fields set to a reserved value must cause an illegal instruction trap,
to allow emulation of future instruction sets.
-This is unlike OpenPower ISA v3.1, which in many instances does not require a trap.
+This is unlike OpenPower ISA v3.1, which in many instances does not require a trap if reserved fields are nonzero.
# Identity Behaviour
-SVP64 is designed so that when the prefix is all zeros, no effect or
+SVP64 is designed so that when the prefix is all zeros, and
+ VL=1, no effect or
influence occurs (no augmentation) such that all standard OpenPOWER
v3.0/1B instructions covered by the prefix are "unaltered". This is termed `scalar identity behaviour` (based on the mathematical definition for "identity", as in, "identity matrix" or better "identity transformation").
-Note that this is completely different from when VL=0. VL=0 turns all operations under its influence into `nops`, whereas when VL=1 and the SV prefix is all zeros, the operation simply acts as if SV had not been applied at all to the instruction (an "identity operation").
+Note that this is completely different from when VL=0. VL=0 turns all operations under its influence into `nops` (regardless of the prefix)
+ whereas when VL=1 and the SV prefix is all zeros, the operation simply acts as if SV had not been applied at all to the instruction (an "identity operation").
-# Register Naming
+# XER, SO and other global flags
+
+Vector systems are expected to be high performance. This is achieved
+through parallelism, which requires that elements in the vector be
+independent. XER SO and other global "accumulation" flags (CR.OV) cause
+Read-Write Hazards on single-bit global resources, having a significant
+detrimental adverse effect.
+
+Consequently in SV, XER.SO and CR.OV behaviour is disregarded. XER is
+simply neither read nor written. This includes when `scalar identity behaviour` occurs. If OpenPOWER v3.0/1 scalar behaviour is desired then OpenPOWER v3.0/1 instructions should be used, not SV Prefixed ones.
+
+An interesting side-effect of this decision is that the OE flag is now free for other uses when SV Prefixing is used.
+
+# Additional instructions: v3.0B/v3.1B overrides
+
+SV is primarily designed for use as an efficient hybrid 3D GPU / VPU / CPU ISA.
+
+As mentioned above, OE=1 is not applicable in SV, freeing this bit for alternative uses. Additionally, Vectorisation of the VSX SIMD system likewise makes no sense whatsoever: SV replaces VSX and provides, at the very minimum, predication (which VSX was not designed to incorporate). Thus all VSX Major Opcodes - all of them - are "unused" and raise illegal instruction exceptions in SV Prefix Mode.
+
+This leaves several Major Opcodes free for use by SV to fit alternative instructions: Vector Product, Vector Normalise, [[sv/mv.swizzle]], Texture LD/ST operations, and others critical to an efficient, effective 3D GPU and VPU ISA, and included as standard in other commercially-successful GPU ISAs.
+
+# Register Naming and size
SV Registers are simply the INT, FP and CR register files extended
-linearly to larger sizes. Where the integer regfile in standard scalar
-OpenPOWER v3.0B and v3.1B is r0 to r31, SV extends this as r0 to r127.
+linearly to larger sizes; SV Vectorisation iterates sequentially through these registers.
+
+Where the integer regfile in standard scalar
+OpenPOWER v3.0B/v3.1B is r0 to r31, SV extends this as r0 to r127.
Likewise FP registers are extended to 128 (fp0 to fp127), and CRs are
extended to 64 entries, CR0 thru CR63.
RB etc. are interpreted and treated as v3.0B / v3.1B scalar registers. This is part of
`scalar identity behaviour` described above.
+## Future expansion.
+
+With the way that EXTRA fields are defined and applied to register fields,
+future versions of SV may involve 256 or greater registers. To accommodate 256 registers, numbering of Vectors will simply shift up by one bit, without
+requiring additional prefix bits. Backwards binary compatibility may be achieved with a PCR bit (Program Compatibility Register). Beyond this, further discussion is out of scope for this version of svp64.
