SV Load and Store

Links:

• https://bugs.libre-soc.org/show_bug.cgi?id=561
• https://bugs.libre-soc.org/show_bug.cgi?id=572
• https://bugs.libre-soc.org/show_bug.cgi?id=571
• https://llvm.org/devmtg/2016-11/Slides/Emerson-ScalableVectorizationinLLVMIR.pdf
• https://github.com/riscv/riscv-v-spec/blob/master/v-spec.adoc#vector-loads-and-stores
• ld.x

Rationale

All Vector ISAs dating back fifty years have extensive and comprehensive Load and Store operations that go far beyond the capabilities of Scalar RISC or CISC processors, yet at their heart on an individual element basis may be found to be no different from RISC Scalar equivalents.

The resource savings from Vector LD/ST are significant and stem from the fact that one single instruction can trigger a dozen (or in some microarchitectures such as Cray or NEC SX Aurora) hundreds of element-level Memory accesses.

Additionally, and simply: if the Arithmetic side of an ISA supports Vector Operations, then in order to keep the ALUs 100% occupied the Memory infrastructure (and the ISA itself) correspondingly needs Vector Memory Operations as well.

Vectorised Load and Store also presents an extra dimension (literally) which creates scenarios unique to Vector applications, that a Scalar (and even a SIMD) ISA simply never encounters. SVP64 endeavours to add such modes without changing the behaviour of the underlying Base (Scalar) v3.0B operations.

Modes overview

Vectorisation of Load and Store requires creation, from scalar operations, a number of different modes:

• fixed stride (contiguous sequence with no gaps) aka "unit" stride
• element strided (sequential but regularly offset, with gaps)
• vector indexed (vector of base addresses and vector of offsets)
• Speculative fail-first (where it makes sense to do so)
• Structure Packing (covered in SV by remap).

Also included in SVP64 LD/ST is both signed and unsigned Saturation, as well as Element-width overrides and Twin-Predication.

Vectorisation of Scalar Power ISA v3.0B

OpenPOWER Load/Store operations may be seen from fixedload and fixedstore pseudocode to be of the form:

lbux RT, RA, RB
EA <- (RA) + (RB)
RT <- MEM(EA)

and for immediate variants:

lb RT,D(RA)
EA <- RA + EXTS(D)
RT <- MEM(EA)

Thus in the first example, the source registers may each be independently marked as scalar or vector, and likewise the destination; in the second example only the one source and one dest may be marked as scalar or vector.

Thus we can see that Vector Indexed may be covered, and, as demonstrated with the pseudocode below, the immediate can be used to give unit stride or element stride. With there being no way to tell which from the OpenPOWER v3.0B Scalar opcode alone, the choice is provided instead by the SV Context.

# LD not VLD!  format - ldop RT, immed(RA)
# op_width: lb=1, lh=2, lw=4, ld=8
op_load(RT, RA, RC, op_width, immed, svctx, RAupdate):
  ps = get_pred_val(FALSE, RA); # predication on src
  pd = get_pred_val(FALSE, RT); # ... AND on dest
  for (i=0, j=0, u=0; i < VL && j < VL;):
    # skip nonpredicates elements
    if (RA.isvec) while (!(ps & 1<<i)) i++;
    if (RAupdate.isvec) while (!(ps & 1<<u)) u++;
    if (RT.isvec) while (!(pd & 1<<j)) j++;
    if svctx.ldstmode == shifted: # for FFT/DCT
      # FFT/DCT shifted mode
      if (RA.isvec)
        srcbase = ireg[RA+i]
      else
        srcbase = ireg[RA]
      offs = (i * immed) << RC
    elif svctx.ldstmode == elementstride:
      # element stride mode
      srcbase = ireg[RA]
      offs = i * immed              # j*immed for a ST
    elif svctx.ldstmode == unitstride:
      # unit stride mode
      srcbase = ireg[RA]
      offs = immed + (i * op_width) # j*op_width for ST
    elif RA.isvec:
      # quirky Vector indexed mode but with an immediate
      srcbase = ireg[RA+i]
      offs = immed;
    else
      # standard scalar mode (but predicated)
      # no stride multiplier means VSPLAT mode
      srcbase = ireg[RA]
      offs = immed

