+[[!tag standards]]
+
# Appendix
-* <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=697>
+* <https://bugs.libre-soc.org/show_bug.cgi?id=574> Saturation
+* <https://bugs.libre-soc.org/show_bug.cgi?id=558#c47> Parallel Prefix
+* <https://bugs.libre-soc.org/show_bug.cgi?id=697> Reduce Modes
+* <https://bugs.libre-soc.org/show_bug.cgi?id=809> OV sv.addex discussion
This is the appendix to [[sv/svp64]], providing explanations of modes
etc. leaving the main svp64 page's primary purpose as outlining the
[[!toc]]
+# Partial Implementations
+
+It is perfectly legal to implement subsets of SVP64 as long as illegal
+instruction traps are always raised on unimplemented features,
+so that soft-emulation is possible,
+even for future revisions of SVP64. With SVP64 being partly controlled
+through contextual SPRs, a little care has to be taken.
+
+**All** SPRs
+not implemented including reserved ones for future use must raise an illegal
+instruction trap if read or written. This allows software the
+opportunity to emulate the context created by the given SPR.
+
+**Embedded Scalar Scenario**
+
+In this scenario an implementation does not wish to implement the Vectorisation
+but simply wishes to take advantage of predication or other feature
+of SVP64, such as instructions that might only be available if prefixed.
+Such an implementation would be entirely free to do so with the proviso
+that:
+
+* any attempts to call `setvl` shall either raise an illegal instruction
+ or be partially implemented to set SVSTATE correctly.
+* if SVSTATE contains any value in any bit that is not supported
+ in hardware, an illegal instruction shall be raised when an SVP64
+ prefixed instruction is executed.
+* if SVSTATE contains values requesting supported features at the time
+ that the prefixed instruction is executed then it is executed in
+ hardware as per specification, with no illegal exception trap raised.
+
+Example, assuming that hardware implements predication but not
+elwidth overrides:
+
+ setvli r0, 4 # sets VL equal to 4
+ sv.addi r5, r0, 1 # raises an 0x700 trap
+ setvli r0, 1 # sets VL equal to 1
+ sv.addi r5, r0, 1 # gets executed by hardware
+ sv.addi/ew=8 r5, r0, 1 # raises an 0x700 trap
+ sv.ori/sm=EQ r5, r0, 1 # executed by hardware
+
+The first
+
# 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
+independent. XER SO/OV and other global "accumulation" flags (CR.SO) cause
Read-Write Hazards on single-bit global resources, having a significant
detrimental effect.
-Consequently in SV, XER.SO and CR.OV behaviour is disregarded (including
-in `cmp` instructions). XER is simply neither read nor written.
+Consequently in SV, XER.SO behaviour is disregarded (including
+in `cmp` instructions). XER.SO is not read, but XER.OV may be written,
+breaking the Read-Modify-Write Hazard Chain that complicates
+microarchitectural implementations.
This includes when `scalar identity behaviour` occurs. If precise
OpenPOWER v3.0/1 scalar behaviour is desired then OpenPOWER v3.0/1
instructions should be used without an SV Prefix.
-An interesting side-effect of this decision is that the OE flag is now
-free for other uses when SV Prefixing is used.
-
-Regarding XER.CA: this does not fit either: it was designed for a scalar
-ISA. Instead, both carry-in and carry-out go into the CR.so bit of a given
-Vector element. This provides a means to perform large parallel batches
-of Vectorised carry-capable additions. crweird instructions can be used
-to transfer the CRs in and out of an integer, where bitmanipulation
-may be performed to analyse the carry bits (including carry lookahead
-propagation) before continuing with further parallel additions.
+TODO jacob add about OV https://www.intel.com/content/dam/www/public/us/en/documents/white-papers/ia-large-integer-arithmetic-paper.pdf
+
+Of note here is that XER.SO and OV may already be disregarded in the
+Power ISA v3.0/1 SFFS (Scalar Fixed and Floating) Compliancy Subset.
+SVP64 simply makes it mandatory to disregard XER.SO even for other Subsets,
+but only for SVP64 Prefixed Operations.