+
# Remapped Encoding (`RM[0:23]`)
To allow relatively easy remapping of which portions of the Prefix Opcode
(shows both PowerISA v3.1 instructions as well as new SVP instructions; empty spaces are yet-to-be-allocated Illegal Instructions)
| 6:11 | ---000 | ---001 | ---010 | ---011 | ---100 | ---101 | ---110 | ---111 |
-| ---- | ------ | ------ | ------ | ------ | ------ | ------ | ------ | ------ |
+|------|--------|--------|--------|--------|--------|--------|--------|--------|
|000---| 8LS | 8LS | 8LS | 8LS | 8LS | 8LS | 8LS | 8LS |
|001---| | | | | | | | |
|010---| 8RR | | | | `SVP64`| `SVP64`| `SVP64`| `SVP64`|
## Prefix Fields
-| Prefix Field Name | Field | Value | Description |
-|---------------------|---------|-------|--------------------------------------------|
-| PO (Primary Opcode) | `0:5` | `1` | Indicates this is Prefixed 64-bit |
-| `RM[0]` | `6` | | Bit 0 of the Remapped Encoding |
-| SVP64_7 | `7` | `1` | Indicates this is SVP64 |
-| `RM[1]` | `8` | | Bit 1 of the Remapped Encoding |
-| SVP64_9 | `9` | `1` | Indicates this is SVP64 |
-| `RM[2:23]` | `10:31` | | Bits 2-23 of the Remapped Encoding |
+| Name | Bits | Value | Description |
+|------------|---------|-------|--------------------------------|
+| EXT01 | `0:5` | `1` | Indicates Prefixed 64-bit |
+| `RM[0]` | `6` | | Bit 0 of Remapped Encoding |
+| SVP64_7 | `7` | `1` | Indicates this is SVP64 |
+| `RM[1]` | `8` | | Bit 1 of Remapped Encoding |
+| SVP64_9 | `9` | `1` | Indicates this is SVP64 |
+| `RM[2:23]` | `10:31` | | Bits 2-23 of Remapped Encoding |
# Remapped Encoding Fields
## RM-1P-3S1D
-| Field Name | Field bits | Description |
-|------------|------------|------------------------------------------------|
-| MASK\_KIND | `0` | Execution Mask Kind |
-| MASK | `1:3` | Execution Mask |
-| ELWIDTH | `4:5` | Element Width |
-| SUBVL | `6:7` | Sub-vector length |
-| Rdest\_EXTRA2 | `8:9` | extra bits for Rdest (R\*\_EXTRA2 Encoding) |
-| Rsrc1\_EXTRA2 | `10:11` | extra bits for Rsrc1 (R\*\_EXTRA2 Encoding) |
-| Rsrc2\_EXTRA2 | `12:13` | extra bits for Rsrc2 (R\*\_EXTRA2 Encoding) |
-| Rsrc3\_EXTRA2 | `14:15` | extra bits for Rsrc3 (R\*\_EXTRA2 Encoding|
-| reserved | `16` | reserved |
-| MODE | `19:23` | changes Vector behaviour |
+| Field Name | Field bits | Description |
+|------------|------------|----------------------------------------|
+| MASK\_KIND | `0` | Execution Mask Kind |
+| MASK | `1:3` | Execution Mask |
+| ELWIDTH | `4:5` | Element Width |
+| SUBVL | `6:7` | Sub-vector length |
+| Rdest\_EXTRA2 | `8:9` | extends Rdest (R\*\_EXTRA2 Encoding) |
+| Rsrc1\_EXTRA2 | `10:11` | extends Rsrc1 (R\*\_EXTRA2 Encoding) |
+| Rsrc2\_EXTRA2 | `12:13` | extends Rsrc2 (R\*\_EXTRA2 Encoding) |
+| Rsrc3\_EXTRA2 | `14:15` | extends Rsrc3 (R\*\_EXTRA2 Encoding) |
+| reserved | `16` | reserved |
+| MODE | `19:23` | changes Vector behaviour |
## RM-1P-2S1D
-| Field Name | Field bits | Description |
-|------------|------------|------------------------------------------------|
-| MASK\_KIND | `0` | Execution Mask Kind |
-| MASK | `1:3` | Execution Mask |
-| ELWIDTH | `4:5` | Element Width |
-| SUBVL | `6:7` | Sub-vector length |
-| Rdest\_EXTRA3 | `8:10` | extra bits for Rdest (Uses R\*\_EXTRA3 Encoding) |