    # compute EA
    EA = srcbase + offs
    # update RA?
    if RAupdate: ireg[RAupdate+u] = EA;
    # load from memory
    ireg[RT+j] <= MEM[EA];
    if (!RT.isvec)
        break # destination scalar, end now
    if (RA.isvec) i++;
    if (RAupdate.isvec) u++;
    if (RT.isvec) j++;

# reverses the bitorder up to "width" bits
def bitrev(val, VL):
  width = log2(VL)
  result = 0
  for _ in range(width):
    result = (result << 1) | (val & 1)
    val >>= 1
  return result

Indexed LD is:

# format: ldop RT, RA, RB
function op_ldx(RT, RA, RB, RAupdate=False) # LD not VLD!
  ps = get_pred_val(FALSE, RA); # predication on src
  pd = get_pred_val(FALSE, RT); # ... AND on dest
  for (i=0, j=0, k=0, u=0; i < VL && j < VL && k < VL):
    # skip nonpredicated RA, RB and RT
    if (RA.isvec) while (!(ps & 1<<i)) i++;
    if (RAupdate.isvec) while (!(ps & 1<<u)) u++;
    if (RB.isvec) while (!(ps & 1<<k)) k++;
    if (RT.isvec) while (!(pd & 1<<j)) j++;
    if svctx.ldstmode == elementstride:
        EA = ireg[RA] + ireg[RB]*j   # register-strided
    else
        EA = ireg[RA+i] + ireg[RB+k] # indexed address
    if RAupdate: ireg[RAupdate+u] = EA
    ireg[RT+j] <= MEM[EA];
    if (!RT.isvec)
        break # destination scalar, end immediately
    if svctx.ldstmode != elementstride:
        if (!RA.isvec && !RB.isvec)
            break # scalar-scalar
    if (RA.isvec) i++;
    if (RAupdate.isvec) u++;
    if (RB.isvec) k++;
    if (RT.isvec) j++;

Note in both cases that svp64 allows RA-as-a-dest in "update" mode (ldux) to be effectively a completely different register from RA-as-a-source. This because there is room in svp64 to extend RA-as-src as well as RA-as-dest, both independently as scalar or vector and independently extending their range.

Determining the LD/ST Modes

A minor complication (caused by the retro-fitting of modern Vector features to a Scalar ISA) is that certain features do not exactly make sense or are considered a security risk. Fail-first on Vector Indexed would allow attackers to probe large numbers of pages from userspace, where strided fail-first (by creating contiguous sequential LDs) does not.

In addition, reduce mode makes no sense, and for LD/ST with immediates Vector source RA makes no sense either (or, is a quirk). Realistically we need an alternative table meaning for svp64 mode. The following modes make sense:

• saturation
• predicate-result (mostly for cache-inhibited LD/ST)
• normal
• fail-first (where Vector Indexed is banned)
• Signed Effective Address computation (Vector Indexed only)

Also, given that FFT, DCT and other related algorithms are of such high importance in so many areas of Computer Science, a special "shift" mode has been added which allows part of the immediate to be used instead as RC, a register which shifts the immediate DS << GPR(RC).

The table for svp64 for immed(RA) is:

The els bit is only relevant when RA.isvec is clear: this indicates whether stride is unit or element:

if bitreversed:
    svctx.ldstmode = bitreversed
elif RA.isvec:
    svctx.ldstmode = indexed
elif els == 0:
    svctx.ldstmode = unitstride
elif immediate != 0:
    svctx.ldstmode = elementstride

An immediate of zero is a safety-valve to allow LD-VSPLAT: in effect the multiplication of the immediate-offset by zero results in reading from the exact same memory location.

For LD-VSPLAT, on non-cache-inhibited Loads, the read can occur just the once and be copied, rather than hitting the Data Cache multiple times with the same memory read at the same location. This would allow for memory-mapped peripherals to have multiple data values read in quick succession and stored in sequentially numbered registers.

For non-cache-inhibited ST from a vector source onto a scalar destination: with the Vector loop effectively creating multiple memory writes to the same location, we can deduce that the last of these will be the "successful" one. Thus, implementations are free and clear to optimise out the overwriting STs, leaving just the last one as the "winner". Bear in mind that predicate masks will skip some elements (in source non-zeroing mode). Cache-inhibited ST operations on the other hand MUST write out a Vector source multiple successive times to the exact same Scalar destination.