+
+XER.CA/CA32 on the other hand is expected and required to be implemented
+according to standard Power ISA Scalar behaviour. Interestingly, due
+to SVP64 being in effect a hardware for-loop around Scalar instructions
+executing in precise Program Order, a little thought shows that a Vectorised
+Carry-In-Out add is in effect a Big Integer Add, taking a single bit Carry In
+and producing, at the end, a single bit Carry out. High performance
+implementations may exploit this observation to deploy efficient
+Parallel Carry Lookahead.
+
+ # assume VL=4, this results in 4 sequential ops (below)
+ sv.adde r0.v, r4.v, r8.v
+
+ # instructions that get executed in backend hardware:
+ adde r0, r4, r8 # takes carry-in, produces carry-out
+ adde r1, r5, r9 # takes carry from previous
+ ...
+ adde r3, r7, r11 # likewise
+
+It can clearly be seen that the carry chains from one
+64 bit add to the next, the end result being that a
+256-bit "Big Integer Add" has been performed, and that
+CA contains the 257th bit. A one-instruction 512-bit Add
+may be performed by setting VL=8, and a one-instruction
+1024-bit add by setting VL=16, and so on.
# v3.0B/v3.1 relevant instructions
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,
+Vectorisation of the VSX Packed SIMD system makes no sense whatsoever,
+the sole exceptions potentially being any operations with 128-bit
+operands such as `vrlq` (Rotate Quad Word) and `xsaddqp` (Scalar
+Quad-precision Add).
+SV effectively *replaces* VSX requiring far less instructions, and provides,
at the very minimum, predication (which VSX was designed without).
Thus all VSX Major Opcodes - all of them - are "unused" and must raise
illegal instruction exceptions in SV Prefix Mode.
This table is taken from v3.0B.
Table 9: Primary Opcode Map (opcode bits 0:5)
- | 000 | 001 | 010 | 011 | 100 | 101 | 110 | 111
- 000 | | | tdi | twi | EXT04 | | | mulli | 000
- 001 | subfic | | cmpli | cmpi | addic | addic. | addi | addis | 001
- 010 | bc/l/a | EXT17 | b/l/a | EXT19 | rlwimi| rlwinm | | rlwnm | 010
- 011 | ori | oris | xori | xoris | andi. | andis. | EXT30 | EXT31 | 011
- 100 | lwz | lwzu | lbz | lbzu | stw | stwu | stb | stbu | 100
- 101 | lhz | lhzu | lha | lhau | sth | sthu | lmw | stmw | 101
- 110 | lfs | lfsu | lfd | lfdu | stfs | stfsu | stfd | stfdu | 110
- 111 | lq | EXT57 | EXT58 | EXT59 | EXT60 | EXT61 | EXT62 | EXT63 | 111
- | 000 | 001 | 010 | 011 | 100 | 101 | 110 | 111
+```
+ | 000 | 001 | 010 | 011 | 100 | 101 | 110 | 111
+000 | | | tdi | twi | EXT04 | | | mulli | 000
+001 | subfic | | cmpli | cmpi | addic | addic. | addi | addis | 001
+010 | bc/l/a | EXT17 | b/l/a | EXT19 | rlwimi| rlwinm | | rlwnm | 010
+011 | ori | oris | xori | xoris | andi. | andis. | EXT30 | EXT31 | 011
+100 | lwz | lwzu | lbz | lbzu | stw | stwu | stb | stbu | 100
+101 | lhz | lhzu | lha | lhau | sth | sthu | lmw | stmw | 101
+110 | lfs | lfsu | lfd | lfdu | stfs | stfsu | stfd | stfdu | 110
+111 | lq | EXT57 | EXT58 | EXT59 | EXT60 | EXT61 | EXT62 | EXT63 | 111
+ | 000 | 001 | 010 | 011 | 100 | 101 | 110 | 111
+```
## Suitable for svp64-only
make no sense in a Vectorisation Context have been removed. These removed
POs can, *in the SV Vector Context only*, be assigned to alternative
(Vectorised-only) instructions, including future extensions.