-| Rsrc1\_EXTRA3 | `11:13` | extra bits for Rsrc1 (Uses R\*\_EXTRA3 Encoding) |
-| Rsrc2\_EXTRA3 | `14:16` | extra bits for Rsrc3 (Uses R\*\_EXTRA3 Encoding) |
-| MODE | `19:23` | changes Vector behaviour |
+| Field Name | Field bits | Description |
+|------------|------------|-------------------------------------------|
+| MASK\_KIND | `0` | Execution Mask Kind |
+| MASK | `1:3` | Execution Mask |
+| ELWIDTH | `4:5` | Element Width |
+| SUBVL | `6:7` | Sub-vector length |
+| Rdest\_EXTRA3 | `8:10` | extends Rdest |
+| Rsrc1\_EXTRA3 | `11:13` | extends Rsrc1 |
+| Rsrc2\_EXTRA3 | `14:16` | extends Rsrc3 |
+| ELWIDTH_SRC | `17:18` | Element Width for Source |
+| MODE | `19:23` | changes Vector behaviour |
These are for 2 operand 1 dest instructions, such as `add RT, RA,
-RB`. However also included are unusual instructions with the same src
-and dest, such as `rlwinmi`.
+RB`. However also included are unusual instructions with an implicit dest
+that is identical to its src reg, such as `rlwinmi`.
-Normally, with instructions such as `rlwinmi`, the scalar v3.0B ISA would not have sufficient bits to allow
+Normally, with instructions such as `rlwinmi`, the scalar v3.0B ISA would not have sufficient bit fields to allow
an alternative destination. With SV however this becomes possible.
Therefore, the fact that the dest is implicitly also a src should not
mislead: due to the *prefix* they are different SV regs.
each may be *independently* made vector or scalar, and be independently
augmented to 7 bits in length.
+Note that if ELWIDTH != ELWIDTH_SRC this may result in reduced performance or increased latency in some implementations due to lane-crossing.
+
## RM-2P-1S1D
| Field Name | Field bits | Description |
| MASK | `1:3` | Execution Mask |
| ELWIDTH | `4:5` | Element Width |
| SUBVL | `6:7` | Sub-vector length |
-| Rdest_EXTRA3 | `8:10` | extra bits for Rdest |
-| Rsrc1_EXTRA3 | `11:13` | extra bits for Rsrc1 |
+| Rdest_EXTRA3 | `8:10` | extends Rdest |
+| Rsrc1_EXTRA3 | `11:13` | extends Rsrc1 |
| MASK_SRC | `14:16` | Execution Mask for Source |
| ELWIDTH_SRC | `17:18` | Element Width for Source |
| MODE | `19:23` | changes Vector behaviour |
-note in [[discussion]]: TODO, evaluate if 2nd SUBVL should be added.
-conclusion: no. 2nd SUBVL makes no sense except for mv, and that is
-covered by [[mv.vec]]
+Note that if ELWIDTH != ELWIDTH_SRC this may result in reduced performance or increased latency in some implementations due to lane-crossing.
## RM-2P-2S1D/1S2D
| MASK | `1:3` | Execution Mask |
| ELWIDTH | `4:5` | Element Width |
| SUBVL | `6:7` | Sub-vector length |
-| Rdest_EXTRA2 | `8:9` | extra bits for Rdest (R\*\_EXTRA2 Encoding) |
-| Rsrc1_EXTRA2 | `10:11` | extra bits for Rsrc1 (R\*\_EXTRA2 Encoding) |
-| Rsrc2_EXTRA2 | `12:13` | extra bits for Rsrc2 (R\*\_EXTRA2 Encoding) |
+| Rdest_EXTRA2 | `8:9` | extends Rdest (R\*\_EXTRA2 Encoding) |
+| Rsrc1_EXTRA2 | `10:11` | extends Rsrc1 (R\*\_EXTRA2 Encoding) |
+| Rsrc2_EXTRA2 | `12:13` | extends Rsrc2 (R\*\_EXTRA2 Encoding) |
| MASK_SRC | `14:16` | Execution Mask for Source |
| ELWIDTH_SRC | `17:18` | Element Width for Source |
| MODE | `19:23` | changes Vector behaviour |
Twin Predication. therefore these are treated as RM-2P-2S1D and the
src spec for RA is also used for the same RA as a dest.
