* <https://github.com/riscv/riscv-v-spec/blob/master/v-spec.adoc#vector-loads-and-stores>
* [[simple_v_extension/specification/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)
-* fail-first on the same (where it makes sense to do so)
+* Speculative fail-first (where it makes sense to do so)
* Structure Packing (covered in SV by [[sv/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 [[isa/fixedload]] and
[[isa/fixedstore]] pseudocode to be of the form:
ireg[RT+j] <= MEM[EA];
if (!RT.isvec)
break # destination scalar, end immediately
- if (!RA.isvec && !RB.isvec)
- break # scalar-scalar
+ if svctx.ldstmode != elementstride:
+ if (!RA.isvec && !RB.isvec)
+ break # scalar-scalar
if (RA.isvec) i++;
if (RAupdate.isvec) u++;
if (RB.isvec) k++;
| 0-1 | 2 | 3 4 | description |
| --- | --- |---------|--------------------------- |
-| 00 | 0 | dz els | normal mode |
-| 00 | 1 | dz shf | shift mode |
+| 00 | 0 | zz els | normal mode |
+| 00 | 1 | zz shf | shift mode |
| 01 | inv | CR-bit | Rc=1: ffirst CR sel |
| 01 | inv | els RC1 | Rc=0: ffirst z/nonz |
-| 10 | N | dz els | sat mode: N=0/1 u/s |
+| 10 | N | zz els | sat mode: N=0/1 u/s |
| 11 | inv | CR-bit | Rc=1: pred-result CR sel |
| 11 | inv | els RC1 | Rc=0: pred-result z/nonz |
| 01 | SEA | dz sz | Strided (scalar only source) |
| 10 | N | dz sz | sat mode: N=0/1 u/s |
| 11 | inv | CR-bit | Rc=1: pred-result CR sel |
-| 11 | inv | dz RC1 | Rc=0: pred-result z/nonz |
+| 11 | inv | zz RC1 | Rc=0: pred-result z/nonz |
+
+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:
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 is unsigned.
+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*
## LD/ST ffirst
-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 in these special circumstances requesting ffirst on Indexed LD/ST is instead interpreted as element-strided LD/ST. See <https://bugs.libre-soc.org/show_bug.cgi?id=561>
+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]];
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 <a name="elwidth"></a>
Loads and Stores are almost unique in that the OpenPOWER Scalar ISA
projects / libreriscv.git / blobdiff