+EXT04 retains the scalar `madd*` operations but would have all PackedSIMD
+(aka VSX) operations removed.
Note, again, to emphasise: outside of svp64 these opcodes **do not**
change. When not prefixed with svp64 these opcodes **specifically**
retain their v3.0B / v3.1B OpenPOWER Standard compliant meaning.
- | 000 | 001 | 010 | 011 | 100 | 101 | 110 | 111
- 000 | | | | | | | | mulli | 000
- 001 | subfic | | cmpli | cmpi | addic | addic. | addi | addis | 001
- 010 | | | | EXT19 | rlwimi| rlwinm | | rlwnm | 010
- 011 | ori | oris | xori | xoris | andi. | andis. | EXT30 | EXT31 | 011
- 100 | lwz | lwzu | lbz | lbzu | stw | stwu | stb | stbu | 100
- 101 | lhz | lhzu | lha | lhau | sth | sthu | | | 101
- 110 | lfs | lfsu | lfd | lfdu | stfs | stfsu | stfd | stfdu | 110
- 111 | | | EXT58 | EXT59 | | EXT61 | | EXT63 | 111
- | 000 | 001 | 010 | 011 | 100 | 101 | 110 | 111
+```
+ | 000 | 001 | 010 | 011 | 100 | 101 | 110 | 111
+000 | | | | | EXT04 | | | mulli | 000
+001 | subfic | | cmpli | cmpi | addic | addic. | addi | addis | 001
+010 | bc/l/a | | | EXT19 | rlwimi| rlwinm | | rlwnm | 010
+011 | ori | oris | xori | xoris | andi. | andis. | EXT30 | EXT31 | 011
+100 | lwz | lwzu | lbz | lbzu | stw | stwu | stb | stbu | 100
+101 | lhz | lhzu | lha | lhau | sth | sthu | | | 101
+110 | lfs | lfsu | lfd | lfdu | stfs | stfsu | stfd | stfdu | 110
+111 | | | EXT58 | EXT59 | | EXT61 | | EXT63 | 111
+ | 000 | 001 | 010 | 011 | 100 | 101 | 110 | 111
+```
It is important to note that having a different v3.0B Scalar opcode
that is different from an SVP64 one is highly undesirable: the complexity
# EXTRA Field Mapping
The purpose of the 9-bit EXTRA field mapping is to mark individual
-registers (RT, RA, BFA) as either scalat or vector, and to extend
+registers (RT, RA, BFA) as either scalar or vector, and to extend
their numbering from 0..31 in Power ISA v3.0 to 0..127 in SVP64.
Three of the 9 bits may also be used up for a 2nd Predicate (Twin
Predication) leaving a mere 6 bits for qualifying registers. As can
-be seen there is significant pressure on these (and all) SVP64 bits.
+be seen there is significant pressure on these (and in fact all) SVP64 bits.
In Power ISA v3.1 prefixing there are bits which describe and classify
the prefix in a fashion that is independent of the suffix. MLSS for
are neither `UNDEFINED` nor prohibited, despite them not making much
sense at first glance.
Scalar reduce is strictly defined behaviour, and the cost in
-hardware terms of prohibition of seemingly non-sensical operations is too great.
+hardware terms of prohibition of seemingly non-sensical operations is too great.
Therefore it is permitted and required to be executed successfully.
Implementors **MAY** choose to optimise such instructions in instances
where their use results in "extraneous execution", i.e. where it is clear
behaviour (no "accumulator"), may discard all but the last element
operation. Identification
of such is trivial to do for `setb` and `cmp`: the source register type is
-a completely different register file from the destination*
+a completely different register file from the destination.
+Likewise Scalar reduction when the destination is a Vector
+is as if the Reduction Mode was not requested.*
Typical applications include simple operations such as `ADD r3, r10.v,
r3` where, clearly, r3 is being used to accumulate the addition of all
-elements is the vector starting at r10.
+elements of the vector starting at r10.
# add RT, RA,RB but when RT==RA
for i in range(VL):
the underlying hardware.
