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# Normal Mode for SVP64

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

Normal SVP64 Mode covers Arithmetic and Logical operations
to provide suitable additional behaviour.

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:

* **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.
  *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**. 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 [[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 testing) and if the test fails it is as if the 
*destination* predicate bit was zero.  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.  normal, 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 | normal mode                      |
| 00  |   1 | 0  RG   | scalar reduce mode (mapreduce), SUBVL=1 |
| 00  |   1 | 1  CRM  | parallel reduce mode (mapreduce), SUBVL=1 |
| 00  |   1 | SVM RG  | subvector reduce mode, SUBVL>1   |
| 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 | dz  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.
* **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
* **CRM** affects the CR on reduce mode when Rc=1
* **SVM** sets "subvector" reduce mode
* **N** sets signed/unsigned saturation.
* **RC1** as if Rc=1, stores CRs *but not the result*
* **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 produce a carry output are
prohibited due to the conflicting use of the CR.so bit for storing if
saturation occurred.

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

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.
Details are in the [[svp64/appendix]]

# 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 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).

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.  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.
Note that when RC1=1 the result elements are never stored, only the CRs.

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 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*.

Another aspect is that 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.  Likewise, to reduce
workloads or balance resources.

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 test on which algorithms
will rely.

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

Operations that actually produce or alter CR Field as a result
do not also in turn have an Rc=1 mode.  However it makes no
sense to try to test the 4 bits of a CR Field for being equal
or not equal to zero. Moreover, the result is already in the
form that is desired: it is a CR field.  Therefore,
CR-based operations have their own SVP64 Mode, described
in [[sv/cr_ops]]

There are two primary different types of CR operations:

* Those which have a 3-bit operand field (referring to a CR Field)
* Those which have a 5-bit operand (referring to a bit within the
   whole 32-bit CR)

More details can be found 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 paeudocode 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.

## pred-result mode on CR ops

CR operations (mtcr, crand, cror) may be Vectorised,
predicated, and also pred-result mode applied to it.  
Vectorisation applies to 4-bit CR Fields which are treated as
elements, not the individual bits of the 32-bit CR.
CR ops and how to identify them is described in [[sv/cr_ops]]