From: lkcl <lkcl@web>
Date: Thu, 20 Apr 2023 15:16:51 +0000 (+0100)
Subject: (no commit message)
X-Git-Url: https://git.libre-soc.org/?a=commitdiff_plain;h=4c2f44f82ed44c1b9d1e2dbc3615c99277e4e4e1;p=libreriscv.git

---

diff --git a/openpower/sv/normal.mdwn b/openpower/sv/normal.mdwn
index f74154f3f..e913b1253 100644
--- a/openpower/sv/normal.mdwn
+++ b/openpower/sv/normal.mdwn
@@ -34,11 +34,12 @@ the following Modes apply to Arithmetic and Logical SVP64 operations:
   is performed. See [[svp64/appendix]].
   Note that there are comprehensive caveats when using this mode,
   and it should not be confused with the Parallel Reduction [[sv/remap]].
+  Also care is needed with `hphint`.
 
 Note that ffirst and reduce modes are not anticipated to be
 high-performance in some implementations.  ffirst due to interactions
-with VL, and reduce due to it requiring additional operations to produce
-a result.  simple and saturate are however inter-element
+with VL, and reduce due to it creating overlapping operations in
+many of its uses.  simple and saturate are however inter-element
 independent and may easily be parallelised to give high performance,
 regardless of the value of VL.
 
@@ -146,6 +147,15 @@ be parallelised.
 Dependency chain as long as Sequential Program Execution Order is preserved.
 Easy examples include Reduction on Logical OR or AND operations.*
 
+**Horizontal Parallelism Hint**
+
+`SVSTATE.hphint` declares to hardware that groups of elements up to this
+size are 100% independent.  With Reduction literally creating Dependency
+Hazards on every element-level sub-instruction it is pretty clear that setting
+`hphint` *at all* would cause data corruption.  However `sv.add *r0, *r4, *r0`
+for example clearly leaves room for four parallel elements. Programmers must
+be aware of this and exercise caution.
+
 ## Data-dependent Fail-on-first
 
 Data-dependent fail-on-first is CR-field-driven and is completely separate