+Note that if ELWIDTH != ELWIDTH_SRC this may result in reduced performance or increased latency in some implementations due to lane-crossing.
# Mode
* **normal** mode is straight vectorisation. no augmentations: the vector comprises an array of independently created results.
* **ffirst** or data-dependent fail-on-first: see separate section. the vector may be truncated depending on certain criteria.
-* **sat mode** or saturation: clamps each elemrnt result to a min/max rather than overflows / wraps. allows signed and unsigned clamping.
+ *VL is altered as a result*.
+* **sat mode** or saturation: clamps each elemrnt result to a min/max rather than overflows / wraps. allows signed and unsigned clamping.
* **reduce mode**. a mapreduce is performed. the result is a scalar. a result vector however is required, as the upper elements may be used to store intermediary computations. the result of the mapreduce is in the first element with a nonzero predicate bit. see separate section below.
- note that reduce mode only applies to 2 src operations.
-* **pred-result** will test the result (CR testing selects a bit of CR and inverts it, just like branch testing) and if the test fails it is as if the predicate bit was zero. When Rc=1 the CR element (CR0) however is still stored in the CR regfile. This scheme does not apply to crops (crand, cror).
+ note that there are comprehensive caveats when using this mode.
+* **pred-result** will test the result (CR testing selects a bit of CR and inverts it, just like branch testing) and if the test fails it is as if the predicate bit was zero. When Rc=1 the CR element however is still stored in the CR regfile, even if the test failed. This scheme does not apply to crops (crand, cror). See appendix for details.
Note that ffirst and reduce modes are not anticipated to be high-performance in some implementations. ffirst due to interactions with VL, and reduce due to it requiring additional operations to produce a result. normal, saturate and pred-result are however independent and may easily be parallelised to give high performance, regardless of the value of VL.
| 0-1 | 2 | 3 4 | description |
| --- | --- |---------|-------------------------- |
-| 00 | 0 | sz dz | normal mode |
+| 00 | 0 | sz dz | normal mode |
| 00 | 1 | sz CRM | reduce mode (mapreduce), SUBVL=1 |
-| 00 | 1 | SVM CRM | reduce mode (mapreduce), SUBVL>1 |
-| 01 | inv | CR-bit | Rc=1: ffirst CR sel |
+| 00 | 1 | SVM CRM | subvector reduce mode, SUBVL>1 |
+| 01 | inv | CR-bit | Rc=1: ffirst CR sel |
| 01 | inv | sz dz | Rc=0: ffirst z/nonz |
| 10 | N | sz dz | sat mode: N=0/1 u/s |
| 11 | inv | CR-bit | Rc=1: pred-result CR sel |
* **SVM** sets "subvector" reduce mode
* **N** sets signed/unsigned saturation.
-## Rounding, clamp and saturate
-
-One of the issues with vector ops is that in integer DSP ops for example in Audio the operation must clamp or saturate rather than overflow or ignore the upper bits and become a modulo operation. This for Audio is extremely important, also to provide an indicator as to whether saturation occurred. see [[av_opcodes]].
-
-To help ensure that audio quality is not compromised by overflow, "saturation" is provided, as well as a way to detect when saturation occurred (Rc=1). When Rc=1 there will be a *vector* of CRs, one CR per element in the result (Note: this is different from VSX which has a single CR per block).
-
-When N=0 the result is saturated to within the maximum range of an unsigned value. For integer ops this will be 0 to 2^elwidth-1. Similar logic applies to FP operations, with the result being saturated to maximum rather than returning INF.
-
-When N=1 the same occurs except that the result is saturated to the min or max of a signed result.
