From: lkcl <lkcl@web>
Date: Wed, 23 Dec 2020 01:06:59 +0000 (+0000)
Subject: (no commit message)
X-Git-Tag: convert-csv-opcode-to-binary~1021
X-Git-Url: https://git.libre-soc.org/?a=commitdiff_plain;h=74b926479ed177c522aaeac68d67d345bf8e5e99;p=libreriscv.git

---

diff --git a/openpower/sv/svp_rewrite/svp64.mdwn b/openpower/sv/svp_rewrite/svp64.mdwn
index d90c33e30..9548f67d8 100644
--- a/openpower/sv/svp_rewrite/svp64.mdwn
+++ b/openpower/sv/svp_rewrite/svp64.mdwn
@@ -681,14 +681,19 @@ are mapreduced per *sub-element* as a result.  illustration with a vec2:
         result.x = op(result.x, iregs[RA+i].x)
         result.y = op(result.y, iregs[RA+i].y)
 
-When SVM is set and SUBVL!=1, another variant is enabled.
+Note here that Rc=1 does not make sense when SVM is clear and SUBVL!=1.
+
+
+When SVM is set and SUBVL!=1, another variant is enabled: horizontal subvector mode.  Example for a vec3:
 
     for i in range(VL):
         result = op(iregs[RA+i].x, iregs[RA+i].x)
-        result = op(result, iregs[RA+i].z)
+        result = op(result, iregs[RA+i].y)
         result = op(result, iregs[RA+i].z)
         iregs[RT+i] = result
 
+In this mode, when Rc=1 the Vector of CRs is as normal: each result element creates a corresponding CR element.
+
 ## Fail-on-first
 
 Data-dependent fail-on-first has two distinct variants: one for LD/ST,
@@ -730,6 +735,10 @@ One extremely important aspect of ffirst is:
   vectorised operations are effectively `nops` which is
   *precisely the desired and intended behaviour*.
 
+Another aspect is that for ffirst LD/STs, VL may be truncated arbitrarily to a nonzero value for any implementation-specific reason.  For example: it is perfectly reasonable for implementations to alter VL when ffirst LD or ST operations are initiated on a nonaligned boundary, such that within a loop the subsequent iteration of that loop begins subsequent ffirst LD/ST operations on an aligned boundary.  Likewise, to reduce workloads or balance resources.
+
+CR-based data-dependent first on the other hand MUST not truncate VL arbitrarily.  This because it is a precise test on which algorithms will rely.
+
 ## pred-result mode
 
 This mode merges common CR testing with predication, saving on instruction count. Below is the pseudocode excluding predicate zeroing and elwidth overrides.