From: lkcl <lkcl@web>
Date: Wed, 5 Apr 2023 08:45:11 +0000 (+0100)
Subject: (no commit message)
X-Git-Tag: opf_rfc_ls012_v1~124
X-Git-Url: https://git.libre-soc.org/?a=commitdiff_plain;h=357bf4aa908fae9197656ce666389d9605256cac;p=libreriscv.git

---

diff --git a/openpower/sv/ldst.mdwn b/openpower/sv/ldst.mdwn
index 943fd0095..62b7d3470 100644
--- a/openpower/sv/ldst.mdwn
+++ b/openpower/sv/ldst.mdwn
@@ -218,7 +218,7 @@ LD-VSPLAT is required then a single *scalar* cache-inhibited LD should
 be performed, followed by a VSPLAT-augmented mv, copying the one *scalar*
 value into multiple register destinations.
 
-Note also that cache-inhibited VSPLAT with Predicate-result is possible.
+Note also that cache-inhibited VSPLAT with Data-Dependent Fail-First is possible.
 This allows for example to issue a massive batch of memory-mapped
 peripheral reads, stopping at the first NULL-terminated character and
 truncating VL to that point. No branch is needed to issue that large
@@ -494,14 +494,21 @@ one Element earlier.
            break                     # stop looping
 ```
 
-**Data-Dependent Fault-First on Store-Conditional**
+**Data-Dependent Fault-First on Store-Conditional (Rc=1)**
 
 There are very few instructions that allow Rc=1 for Load/Store:
 one of those is the `stdcx.` and other Atomic Store-Conditional
-instructions.  It should be self-evident that being able to
-Vectorise and then truncate a sequence of Atomic Store-Conditional
-operations at the point where a store was not performed, should
-be pretty important.
+instructions.  With Simple-V being a loop around Scalar instructions
+strictly obeying Scalar Program Order a Fail-First loop on an
+Atomic Store-Conditional will always fail the second and all other
+Store-Conditional instructions in Horizontal-First Mode because
+Load-Reservation and Store-Conditional are required to be executed
+in pairs.
+
+By contrast, in Vertical-First Mode it is in fact possible to issue
+the pairs, and consequently allowing Vectorised Data-Dependent Fail-First is
+useful.  Care should be taken however when VL is truncated in Vertical-First
+Mode.
 
 ## LOAD/STORE Elwidths <a name="elwidth"></a>