-
-When Rc=1, the CR "overflow" bit is set on the CR associated with the element, to indicate whether saturation occurred. Note that due to the hugely detrimental effect it has on parallel processing, XER.SO is **ignored** completely and is **not** brought into play here. The CR overflow bit is therefore simply set to zero if saturation did not occur, and to one if it did.
-
-Post-analysis of the Vector of CRs to find out if any given element hit saturation may be done using a mapreduced CR op (cror), or by using the new crweird instruction, transferring the relevant CR bits to a scalar integer and testing it for nonzero. see [[sv/cr_int_predication]]
-
-
-## Reduce mode
-
-1. limited to single predicated dual src operations (add RT, RA, RB) and to triple source operations where one of the inputs is set to a scalar (these are rare)
-2. limited to operations that make sense. divide is excluded, as is subtract (X - Y - Z produces different answers depending on the order). sane operations: multiply, add, logical bitwise OR, CR operations. operations that do not return the same register type are also excluded (isel, cmp)
-3. the destination is a vector but the result is stored, ultimately, in the first nonzero predicated element. all other nonzero predicated elements are undefined. *this includes the CR vector* when Rc=1
-4. implementations may use any ordering and any algorithm to reduce down to a single result. However it must be equivalent to a straight application of mapreduce. The destination vector (except masked out elements) may be used for storing any intermediate results. these may be left in the vector (undefined).
-5. CRM applies when Rc=1. When CRM is zero, the CR associated with the result is regarded as a "some results met standard CR result criteria". When CRM is one, this changes to "all results met standard CR criteria".
-6. implementations MAY use destoffs as well as srcoffs (see [[sv/sprs]]) in order to store sufficient state to resume operation should an interrupt occur. this is also why implementations are permitted to use the destination vector to store intermediary computations
-
-TODO: Rc=1 on Scalar Logical Operations? is this possible? was space reserved in Logical Ops?
-
-Pseudocode for the case where RA==RB:
-
- result = op(iregs[RA], iregs[RA+1])
- CR = analyse(result)
- for i in range(2, VL):
- result = op(result, iregs[RA+i])
- CRnew = analyse(result)
- if Rc=1
- if CRM:
- CR = CR bitwise or CRnew
- else:
- CR = CR bitwise AND CRnew
-
-TODO: case where RA!=RB which involves first a vector of 2-operand results followed by a mapreduce on the intermediates.
-
-Note that when SUBVL!=1 the sub-elements are *independent*, i.e. they are mapreduced per *sub-element* as a result. illustration with a vec2:
-
- result.x = op(iregs[RA].x, iregs[RA+1].x)
- result.y = op(iregs[RA].y, iregs[RA+1].y)
- for i in range(2, VL):
- result.x = op(result.x, iregs[RA+i].x)
- result.y = op(result.y, iregs[RA+i].y)
-
-When SVM is set and SUBVL!=1, another variant is enabled, which switches to `RM-2P-2S1D` such that different elwidths may be applied to src and dest.
-
- for i in range(VL):
- result = op(iregs[RA+i].x, iregs[RA+i].x)
- result = op(result, iregs[RA+i].z)
- result = op(result, iregs[RA+i].z)
- iregs[RT+i] = result
-
-
-
-## Fail-on-first
-
-Data-dependent fail-on-first has two distinct variants: one for LD/ST, the other for arithmetic operations (actually, CR-driven). Note in each case the assumption is that vector elements are required appear to be executed in sequential Program Order, element 0 being the first.
-
-* 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.
-* Data-driven (CR-driven) fail-on-first activates when Rc=1 or other CR-creating operation produces a result (including cmp). Similar to branch, an analysis of the CR is performed and if the test fails, the vector operation terminates and discards all element operations at and above the current one, and VL is truncated to the *previous* element. Thus the new VL comprises a contiguous vector of results, all of which pass the testing criteria (equal to zero, less than zero).
-
-The CR-based data-driven fail-on-first is new and not found in ARM SVE or RVV. It is extremely useful for reducing instruction count, however requires speculative execution involving modifications of VL to get high performance implementations.
