ROTL64(prod2, XLEN-SH)
+    RT <- (RT) + res1
+    RS <- (RS) + res2
+```
+
+Special Registers Altered:
+
+```
+    None
+```
+
+Where DCTI-Form is defined in fields.txt:
+
+```
+# 1.6.7.2 DCTI-FORM
+    |0     | 6    |11      |16     |21    |25      |31  |
+    | PO   |  RT  |   RA   |   RB  |   SH |   XO   | Rc |
+
+```
+
+The instruction has been added to `minor_22.csv`:
+
+```
+------01000,ALU,OP_MADDSUBRS,RT,CONST_UI,RB,RT,NONE,CR0,0,0,ZERO,0,NONE,0,0,0,0,1,0,RC_ONLY,0,0,maddsubrs,DCTI,,1,unofficial until submitted and approved/renumbered by the opf isa wg
+```
+