From: Jacob Lifshay <programmerjake@gmail.com>
Date: Mon, 3 Oct 2022 23:40:36 +0000 (-0700)
Subject: work on SVP64 scalar overriding VL to 1 question
X-Git-Tag: opf_rfc_ls005_v1~205
X-Git-Url: https://git.libre-soc.org/?a=commitdiff_plain;h=fdfbe2f064e73c1109c2a56e352179fdffb00e86;p=libreriscv.git

work on SVP64 scalar overriding VL to 1 question
---

diff --git a/openpower/sv/svp64/discussion.mdwn b/openpower/sv/svp64/discussion.mdwn
index c97625660..894d8b3ff 100644
--- a/openpower/sv/svp64/discussion.mdwn
+++ b/openpower/sv/svp64/discussion.mdwn
@@ -210,7 +210,7 @@ therefore it makes no sense to have DEST SUBVL, and instead to have special mv o
 
 full review needed, answering question:
 
-    if sv.op RT.scalar RA.scalar RB.scalar is set to "VL=1" is anything lost?
+    if sv.op RT.scalar RA.scalar RB.scalar is changed to temporarily override VL to be 1, is anything lost?
 
 four aspects:
 
@@ -223,7 +223,7 @@ four aspects:
 
 **Normal Mode:**
 
-* simple mode is straight vectorisation.
+* simple mode is straight vectorization.
 * reduce mode
 * ffirst or data-dependent fail-on-first: 
 * sat mode or saturation:
@@ -243,7 +243,7 @@ predicate-result should be fine as well (aside from change in predicate behaviou
 
 [[sv/ldst]]
 
-First critical observation, RA.isvec is utilised to detect element-stride.
+First critical observation, RA.isvec is utilized to detect element-stride.
 
 ```
 if RA.isvec:
@@ -256,42 +256,43 @@ elif immediate != 0:
 
 thus it is actually legitimate to have scalar src *and* dest especially
 with predicate masks.  the trick noted in (4) below of setting `RA.isvec`
-would therefore activate the *Vector Indexed* mode, with associated predication-based offsets (and REMAP) which is **NOT** the same as
-`VSPLAT` mode.
+would therefore activate the *Vector Indexed* mode, with associated
+predication-based offsets (and REMAP) (not to be confused with VSPLAT mode).
 
     elif RA.isvec:
-      # quirky Vector indexed mode but with an immediate
-      srcbase = ireg[RA+i]
-      offs = immed;
+        # quirky Vector indexed mode but with an immediate
+        srcbase = ireg[RA+i]
+        offs = immed;
     else
-      # standard scalar mode (but predicated)
-      # no stride multiplier means VSPLAT mode
-      srcbase = ireg[RA]
-      offs = immed
+        # standard scalar mode (but predicated)
+        # no stride multiplier means VSPLAT mode (FIXME(lkcl): unclear --
+        # VSPLAT requires RT.isvec=1 and all sources to be scalar)
+        srcbase = ireg[RA]
+        offs = immed
 
-Question:
+* Question:
 
     is RA.isvec=0 RT.isvec=1 a substitute?
 
-Answer:
+    Answer:
 
-     no because this is VSPLAT mode
+    no because RA.isvec=0 RT.isvec=1 is VSPLAT mode
 
-Question:
+* Question:
 
-     are there any other equivalent modes?
+    are there any other equivalent modes?
 
-Answer:
+    Answer:
 
     no, because immediate=0 selects Indexed mode
 
-Question:
+* Question:
 
     can the VSPLAT mode be actually used in proposed scalar-mode?
 
-Answer:
+    Answer:
 
-    possibly.
+    No, scalar-mode requires RA.isvec=0 RT.isvec=0, but VSPLAT is RA.isvec=0 RT.isvec=1.
 
 VL>1 at the moment, with a scalar source and scalar dest, will
 not undergo any changes to the EA compared to if VL=1.