Other examples include shift-mask operations where a Vector of inserts
-into a single destination register is required, as a way to construct
+into a single destination register is required (see [[sv/bitmanip]], bmset),
+as a way to construct
a value quickly from multiple arbitrary bit-ranges and bit-offsets.
Using the same register as both the source and destination, with Vectors
of different offsets masks and values to be inserted has multiple
applications including Video, cryptography and JIT compilation.
+ # assume VL=4:
+ # * Vector of shift-offsets contained in RC (r12.v)
+ # * Vector of masks contained in RB (r8.v)
+ # * Vector of values to be masked-in in RA (r4.v)
+ # * Scalar destination RT (r0) to receive all mask-offset values
+ sv.bmset/mr r0, r4.v, r8.v, r12.v
+
+Due to the Deterministic Scheduling,
Subtract and Divide are still permitted to be executed in this mode,
although from an algorithmic perspective it is strongly discouraged.
It would be better to use addition followed by one final subtract,
* 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
+ vector operation terminates and discards all element operations
+ above the current one (and the current one if VLi is not set),
+ and VL is truncated to either
the *previous* element or the current one, depending on whether
VLi (VL "inclusive") is set.
*as if* the destination predicate bit was zero.
Arithmetic and Logical Pred-result is covered in [[sv/normal]]
-## pred-result mode on CR ops
+Ped-result mode may not be applied 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]]
+Although CR operations (mtcr, crand, cror) may be Vectorised,
+predicated, pred-result mode applies to operations that have
+an Rc=1 mode, or make sense to add an RC1 option.
# CR Operations
### 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.
+CRn is the notation used by the OpenPower spec to refer to CR field #i,
+so FP instructions with Rc=1 write to CR1 (n=1).
CRs are not stored in SPRs: they are registers in their own right.
Therefore context-switching the full set of CRs involves a Vectorised
-mfcr or mtcr, using VL=64, elwidth=8 to do so. This is exactly as how
+mfcr or mtcr, using VL=8 to do so. This is exactly as how
scalar OpenPOWER context-switches CRs: it is just that there are now
more of them.
overrides not included. if there is no predicate, it is set to all 1s
function op_add(rd, rs1, rs2) # add not VADD!
- int i, id=0, irs1=0, irs2=0; predval = get_pred_val(FALSE, rd);
+ int i, id=0, irs1=0, irs2=0;
+ predval = get_pred_val(FALSE, rd);
for (i = 0; i < VL; i++)
- STATE.srcoffs = i # save context if (predval & 1<<i) # predication
- uses intregs
- ireg[rd+id] <= ireg[rs1+irs1] + ireg[rs2+irs2]; if (!int_vec[rd
- ].isvec) break;
- if (rd.isvec) { id += 1; } if (rs1.isvec) { irs1 += 1; } if
- (rs2.isvec) { irs2 += 1; } if (id == VL or irs1 == VL or irs2 ==
- VL) {
- # end VL hardware loop STATE.srcoffs = 0; # reset return;
+ STATE.srcoffs = i # save context
+ if (predval & 1<<i) # predication uses intregs
+ ireg[rd+id] <= ireg[rs1+irs1] + ireg[rs2+irs2];
+ if (!int_vec[rd].isvec) break;
+ if (rd.isvec) { id += 1; }
+ if (rs1.isvec) { irs1 += 1; }
+ if (rs2.isvec) { irs2 += 1; }
+ if (id == VL or irs1 == VL or irs2 == VL)
+ {
+ # end VL hardware loop
+ STATE.srcoffs = 0; # reset
+ return;
}
This has several modes:
-* RT.v = RA.v RB.v * RT.v = RA.v RB.s (and RA.s RB.v) * RT.v = RA.s RB.s *
-RT.s = RA.v RB.v * RT.s = RA.v RB.s (and RA.s RB.v) * RT.s = RA.s RB.s
+* RT.v = RA.v RB.v
+* RT.v = RA.v RB.s (and RA.s RB.v)
+* RT.v = RA.s RB.s
+* RT.s = RA.v RB.v
+* RT.s = RA.v RB.s (and RA.s RB.v)
+* RT.s = RA.s RB.s
All of these may be predicated. Vector-Vector is straightfoward.
