From fa163b71f736c37a8e749530496032ef2fc0fe83 Mon Sep 17 00:00:00 2001
From: lkcl <lkcl@web>
Date: Fri, 19 Aug 2022 06:43:13 +0100
Subject: [PATCH]

---
 openpower/sv/ldst/discussion.mdwn | 58 +++++++++++++++++++++++++++++++
 1 file changed, 58 insertions(+)
 create mode 100644 openpower/sv/ldst/discussion.mdwn

diff --git a/openpower/sv/ldst/discussion.mdwn b/openpower/sv/ldst/discussion.mdwn
new file mode 100644
index 000000000..a6a113e3d
--- /dev/null
+++ b/openpower/sv/ldst/discussion.mdwn
@@ -0,0 +1,58 @@
+# notes from lxo
+
+this section covers assembly notation for the immediate and indexed LD/ST.
+the summary is that in immediate mode for LD it is not clear that if the 
+destination register is Vectorised `RT.v` but the source `imm(RA)` is scalar
+the memory being read is *still a vector load*, known as "unit or element strides".
+
+This anomaly is made clear with the following notation:
+
+    sv.ld RT.v, imm(RA).v
+
+The following notation, although technically correct due to being implicitly identical to the above, is prohibited and is a syntax error:
+ 
+    sv.ld RT.v, imm(RA)
+
+Notes taken from IRC conversation
+
+    <lxo> sv.ld r#.v, ofst(r#).v -> the whole vector is at ofst+r#
+    <lxo> sv.ld r#.v, ofst(r#.v) -> r# is a vector of addresses
+    <lxo> similarly sv.ldx r#.v, r#, r#.v -> whole vector at r#+r#
+    <lxo> whereas sv.ldx r#.v, r#.v, r# -> vector of addresses
+    <lxo> point being, you take an operand with the "m" constraint (or other memory-operand constraints), append .v to it and you're done addressing the in-memory vector
+    <lxo> as in asm ("sv.ld1 %0.v, %1.v" : "=r"(vec_in_reg) : "m"(vec_in_mem));
+    <lxo> (and ld%U1 got mangled into underline; %U expands to x if the address is a sum of registers
+
+permutations of vector selection, to identify above asm-syntax:
+
+     imm(RA)  RT.v   RA.v   nonstrided
+         sv.ld r#.v, ofst(r#2.v) -> r#2 is a vector of addresses
+           mem@     0+r#2   offs+(r#2+1)  offs+(r#2+2)
+           destreg  r#      r#+1          r#+2
+     imm(RA)  RT.s   RA.v   nonstrided
+         sv.ld r#, ofst(r#2.v) -> r#2 is a vector of addresses
+           (dest r# is scalar) -> VSELECT mode
+     imm(RA)  RT.v   RA.s   fixed stride: unit or element
+         sv.ld r#.v, ofst(r#2).v -> whole vector is at ofst+r#2
+           mem@r#2  +0   +1   +2
+           destreg  r#   r#+1 r#+2
+         sv.ld/els r#.v, ofst(r#2).v -> vector at ofst*elidx+r#2
+           mem@r#2  +0 ...   +offs ...  +offs*2
+           destreg  r#       r#+1       r#+2
+     imm(RA)  RT.s   RA.s   not vectorised
+         sv.ld r#, ofst(r#2)
+
+indexed mode:
+
+     RA,RB    RT.v  RA.v  RB.v
+        sv.ldx r#.v, r#2, r#3.v -> whole vector at r#2+r#3
+     RA,RB    RT.v  RA.s  RB.v
+        sv.ldx r#.v, r#2.v, r#3.v -> whole vector at r#2+r#3
+     RA,RB    RT.v  RA.v  RB.s
+        sv.ldx r#.v, r#2.v, r#3 -> vector of addresses
+     RA,RB    RT.v  RA.s  RB.s
+        sv.ldx r#.v, r#2, r#3 -> VSPLAT mode
+     RA,RB    RT.s  RA.v  RB.v
+     RA,RB    RT.s  RA.s  RB.v
+     RA,RB    RT.s  RA.v  RB.s
+     RA,RB    RT.s  RA.s  RB.s not vectorised
-- 
2.30.2