Note that there are no immediate versions of cache-inhibited LD/ST.

The modes for RA+RB indexed version are slightly different:

Vector Indexed Strided Mode is qualified as follows:

if mode = 0b01 and !RA.isvec and !RB.isvec:
    svctx.ldstmode = elementstride

A summary of the effect of Vectorisation of src or dest:

 imm(RA)  RT.v   RA.v   no stride allowed
 imm(RA)  RT.s   RA.v   no stride allowed
 imm(RA)  RT.v   RA.s   stride-select allowed
 imm(RA)  RT.s   RA.s   not vectorised
 RA,RB    RT.v  {RA|RB}.v UNDEFINED
 RA,RB    RT.s  {RA|RB}.v UNDEFINED
 RA,RB    RT.v  {RA&RB}.s VSPLAT possible. stride selectable
 RA,RB    RT.s  {RA&RB}.s not vectorised

Signed Effective Address computation is only relevant for Vector Indexed Mode, when elwidth overrides are applied. The source override applies to RB, and before adding to RA in order to calculate the Effective Address, if SEA is set RB is sign-extended from elwidth bits to the full 64 bits. For other Modes (ffirst, saturate), all EA computation with elwidth overrides is unsigned.

Note that cache-inhibited LD/ST (ldcix) when VSPLAT is activated will perform multiple LD/ST operations, sequentially. ldcix even with scalar src will read the same memory location multiple times, storing the result in successive Vector destination registers. This because the cache-inhibit instructions are used to read and write memory-mapped peripherals. If a genuine cache-inhibited LD-VSPLAT is required then a scalar cache-inhibited LD should be performed, followed by a VSPLAT-augmented mv.

LD/ST ffirst

LD/ST ffirst treats the first LD/ST in a vector (element 0) as an ordinary one. Exceptions occur "as normal". However for elements 1 and above, if an exception would occur, then VL is truncated to the previous element: the exception is not then raised because the LD/ST was effectively speculative.

ffirst LD/ST to multiple pages via a Vectorised Index base is considered a security risk due to the abuse of probing multiple pages in rapid succession and getting feedback on which pages would fail. Therefore Vector Indexed LD/ST is prohibited entirely, and the Mode bit instead used for element-strided LD/ST. See https://bugs.libre-soc.org/show_bug.cgi?id=561

for(i = 0; i < VL; i++)
    reg[rt + i] = mem[reg[ra] + i * reg[rb]];

High security implementations where any kind of speculative probing of memory pages is considered a risk should take advantage of the fact that implementations may truncate VL at any point, without requiring software to be rewritten and made non-portable. Such implementations may choose to always set VL=1 which will have the effect of terminating any speculative probing (and also adversely affect performance), but will at least not require applications to be rewritten.

Low-performance simpler hardware implementations may choose (always) to also set VL=1 as the bare minimum compliant implementation of LD/ST Fail-First. It is however critically important to remember that the first element LD/ST MUST be treated as an ordinary LD/ST, i.e. MUST raise exceptions exactly like an ordinary LD/ST.

For ffirst LD/STs, VL may be truncated arbitrarily to a nonzero value for any implementation-specific reason. For example: it is perfectly reasonable for implementations to alter VL when ffirst LD or ST operations are initiated on a nonaligned boundary, such that within a loop the subsequent iteration of that loop begins subsequent ffirst LD/ST operations on an aligned boundary such as the beginning of a cache line, or beginning of a Virtual Memory page. Likewise, to reduce workloads or balance resources.

Vertical-First Mode is slightly strange in that only one element at a time is ever executed anyway. Given that programmers may legitimately choose to alter srcstep and dststep in non-sequential order as part of explicit loops, it is neither possible nor safe to make speculative assumptions about future LD/STs. Therefore, Fail-First LD/ST in Vertical-First is UNDEFINED. This is very different from Arithmetic (Data-dependent) FFirst where Vertical-First Mode is deterministic, not speculative.

