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[[!tag standards]]

# Normal SVP64 Modes, for Arithmetic and Logical Operations

* <https://bugs.libre-soc.org/show_bug.cgi?id=574>
* <https://bugs.libre-soc.org/show_bug.cgi?id=558#c47>
* <https://bugs.libre-soc.org/show_bug.cgi?id=936>
* [[svp64]]

Normal SVP64 Mode covers Arithmetic and Logical operations
to provide suitable additional behaviour.  The Mode
field is bits 19-23 of the [[svp64]] RM Field.

Table of contents:

[[!toc]]

# Mode

Mode is an augmentation of SV behaviour, providing additional
functionality.  Some of these alterations are element-based (saturation), others involve post-analysis (predicate result) and others are Vector-based (mapreduce, fail-on-first).

[[sv/ldst]],
[[sv/cr_ops]] and [[sv/branches]] are covered separately: the following
Modes apply to Arithmetic and Logical SVP64 operations:

* **simple** 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.
  *VL is altered as a result*.
* **sat mode** or saturation: clamps each element result to a min/max rather than overflows / wraps.  allows signed and unsigned clamping for both INT
and FP.
* **reduce mode**. if used correctly, a mapreduce (or a prefix sum)
  is performed.    see [[svp64/appendix]].
  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 conditional testing) and if the test fails it
is as if the 
*destination* predicate bit was zero even before starting the operation. 
When Rc=1 the CR element however is still stored in the CR regfile, even if the test failed.  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.  simple, saturate and pred-result are however inter-element independent and may easily be parallelised to give high performance, regardless of the value of VL.

The Mode table for Arithmetic and Logical operations
 is laid out as follows:

| 0-1 |  2  |  3   4  |  description              |
| --- | --- |---------|-------------------------- |
| 00  |   0 |  dz  sz | simple mode                      |
| 00  |   1 | 0  RG   | scalar reduce mode (mapreduce) |
| 00  |   1 | 1  /    | reserved     |
| 01  | inv | CR-bit  | Rc=1: ffirst CR sel              |
| 01  | inv | VLi RC1 |  Rc=0: ffirst z/nonz |
| 10  |   N | dz   sz |  sat mode: N=0/1 u/s |
| 11  | inv | CR-bit  |  Rc=1: pred-result CR sel |
| 11  | inv | zz  RC1 |  Rc=0: pred-result z/nonz |

Fields:

* **sz / dz**  if predication is enabled will put zeros into the dest (or as src in the case of twin pred) when the predicate bit is zero.  otherwise the element is ignored or skipped, depending on context.
* **zz**: both sz and dz are set equal to this flag
* **inv CR bit** just as in branches (BO) these bits allow testing of a CR bit and whether it is set (inv=0) or unset (inv=1)
* **RG** inverts the Vector Loop order (VL-1 downto 0) rather
than the normal 0..VL-1
* **N** sets signed/unsigned saturation.
* **RC1** as if Rc=1, enables access to `VLi`.
* **VLi** VL inclusive: in fail-first mode, the truncation of
  VL *includes* the current element at the failure point rather
  than excludes it from the count.

For LD/ST Modes, see [[sv/ldst]].  For Condition Registers
see [[sv/cr_ops]].
For Branch modes, see [[sv/branches]].

# Rounding, clamp and saturate

See [[av_opcodes]] for relevant opcodes and use-cases.

To help ensure that audio quality is not compromised by overflow,
"saturation" is provided, as well as a way to detect when saturation
occurred if desired (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, and the minimum to +0.0

When N=1 the same occurs except that the result is saturated to the min
or max of a signed result, and for FP to the min and max value rather
than returning +/- INF.

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.

Note also that saturate on operations that set OE=1 must raise an
Illegal Instruction due to the conflicting use of the CR.so bit for
storing if
saturation occurred. Integer Operations that produce a Carry-Out (CA, CA32):
these two bits will be `UNDEFINED` if saturation is also requested.

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.