-
-In CR-based data-driven fail-on-first there is only the option to select and test one bit of each CR (just as with branch BO). For more complex tests this may be insufficient. If that is the case, a vectorised crops (crand, cror) may be used, and ffirst applied to the crop instead of to the arithmetic vector.
-
-One extremely important aspect of ffirst is:
-
-* LDST ffirst may never set VL equal to zero. This because on the first element an exception must be raised "as normal".
-* CR-based data-dependent ffirst on the other hand **can** set VL equal to zero. This is the only means in the entirety of SV that VL may be set to zero (with the exception of via the SV.STATE SPR). When VL is set zero due to the first element failing the CR bit-test, all subsequent vectorised operations are effectively `nops` which is *precisely the desired and intended behaviour*.
-
-# R\*_EXTRA2 and R\*_EXTRA3 Encoding
+# R\*\_EXTRA2 and R\*\_EXTRA3 Encoding
EXTRA is the means by which two things are achieved:
where overlap will occur, *or* that they use the same starting point
but select different *bits* of the same CRs
-`offs` is defined as CR48 (6x8) so as to mesh cleanly with Vectorised Rc=1 operations (see below). Arithmetic Rc=1 operations start from CR16 (TBD); FP Rc=1 from CR32 (TBD).
+`offs` is defined as CR32 (4x8) so as to mesh cleanly with Vectorised Rc=1 operations (see below). Rc=1 operations start from CR8 (TBD).
-# Twin Predication
+# Appendix
+
+## Twin Predication
This is a novel concept that allows predication to be applied to a single
source and a single dest register. The following types of traditional
particularly given that one of the predicate options is `(1<<r3)`
Additional unusual capabilities of Twin Predication include a back-to-back
-version of VCOMPRESS-VEXPAND which is effectively the ability to do an
-ordered multiple VINSERT.
+version of VCOMPRESS-VEXPAND which is effectively the ability to do
+sequentially ordered multiple VINSERTs. The source predicate selects a
+sequentially ordered subset of elements to be inserted; the destination predicate specifies the sequentially ordered recipient locations.
+
+## Rounding, clamp and saturate
+
+One of the issues with vector ops is that in integer DSP ops for example
+in Audio the operation must clamp or saturate rather than overflow or
+ignore the upper bits and become a modulo operation. This for Audio
+is extremely important, also to provide an indicator as to whether
+saturation occurred. see [[av_opcodes]].
+
+To help ensure that audio quality is not compromised by overflow,
+"saturation" is provided, as well as a way to detect when saturation
+occurred (Rc=1). When Rc=1 there will be a *vector* of CRs, one CR per
+element in the result (Note: this is different from VSX which has a
+single CR per block).
+
+When N=0 the result is saturated to within the maximum range of an
+unsigned value. For integer ops this will be 0 to 2^elwidth-1. Similar
+logic applies to FP operations, with the result being saturated to
+maximum rather than returning INF.
+
+When N=1 the same occurs except that the result is saturated to the min
+or max of a signed result.
+
+When Rc=1, the CR "overflow" bit is set on the CR associated with the
+element, to indicate whether saturation occurred. Note that due to
+the hugely detrimental effect it has on parallel processing, XER.SO is
+**ignored** completely and is **not** brought into play here. The CR
+overflow bit is therefore simply set to zero if saturation did not occur,
+and to one if it did.
+
+Post-analysis of the Vector of CRs to find out if any given element hit
+saturation may be done using a mapreduced CR op (cror), or by using the
+new crweird instruction, transferring the relevant CR bits to a scalar
+integer and testing it for nonzero. see [[sv/cr_int_predication]]
+
+Note that the operation takes place at the maximum bitwidth (max of src and dest elwidth) and that truncation occurs to the range of the dest elwidth.
+
+## Reduce mode
+
+1. limited to single predicated dual src operations (add RT, RA, RB).
+ triple source operations are prohibited (fma).
+2. limited to operations that make sense. divide is excluded, as is
+ subtract (X - Y - Z produces different answers depending on the order)
+ and asymmetric CRops (crandc, crorc). sane operations:
+ multiply, min/max, add, logical bitwise OR, most other CR ops.
