From: Luke Kenneth Casson Leighton <lkcl@lkcl.net>
Date: Wed, 12 Apr 2023 13:28:30 +0000 (+0100)
Subject: add section prohibiting cross-over between CR0-7 group and CR8-127 group
X-Git-Tag: opf_rfc_ls010_v1~54
X-Git-Url: https://git.libre-soc.org/?a=commitdiff_plain;h=8a607ea0122bd043125f3318bbc6ef1294255e1b;p=libreriscv.git

add section prohibiting cross-over between CR0-7 group and CR8-127 group
https://bugs.libre-soc.org/show_bug.cgi?id=1053
---

diff --git a/openpower/sv/svp64.mdwn b/openpower/sv/svp64.mdwn
index f662ba2a4..05f48b074 100644
--- a/openpower/sv/svp64.mdwn
+++ b/openpower/sv/svp64.mdwn
@@ -493,13 +493,13 @@ This is part of `scalar identity behaviour` described above.
 
 **Condition Register(s)**
 
-The Scalar Power ISA Condition Register is a 64 bit register where the top
-32 MSBs (numbered 0:31 in MSB0 numbering) are not used.  This convention is
-*preserved*
-in SVP64 and an additional 15 Condition Registers provided in
-order to store the new CR Fields, CR8-CR15, CR16-CR23 etc. sequentially.
-The top 32 MSBs in each new SVP64 Condition Register are *also* not used:
-only the bottom 32 bits (numbered 32:63 in MSB0 numbering).
+The Scalar Power ISA Condition Register is a 64 bit register where
+the top 32 MSBs (numbered 0:31 in MSB0 numbering) are not used.
+This convention is *preserved* in SVP64 and an additional 15 Condition
+Registers provided in order to store the new CR Fields, CR8-CR15,
+CR16-CR23 etc. sequentially.  The top 32 MSBs in each new SVP64 Condition
+Register are *also* not used: only the bottom 32 bits (numbered 32:63
+in MSB0 numbering).
 
 *Programmer's note: using `sv.mfcr` without element-width overrides
 to take into account the fact that the top 32 MSBs are zero and thus
@@ -519,6 +519,54 @@ Condition-Register instructions because all other CR instructions,
 on closer investigation, will be observed to all be CR-bit or CR-Field
 related. Thus a `VL` of 16 must be used*
 
+**Condition Register Fields as Predicate Masks**
+
+Condition Register Fields perform an additional duty in Simple-V: they are
+used for Predicate Masks.  ARM's Scalar Instruction Set calls single-bit
+predication "Conditional Execution", and utilises Condition Codes for
+exactly this purpose to solve the problem caused by Branch Speculation.
+In a Vector ISA context the concept of Predication is naturally extended
+from single-bit to multi-bit, and the (well-known) benefits become all the
+more critical given that parallel branches in Vector ISAs are impossible
+(even a Vector ISA can only have Scalar branches).
+
+However the Scalar Power ISA does not have Conditional Execution (for
+which, if it had ever been considered, Condition Register bits would be
+a perfect natural fit).  Thus, when adding Predication using CR Fields
+via Simple-V it becomes a somewhat disruptive addition to the Power ISA.
+
+To ameliorate this situation, particularly for pre-existing Hardware
+designs implementing up to Scalar Power ISA v3.1, some rules are set that
+allow those pre-existing designs not to require heavy modification to
+their existing Scalar pipelines.  These rules effectively allow Hardware
+Architects to add the additional CR Fields CR8 to CR127 as if they were
+an **entirely separate register file**.
+
+* any instruction involving more than 1 source 1 destination
+  where one of the operands is a Condition Register is prohibited from
+  using registers from both the CR0-7 group and the CR8-127 group at
+  the same time.
+* any instruction involving 1 source 1 destination where either the
+  source or the destination is a Condition Register is prohibited
+  from setting CR0-7 as a Vector.
+* prohibitions are required to be enforced by raising Illegal Instruction
+  Traps
+
+Examples of permitted instructions:
+
+```
+    sv.crand *cr8.eq, *cr16.le, *cr40.so # all CR8-CR127
+    sv.mfcr cr5, *cr40                   # only one source (CR40) copied to CR5
+    sv.mfcr *cr16, cr40                  # Vector-Splat CR40 onto CR16,17,18...
+```
+
+Examples of prohibited instructions:
+
+```
+    sv.mfcr *cr0, cr40        # Vector-Splat onto CR0,1,2
+    sv.crand cr7, cr9, cr10   # crosses over between CR0-7 and CR8-127
+```
+
 ## Future expansion.
 
 With the way that EXTRA fields are defined and applied to register