# 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