+ operations that do have the same source and dest register type are
+ also excluded (isel, cmp)
+3. the destination is a vector but the result is stored, ultimately,
+ in the first nonzero predicated element. all other nonzero predicated
+ elements are undefined. *this includes the CR vector* when Rc=1
+4. implementations may use any ordering and any algorithm to reduce
+ down to a single result. However it must be equivalent to a straight
+ application of mapreduce. The destination vector (except masked out
+ elements) may be used for storing any intermediate results. these may
+ be left in the vector (undefined).
+5. CRM applies when Rc=1. When CRM is zero, the CR associated with
+ the result is regarded as a "some results met standard CR result
+ criteria". When CRM is one, this changes to "all results met standard
+ CR criteria".
+6. implementations MAY use destoffs as well as srcoffs (see [[sv/sprs]])
+ in order to store sufficient state to resume operation should an
+ interrupt occur. this is also why implementations are permitted to use
+ the destination vector to store intermediary computations
+7. *Predication may be applied*. zeroing mode is not an option. masked-out
+ inputs are ignored; masked-out elements in the destination vector are
+ unaltered (not used for the purposes of intermediary storage); the
+ scalar result is placed in the first available unmasked element.
+
+TODO: Rc=1 on Scalar Logical Operations? is this possible? was space
+reserved in Logical Ops?
+
+Pseudocode for the case where RA==RB:
+
+ result = op(iregs[RA], iregs[RA+1])
+ CR = analyse(result)
+ for i in range(2, VL):
+ result = op(result, iregs[RA+i])
+ CRnew = analyse(result)
+ if Rc=1
+ if CRM:
+ CR = CR bitwise or CRnew
+ else:
+ CR = CR bitwise AND CRnew
+
+TODO: case where RA!=RB which involves first a vector of 2-operand
+results followed by a mapreduce on the intermediates.
-# CR Operations
+Note that when SVM is clear and SUBVL!=1 the sub-elements are *independent*, i.e. they
+are mapreduced per *sub-element* as a result. illustration with a vec2:
+
+ result.x = op(iregs[RA].x, iregs[RA+1].x)
+ result.y = op(iregs[RA].y, iregs[RA+1].y)
+ for i in range(2, VL):
+ result.x = op(result.x, iregs[RA+i].x)
+ result.y = op(result.y, iregs[RA+i].y)
+
+When SVM is set and SUBVL!=1, another variant is enabled.
+
+ for i in range(VL):
+ result = op(iregs[RA+i].x, iregs[RA+i].x)
+ result = op(result, iregs[RA+i].z)
+ result = op(result, iregs[RA+i].z)
+ iregs[RT+i] = result
+
+## Fail-on-first
+
+Data-dependent fail-on-first has two distinct variants: one for LD/ST,
+the other for arithmetic operations (actually, CR-driven). Note in each
+case the assumption is that vector elements are required appear to be
+executed in sequential Program Order, element 0 being the first.
+
+* 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.
+* Data-driven (CR-driven) fail-on-first activates when Rc=1 or other
+ CR-creating operation produces a result (including cmp). Similar to
+ branch, an analysis of the CR is performed and if the test fails, the
+ vector operation terminates and discards all element operations at and
+ above the current one, and VL is truncated to the *previous* element.
+ Thus the new VL comprises a contiguous vector of results, all of which
+ pass the testing criteria (equal to zero, less than zero).
+
+The CR-based data-driven fail-on-first is new and not found in ARM SVE
+or RVV. It is extremely useful for reducing instruction count, however
+requires speculative execution involving modifications of VL to get high
+performance implementations.
+
+In CR-based data-driven fail-on-first there is only the option to select
+and test one bit of each CR (just as with branch BO). For more complex
+tests this may be insufficient. If that is the case, a vectorised crops
+(crand, cror) may be used, and ffirst applied to the crop instead of to
+the arithmetic vector.
+
+One extremely important aspect of ffirst is:
+
+* LDST ffirst may never set VL equal to zero. This because on the first
+ element an exception must be raised "as normal".
