From: lkcl <lkcl@web>
Date: Mon, 27 Mar 2023 09:21:56 +0000 (+0100)
Subject: (no commit message)
X-Git-Tag: opf_rfc_ls001_v3~35
X-Git-Url: https://git.libre-soc.org/?a=commitdiff_plain;h=c7f93bcb8a5ff9c69ca4580c45380d7439d62d37;p=libreriscv.git

---

diff --git a/openpower/sv/rfc/ls009.mdwn b/openpower/sv/rfc/ls009.mdwn
index 846ab816d..6e80dfc35 100644
--- a/openpower/sv/rfc/ls009.mdwn
+++ b/openpower/sv/rfc/ls009.mdwn
@@ -453,39 +453,10 @@ not work.
 
 ### Sub-Vector Horizontal Reduction
 
-Note that when SVM is clear and SUBVL!=1 a Parallel Reduction is performed
-on all first Subvector elements, followed by another separate independent
-Parallel Reduction on all the second Subvector elements and so on.
-
-    for selectsubelement in (x,y,z,w):
-       parallelreduce(0..VL-1, selectsubelement)
-
-By contrast, when SVM is set and SUBVL!=1, a Horizontal
-Subvector mode is enabled, applying the Parallel Reduction
-Algorithm to the Subvector Elements. The Parallel Reduction
-is independently applied VL times, to each group of Subvector
-elements. Bear in mind that predication is never applied down
-into individual Subvector elements, but will be applied
-to select whether the *entire* Parallel Reduction on each
-group is performed or not.
-
-      for (i = 0; i < VL; i++)
-        if (predval & 1<<i) # predication
-           el = element[i]
-           parallelreduction([el.x, el.y, el.z, el.w])
-
-Note that as this is a Parallel Reduction, for best results
-it should be an overwrite operation, where the result for
-the Horizontal Reduction of each Subvector will be in the
-first Subvector element.
-Also note that use of Rc=1 is `UNDEFINED` behaviour.
-
-In essence what is happening here is that Structure Packing is being
-combined with Parallel Reduction.  If the Subvector elements may be
-laid out as a 2D matrix, with the Subvector elements on rows,
-and Parallel Reduction is applied per row, then if `SVM` is **clear**
-the Matrix is transposed (like Pack/Unpack)
-before still applying the Parallel Reduction to the **row**.
+To achieve Sub-Vector Horizontal Reduction, Pack/Unpack should be enabled,
+which will turn the Schedule around such that issuing of the Scalar
+Defined Words is done with SUBVL looping as the inner loop not the
+outer loop. Rc=1 with Sub-Vectors (SUBVL=2,3,4) is `UNDEFINED` behaviour.
 
 ## Determining Register Hazards