LOAD/STORE Elwidths

Loads and Stores are almost unique in that the OpenPOWER Scalar ISA provides a width for the operation (lb, lh, lw, ld). Only extsb and others like it provide an explicit operation width. There are therefore three widths involved:

• operation width (lb=8, lh=16, lw=32, ld=64)
• src elelent width override
• destination element width override

Some care is therefore needed to express and make clear the transformations, which are expressly in this order:

• Load at the operation width (lb/lh/lw/ld) as usual
• byte-reversal as usual
• Non-saturated mode:
  • zero-extension or truncation from operation width to source elwidth
  • zero/truncation to dest elwidth
• Saturated mode:
  • Sign-extension or truncation from operation width to source width
  • signed/unsigned saturation down to dest elwidth

In order to respect OpenPOWER v3.0B Scalar behaviour the memory side is treated effectively as completely separate and distinct from SV augmentation. This is primarily down to quirks surrounding LE/BE and byte-reversal in OpenPOWER.

It is unfortunately possible to request an elwidth override on the memory side which does not mesh with the operation width: these result in UNDEFINED behaviour. The reason is that the effect of attempting a 64-bit sv.ld operation with a source elwidth override of 8/16/32 would result in overlapping memory requests, particularly on unit and element strided operations. Thus it is UNDEFINED when the elwidth is smaller than the memory operation width. Examples include sv.lw/sw=16/els which requests (overlapping) 4-byte memory reads offset from each other at 2-byte intervals. Store likewise is also UNDEFINED where the dest elwidth override is less than the operation width.

Note the following regarding the pseudocode to follow:

• scalar identity behaviour SV Context parameter conditions turn this into a straight absolute fully-compliant Scalar v3.0B LD operation
• brev selects whether the operation is the byte-reversed variant (ldbrx rather than ld)
• op_width specifies the operation width (lb, lh, lw, ld) as a "normal" part of Scalar v3.0B LD
• imm_offs specifies the immediate offset ld r3, imm_offs(r5), again as a "normal" part of Scalar v3.0B LD
• svctx specifies the SV Context and includes VL as well as source and destination elwidth overrides.

Below is the pseudocode for Unit-Strided LD (which includes Vector capability).

Note that twin predication, predication-zeroing, saturation and other modes have all been removed, for clarity and simplicity:

# LD not VLD! (ldbrx if brev=True)
# this covers unit stride mode and a type of vector offset
function op_ld(RT, RA, brev, op_width, imm_offs, svctx)
  for (int i = 0, int j = 0; i < svctx.VL && j < svctx.VL;):

    if not svctx.unit/el-strided:
        # strange vector mode, compute 64 bit address which is
        # not polymorphic! elwidth hardcoded to 64 here
        srcbase = get_polymorphed_reg(RA, 64, i)
    else:
        # unit / element stride mode, compute 64 bit address
        srcbase = get_polymorphed_reg(RA, 64, 0)
        # adjust for unit/el-stride
        srcbase += ....

    # takes care of (merges) processor LE/BE and ld/ldbrx
    bytereverse = brev XNOR MSR.LE

    # read the underlying memory
    memread <= mem[srcbase + imm_offs];

    # optionally performs byteswap at op width
    if (bytereverse):
        memread = byteswap(memread, op_width)

    # check saturation.
    if svpctx.saturation_mode:
        ... saturation adjustment...
    else:
        # truncate/extend to over-ridden source width.
        memread = adjust_wid(memread, op_width, svctx.src_elwidth)

    # takes care of inserting memory-read (now correctly byteswapped)
    # into regfile underlying LE-defined order, into the right place
    # within the NEON-like register, respecting destination element
    # bitwidth, and the element index (j)
    set_polymorphed_reg(RT, svctx.dest_bitwidth, j, memread)

    # increments both src and dest element indices (no predication here)
    i++;
    j++;

Remapped LD/ST

In the propagation page the concept of "Remapping" is described. Whilst it is expensive to set up (2 64-bit opcodes minimum) it provides a way to arbitrarily perform 1D, 2D and 3D "remapping" of up to 64 elements worth of LDs or STs. The usual interest in such re-mapping is for example in separating out 24-bit RGB channel data into separate contiguous registers. NEON covers this as shown in the diagram below:

Remap easily covers this capability, and with dest elwidth overrides and saturation may do so with built-in conversion that would normally require additional width-extension, sign-extension and min/max Vectorised instructions as post-processing stages.

Thus we do not need to provide specialist LD/ST "Structure Packed" opcodes because the generic abstracted concept of "Remapping", when applied to LD/ST, will give that same capability, with far more flexibility.

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