When one of source is a Vector and the other a Scalar, it is clear that
* ew=8/16/32 - element width
* sew=8/16/32 - source element width
* vec=2/3/4 - SUBVL
-* mode=reduce/satu/sats/crpred
+* mode=mr/satu/sats/crpred
* pred=1\<\<3/r3/~r3/r10/~r10/r30/~r30/lt/gt/le/ge/eq/ne
-* spred={reg spec}
similar to x86 "rex" prefix.
For modes:
* pred-result:
- - pm=lt/gt/le/ge/eq/ne/so/ns OR
- - pm=RC1 OR pm=~RC1
+ - pm=lt/gt/le/ge/eq/ne/so/ns
+ - RC1 mode
* fail-first
- - ff=lt/gt/le/ge/eq/ne/so/ns OR
- - ff=RC1 OR ff=~RC1
+ - ff=lt/gt/le/ge/eq/ne/so/ns
+ - RC1 mode
* saturation:
- sats
- satu
# Proposed Parallel-reduction algorithm
+**This algorithm contains a MV operation and may NOT be used. Removal
+of the MV operation may be achieved by using index-redirection as was
+achieved in DCT and FFT REMAP**
+
```
/// reference implementation of proposed SimpleV reduction semantics.
///
// reduction operation -- we still use this algorithm even
// if the reduction operation isn't associative or
// commutative.
-/// `temp_pred` is a user-visible Vector Condition register
+ XXX VIOLATION OF SVP64 DESIGN PRINCIPLES XXXX
+/// XXX `pred` is a user-visible Vector Condition register XXXX
+ XXX VIOLATION OF SVP64 DESIGN PRINCIPLES XXXX
///
/// all input arrays have length `vl`
def reduce(vl, vec, pred):
+ pred = copy(pred) # must not damage predicate
step = 1;
while step < vl
step *= 2;
if pred[i] && other_pred
vec[i] += vec[other];
else if other_pred
- vec[i] = vec[other];
+ XXX VIOLATION OF SVP64 DESIGN XXX
+ XXX vec[i] = vec[other]; XXX
+ XXX VIOLATION OF SVP64 DESIGN XXX
pred[i] |= other_pred;
```
-we'd want to use something based on the above pseudo-code
-rather than the below pseudo-code -- reasoning here:
-<https://bugs.libre-soc.org/show_bug.cgi?id=697#c11>
+The first principle in SVP64 being violated is that SVP64 is a fully-independent
+Abstraction of hardware-looping in between issue and execute phases
+that has no relation to the operation it issues. The above pseudocode
+conditionally changes not only the type of element operation issued
+(a MV in some cases) but also the number of arguments (2 for a MV).
+At the very least, for Vertical-First Mode this will result in unanticipated and unexpected behaviour (maximise "surprises" for programmers) in
+the middle of loops, that will be far too hard to explain.
+
+The second principle being violated by the above algorithm is the expectation
+that temporary storage is available for a modified predicate: there is no
+such space, and predicates are read-only to reduce complexity at the
+micro-architectural level.
+SVP64 is founded on the principle that all operations are
+"re-entrant" with respect to interrupts and exceptions: SVSTATE must
+be saved and restored alongside PC and MSR, but nothing more. It is perfectly
+fine to have context-switching back to the operation be somewhat slower,
+through "reconstruction" of temporary internal state based on what SVSTATE
+contains, but nothing more.
+
+An alternative algorithm is therefore required that does not perform MVs,
+and does not require additional state to be saved on context-switching.