+* CR-based data-dependent ffirst on the other hand **can** set VL equal
+ to zero. This is the only means in the entirety of SV that VL may be set
+ to zero (with the exception of via the SV.STATE SPR). When VL is set
+ zero due to the first element failing the CR bit-test, all subsequent
+ vectorised operations are effectively `nops` which is
+ *precisely the desired and intended behaviour*.
+
+## pred-result mode
+
+This mode merges common CR testing with predication, saving on instruction count. Below is the pseudocode excluding predicate zeroing and elwidth overrides.
+
+ for i in range(VL):
+ # predication test, skip all masked out elements.
+ if predicate_masked_out(i):
+ continue
+ result = op(iregs[RA+i], iregs[RB+i])
+ CRnew = analyse(result) # calculates eq/lt/gt
+ # Rc=1 always stores the CR
+ if Rc=1:
+ crregs[offs+i] = CRnew
+ # now test CR, similar to branch
+ if CRnew[BO[0:1]] != BO[2]:
+ continue # test failed: cancel store
+ # result optionally stored but CR always is
+ iregs[RT+i] = result
+
+The reason for allowing the CR element to be stored is so that post-analysis
+of the CR Vector may be carried out. For example: Saturation may have occurred (and been prevented from updating, by the test) but it is desirable to know *which* elements fail saturation.
+
+Note that predication is still respected: predicate zeroing is slightly different: elements that fail the CR test *or* are masked out are zero'd.
+
+## CR Operations
CRs are slightly more involved than INT or FP registers due to the
possibility for indexing individual bits (crops BA/BB/BT). Again however
numbering, with a clear linear relationship and mapping existing when
SV is applied.
-## CR EXTRA mapping table and algorithm
+### CR EXTRA mapping table and algorithm
Numbering relationships for CR fields are already complex due to being
in BE format (*the relationship is not clearly explained in the v3.0B
simplify internal design. If instructions are issued where CR Vectors
do not start on a 32-bit aligned boundary, performance may be affected.
-## CR fields as inputs/outputs of vector operations
+### CR fields as inputs/outputs of vector operations
CRs (or, the arithmetic operations associated with them)
may be marked as Vectorised or Scalar. When Rc=1 in arithmetic operations that have no explicit EXTRA to cover the CR, the CR is Vectorised if the destination is Vectorised. Likewise if the destination is scalar then so is the CR.
When vectorized, the CR inputs/outputs are sequentially read/written
to 4-bit CR fields. Vectorised Integer results, when Rc=1, will begin
-writing to CR16 (TBD evaluate) and increase sequentially from there.
-Vectorised FP results, when Rc=1, start from CR32 (TBD evaluate).
+writing to CR8 (TBD evaluate) and increase sequentially from there.
This is so that:
* implementations may rely on the Vector CRs being aligned to 8. This
(8 CRs per batch), for high performance implementations.
* scalar Rc=1 operation (CR0, CR1) and callee-saved CRs (CR2-4) are not
overwritten by vector Rc=1 operations except for very large VL
-* Vector FP and Integer Rc=1 operations do not overwrite each other
- except for large VL.
-* CR-based predication, from CR48, is also not interfered with
+* CR-based predication, from CR32, is also not interfered with
(except by large VL).
However when the SV result (destination) is marked as a scalar by the
(see [[discussion]]. some alternative schemes are described there)
-## Table of CR fields
+### Rc=1 when SUBVL!=1
+
+sub-vectors are effectively a form of SIMD (length 2 to 4). Only 1 bit of predicate is allocated per subvector; likewise only one CR is allocated
+per subvector.
+
+This leaves a conundrum as to how to apply CR computation per subvector, when normally Rc=1 is exclusively applied to scalar elements. A solution is to perform a bitwise OR or AND of the subvector tests. Given that OE is ignored, rhis field may (when available) be used to select OR or AND behavior.
+
+### Table of CR fields
CR[i] is the notation used by the OpenPower spec to refer to CR field #i,
so FP instructions with Rc=1 write to CR[1] aka SVCR1_000.
-# Register Profiles
+## Register Profiles
**NOTE THIS TABLE SHOULD NO LONGER BE HAND EDITED** see
<https://bugs.libre-soc.org/show_bug.cgi?id=548> for details.
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