From: lkcl <lkcl@web>
Date: Sat, 18 Jun 2022 09:35:06 +0000 (+0100)
Subject: (no commit message)
X-Git-Tag: opf_rfc_ls005_v1~1729
X-Git-Url: https://git.libre-soc.org/?a=commitdiff_plain;h=e345a7335d0f2447b4050338b596143129f214a0;p=libreriscv.git

---

diff --git a/openpower/sv/svp64_quirks.mdwn b/openpower/sv/svp64_quirks.mdwn
index d1bcc4e73..b24fc6d7c 100644
--- a/openpower/sv/svp64_quirks.mdwn
+++ b/openpower/sv/svp64_quirks.mdwn
@@ -249,7 +249,7 @@ easy to identify by looking for the EXT001 pattern. Once identified
 the 32/64 bits may be passed independently to multiple Decoders in
 parallel.
 
-# Predication.
+# Predication
 
 Predication is entirely missing from the Power ISA.
 Adding it would be a costly mistake because it cannot be retrofitted
@@ -267,12 +267,10 @@ they can be avoided by using the crweird instructions to get CR Field
 bits into an Integer GPR (r3, r10 or r30) and use that GPR as a
 Predicate mask instead.
 
-Also, in Vertical-First Mode, usually used for looping in combination
-with svstep, predicates of both types (GPR, CR Field) should never be
-written to. The Predicate masks should be set, then Vertical-First mode
-set, and within the loop all Predicates used should be considered
-loop-invariant.  Given that Vertical-First is a type of Scalar execution
-mode, branches can and should be used on Scalar tests anyway.
+Even in Vertical-First Mode, which is a single Scalar instruction executed
+with "offset" registers (in effect), the rule still applies: don't write
+to the same register being used as the predicate, it's `UNDEFINED`
+behaviour.
 
 ## Single Predication