```
-def reduce( vl, vec, pred, pred,):
+def reduce( vl, vec, pred ):
+ pred = copy(pred) # must not damage predicate
j = 0
vi = [] # array of lookup indices to skip nonpredicated
for i, pbit in enumerate(pred):
halfstep = step // 2
for i in (0..vl).step_by(step)
other = vi[i + halfstep]
- i = vi[i]
+ ir = vi[i]
other_pred = other < vl && pred[other]
if pred[i] && other_pred
- vec[i] += vec[other]
- pred[i] |= other_pred
+ vec[ir] += vec[other]
+ else if other_pred:
+ vi[ir] = vi[other] # index redirection, no MV
+ pred[ir] |= other_pred # reconstructed on context-switch
step *= 2
-
```
+
+In this version the need for an explicit MV is made unnecessary by instead
+leaving elements *in situ*. The internal modifications to the predicate may,
+due to the reduction being entirely deterministic, be "reconstructed"
+on a context-switch. This may make some implementations slower.
+
+*Implementor's Note: many SIMD-based Parallel Reduction Algorithms are
+implemented in hardware with MVs that ensure lane-crossing is minimised.
+The mistake which would be catastrophic to SVP64 to make is to then
+limit the Reduction Sequence for all implementors
+based solely and exclusively on what one
+specific internal microarchitecture does.
+In SIMD ISAs the internal SIMD Architectural design is exposed and imposed on the programmer. Cray-style Vector ISAs on the other hand provide convenient,
+compact and efficient encodings of abstract concepts.
+It is the Implementor's responsibility to produce a design
+that complies with the above algorithm,
+utilising internal Micro-coding and other techniques to transparently
+insert MV operations
+if necessary or desired, to give the level of efficiency or performance
+required.*
+
+# Element-width overrides
+
+Element-width overrides are best illustrated with a packed structure
+union in the c programming language. The following should be taken
+literally, and assume always a little-endian layout:
+
+ typedef union {
+ uint8_t b[];
+ uint16_t s[];
+ uint32_t i[];
+ uint64_t l[];
+ uint8_t actual_bytes[8];
+ } el_reg_t;
+
+ elreg_t int_regfile[128];
+
+ get_polymorphed_reg(reg, bitwidth, offset):
+ el_reg_t res;
+ res.l = 0; // TODO: going to need sign-extending / zero-extending
+ if bitwidth == 8:
+ reg.b = int_regfile[reg].b[offset]
+ elif bitwidth == 16:
+ reg.s = int_regfile[reg].s[offset]
+ elif bitwidth == 32:
+ reg.i = int_regfile[reg].i[offset]
+ elif bitwidth == 64:
+ reg.l = int_regfile[reg].l[offset]
+ return res
+
+ set_polymorphed_reg(reg, bitwidth, offset, val):
+ if (!reg.isvec):
+ # not a vector: first element only, overwrites high bits
+ int_regfile[reg].l[0] = val
+ elif bitwidth == 8:
+ int_regfile[reg].b[offset] = val
+ elif bitwidth == 16:
+ int_regfile[reg].s[offset] = val
+ elif bitwidth == 32:
+ int_regfile[reg].i[offset] = val
+ elif bitwidth == 64:
+ int_regfile[reg].l[offset] = val
+
+In effect the GPR registers r0 to r127 (and corresponding FPRs fp0
+to fp127) are reinterpreted to be "starting points" in a byte-addressable
+memory. Vectors - which become just a virtual naming construct - effectively
+overlap.
+
+It is extremely important for implementors to note that the only circumstance
+where upper portions of an underlying 64-bit register are zero'd out is
+when the destination is a scalar. The ideal register file has byte-level
+write-enable lines, just like most SRAMs, in order to avoid READ-MODIFY-WRITE.
+
+An example ADD operation with predication and element width overrides:
+
+ for (i = 0; i < VL; i++)
+ if (predval & 1<<i) # predication
+ src1 = get_polymorphed_reg(RA, srcwid, irs1)
+ src2 = get_polymorphed_reg(RB, srcwid, irs2)
+ result = src1 + src2 # actual add here
+ set_polymorphed_reg(RT, destwid, ird, result)
+ if (!RT.isvec) break
+ if (RT.isvec) { id += 1; }
+ if (RA.isvec) { irs1 += 1; }
+ if (RB.isvec) { irs2 += 1; }
+
+Thus it can be clearly seen that elements are packed by their
+element width, and the packing starts from the source (or destination)
+specified by the instruction.
