From: Luke Kenneth Casson Leighton <lkcl@lkcl.net>
Date: Wed, 25 Sep 2019 08:13:41 +0000 (+0100)
Subject: add note about swapping src1/src2
X-Git-Tag: convert-csv-opcode-to-binary~3972
X-Git-Url: https://git.libre-soc.org/?a=commitdiff_plain;h=db92f902f981bc8bc7aef42965c066605659af87;p=libreriscv.git

add note about swapping src1/src2
---

diff --git a/simple_v_extension/appendix.mdwn b/simple_v_extension/appendix.mdwn
index 3c07311f4..8d8941bca 100644
--- a/simple_v_extension/appendix.mdwn
+++ b/simple_v_extension/appendix.mdwn
@@ -234,6 +234,13 @@ element tests must be performed (and the result optionally stored in
 the result mask), with a "post-analysis" phase carried out which checks
 whether to branch.
 
+Note also that whilst it may seem excessive to have all four (because
+conditional comparisons may be inverted by swapping src1 and src2),
+data-dependent fail-on-first is *not* invertible and *only* terminates
+on first zero-condition encountered.  Additionally it may be inconvenient
+to have to swap the predicate registers associated with src1 and src2,
+because this involves a new VBLOCK Context.
+
 ### Standard Branch <a name="standard_branch"></a>
 
 Branch operations use standard RV opcodes that are reinterpreted to