*Programmer's Note: 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 crrweird instruction with Rc=1, which will transfer the required
CR bits to a scalar integer and update CR0, which will allow testing
the scalar integer for nonzero.  see [[sv/cr_int_predication]]*

# Reduce mode

Reduction in SVP64 is similar in essence to other Vector Processing
ISAs, but leverages the underlying scalar Base v3.0B operations.
Thus it is more a convention that the programmer may utilise to give
the appearance and effect of a Horizontal Vector Reduction. Due
to the unusual decoupling it is also possible to perform
prefix-sum (Fibonacci Series) in certain circumstances. Details are in the [[svp64/appendix]]

Reduce Mode should not be confused with Parallel Reduction [[sv/remap]].
As explained in the [[sv/appendix]] Reduce Mode switches off the check
which would normally stop looping if the result register is scalar.
Thus, the result scalar register, if also used as a source scalar, 
may be used to perform sequential accumulation.  This *deliberately*
sets up a chain
of Register Hazard Dependencies, whereas Parallel Reduce [[sv/remap]]
deliberately issues a Tree-Schedule of operations that may be parallelised.

# 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 to appear to be
executed in sequential Program Order. When REMAP is not active,
element 0 would be the first.

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 either
the *previous* element or the current one, depending on whether
VLi (VL "inclusive") is set.

Thus the new VL comprises a contiguous vector of results, 
all of which pass the testing criteria (equal to zero, less than zero etc
as defined by the CR-bit test).

The CR-based data-driven fail-on-first is "new" and not found in ARM
SVE or RVV. At the same time it is "old" because it is almost
identical to a generalised form of Z80's `CPIR` instruction.
It is extremely useful for reducing instruction count,
however requires speculative execution involving modifications of VL
to get high performance implementations.  An additional mode (RC1=1)
effectively turns what would otherwise be an arithmetic operation
into a type of `cmp`.  The CR is stored (and the CR.eq bit tested
against the `inv` field).
If the CR.eq bit is equal to `inv` then the Vector is truncated and
the loop ends.

VLi is only available as an option when `Rc=0` (or for instructions
which do not have Rc). When set, the current element is always
also included in the count (the new length that VL will be set to).
This may be useful in combination with "inv" to truncate the Vector
to *exclude* elements that fail a test, or, in the case of implementations
of strncpy, to include the terminating zero.

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 crop
such as crand, cror or [[sv/cr_int_predication]] crweirder may be used,
and ffirst applied to the crop instead of to
the arithmetic vector. Note that crops are covered by 
the [[sv/cr_ops]] Mode format.

*Programmer's note: `VLi` is only accessible in normal operations
which in turn limits the CR field bit-testing to only `EQ/NE`.
[[sv/cr_ops]] are not so limited.  Thus it is possible to use for
example `sv.cror/ff=gt/vli *0,*0,*0`, which is not a `nop` because
it allows Fail-First Mode to perform a test and truncate VL.*

Two extremely important aspects of ffirst are:

* 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*.

The second crucial aspect, compared to LDST Ffirst:

* LD/ST Failfirst may (beyond the initial first element
  conditions) truncate VL for any architecturally
  suitable reason. Beyond the first element LD/ST Failfirst is
  arbitrarily speculative and 100% non-deterministic.
* CR-based data-dependent first on the other hand MUST NOT truncate VL
arbitrarily to a length decided by the hardware: VL MUST only be
truncated based explicitly on whether a test fails.
This because it is a precise Deterministic test on which algorithms
can and will will rely.

**Floating-point Exceptions**

When Floating-point exceptions are enabled VL must be truncated at
the point where the Exception appears not to have occurred. If `VLi`
is set then VL must include the faulting element, and thus the
faulting element will always raise its exception.  If however `VLi`
is clear then VL **excludes** the faulting element and thus the
exception will **never** be raised.

Although very strongly
discouraged the Exception Mode that permits Floating Point Exception
notification to arrive too late to unwind is permitted
(under protest, due it violating
the otherwise 100% Deterministic nature of Data-dependent Fail-first).

**Use of lax FP Exception Notification Mode could result in parallel
computations proceeding with invalid results that have to be explicitly
detected, whereas with the strict FP Execption Mode enabled, FFirst
truncates VL, allows subsequent parallel computation to avoid
the exceptions entirely**

## Data-dependent fail-first on CR operations (crand etc)

Operations that actually produce or alter CR Field as a result
have their own SVP64 Mode, described
in [[sv/cr_ops]].

# 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. Note that the pseudocode for [[sv/cr_ops]] is slightly different.

    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 or RC1:
            crregs[offs+i] = CRnew
        # now test CR, similar to branch
        if RC1 or 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 RC1 Mode basically turns all operations into `cmp`.  The
calculation is performed but it is only the CR that is written. The
element result is *always* discarded, never written (just like `cmp`).

Note that predication is still respected: predicate zeroing is slightly
different: elements that fail the CR test *or* are masked out are zero'd.