+
+# Twin (implicit) result operations
+
+Some operations in the Power ISA already target two 64-bit scalar
+registers: `lq` for example, and LD with update.
+Some mathematical algorithms are more
+efficient when there are two outputs rather than one, providing
+feedback loops between elements (the most well-known being add with
+carry). 64-bit multiply
+for example actually internally produces a 128 bit result, which clearly
+cannot be stored in a single 64 bit register. Some ISAs recommend
+"macro op fusion": the practice of setting a convention whereby if
+two commonly used instructions (mullo, mulhi) use the same ALU but
+one selects the low part of an identical operation and the other
+selects the high part, then optimised micro-architectures may
+"fuse" those two instructions together, using Micro-coding techniques,
+internally.
+
+The practice and convention of macro-op fusion however is not compatible
+with SVP64 Horizontal-First, because Horizontal Mode may only
+be applied to a single instruction at a time, and SVP64 is based on
+the principle of strict Program Order even at the element
+level. Thus it becomes
+necessary to add explicit more complex single instructions with
+more operands than would normally be seen in the average RISC ISA
+(3-in, 2-out, in some cases). If it
+was not for Power ISA already having LD/ST with update as well as
+Condition Codes and `lq` this would be hard to justify.
+
+With limited space in the `EXTRA` Field, and Power ISA opcodes
+being only 32 bit, 5 operands is quite an ask. `lq` however sets
+a precedent: `RTp` stands for "RT pair". In other words the result
+is stored in RT and RT+1. For Scalar operations, following this
+precedent is perfectly reasonable. In Scalar mode,
+`madded` therefore stores the two halves of the 128-bit multiply
+into RT and RT+1.
+
+What, then, of `sv.madded`? If the destination is hard-coded to
+RT and RT+1 the instruction is not useful when Vectorised because
+the output will be overwritten on the next element. To solve this
+is easy: define the destination registers as RT and RT+MAXVL
+respectively. This makes it easy for compilers to statically allocate
+registers even when VL changes dynamically.
+
+Bear in mind that both RT and RT+MAXVL are starting points for Vectors,
+and bear in mind that element-width overrides still have to be taken
+into consideration, the starting point for the implicit destination
+is best illustrated in pseudocode:
+
+ # demo of madded
+ for (i = 0; i < VL; i++)
+ if (predval & 1<<i) # predication
+ src1 = get_polymorphed_reg(RA, srcwid, irs1)
+ src2 = get_polymorphed_reg(RB, srcwid, irs2)
+ src2 = get_polymorphed_reg(RC, srcwid, irs3)
+ result = src1*src2 + src2
+ destmask = (2<<destwid)-1
+ # store two halves of result, both start from RT.
+ set_polymorphed_reg(RT, destwid, ird , result&destmask)
+ set_polymorphed_reg(RT, destwid, ird+MAXVL, result>>destwid)
+ if (!RT.isvec) break
+ if (RT.isvec) { id += 1; }
+ if (RA.isvec) { irs1 += 1; }
+ if (RB.isvec) { irs2 += 1; }
+ if (RC.isvec) { irs3 += 1; }
+
+The significant part here is that the second half is stored
+starting not from RT+MAXVL at all: it is the *element* index
+that is offset by MAXVL, both halves actually starting from RT.
+If VL is 3, MAXVL is 5, RT is 1, and dest elwidth is 32 then the elements
+RT0 to RT2 are stored:
+
+ 0..31 32..63
+ r0 unchanged unchanged
+ r1 RT0.lo RT1.lo
+ r2 RT2.lo unchanged
+ r3 unchanged RT0.hi
+ r4 RT1.hi RT2.hi
+ r5 unchanged unchanged
+
+Note that all of the LO halves start from r1, but that the HI halves
+start from half-way into r3. The reason is that with MAXVL bring
+5 and elwidth being 32, this is the 5th element
+offset (in 32 bit quantities) counting from r1.
+
+Additional DRAFT Scalar instructions in 3-in 2-out form
+with an implicit 2nd destination:
+
+* [[isa/svfixedarith]]
+* [[isa/svfparith]]
+
projects / libreriscv.git / blobdiff