+// This file describes the third of three pattern matcher setups that
+// forms the `xilinx_dsp` pass described in xilinx_dsp.cc
+// At a high level, it works as follows:
+// (1) Starting from a DSP48E1 cell that (a) has the Z multiplexer
+// (controlled by OPMODE[6:4]) set to zero and (b) doesn't already
+// use the 'PCOUT' port
+// (2.1) Match another DSP48E1 cell that (a) does not have the CREG enabled,
+// (b) has its Z multiplexer output set to the 'C' port, which is
+// driven by the 'P' output of the previous DSP cell, and (c) has its
+// 'PCIN' port unused
+// (2.2) Same as (2.1) but with the 'C' port driven by the 'P' output of the
+// previous DSP cell right-shifted by 17 bits
+// (3) For this subequent DSP48E1 match (i.e. PCOUT -> PCIN cascade exists)
+// if (a) the previous DSP48E1 uses either the A2REG or A1REG, (b) this
+// DSP48 does not use A2REG nor A1REG, (c) this DSP48E1 does not already
+// have an ACOUT -> ACIN cascade, (d) the previous DSP does not already
+// use its ACOUT port, then examine if an ACOUT -> ACIN cascade
+// opportunity exists by matching for a $dff-with-optional-clock-enable-
+// or-reset and checking that the 'D' input of this register is the same
+// as the 'A' input of the previous DSP
+// (4) Same as (3) but for BCOUT -> BCIN cascade
+// (5) Recursively go to (2.1) until no more matches possible, keeping track
+// of the longest possible chain found
+// (6) The longest chain is then divided into chunks of no more than
+// MAX_DSP_CASCADE in length (to prevent long cascades that exceed the
+// height of a DSP column) with each DSP in each chunk being rewritten
+// to use [ABP]COUT -> [ABP]CIN cascading as appropriate
+// Notes:
+// - Currently, [AB]COUT -> [AB]COUT cascades (3 or 4) are only considered
+// if a PCOUT -> PCIN cascade is (2.1 or 2.2) first identified; this need
+// not be the case --- [AB] cascades can exist independently of a P cascade
+// (though all three cascades must come from the same DSP). This situation
+// is not handled currently.
+// - In addition, [AB]COUT -> [AB]COUT cascades (3 or 4) are currently
+// conservative in that they examine the situation where (a) the previous
+// DSP has [AB]2REG or [AB]1REG enabled, (b) that the downstream DSP has no
+// registers enabled, and (c) that there exists only one additional register
+// between the upstream and downstream DSPs. This can certainly be relaxed
+// to identify situations ranging from (i) neither DSP uses any registers,
+// to (ii) upstream DSP has 2 registers, downstream DSP has 2 registers, and
+// there exists a further 2 registers between them. This remains a TODO
+// item.
+
pattern xilinx_dsp_cascade
udata <std::function<SigSpec(const SigSpec&)>> unextend
state <SigSpec> clock
state <int> AREG BREG
-// subpattern
+// Variables used for subpatterns
state <SigSpec> argQ argD
state <bool> ffcepol ffrstpol
state <int> ffoffset
#define MAX_DSP_CASCADE 20
endcode
+// (1) Starting from a DSP48E1 cell that (a) has the Z multiplexer
+// (controlled by OPMODE[6:4]) set to zero and (b) doesn't already
+// use the 'PCOUT' port
match first
select first->type.in(\DSP48E1)
select port(first, \OPMODE, Const(0, 7)).extract(4,3) == Const::from_string("000")
select nusers(port(first, \PCOUT, SigSpec())) <= 1
endmatch
+// (6) The longest chain is then divided into chunks of no more than
+// MAX_DSP_CASCADE in length (to prevent long cascades that exceed the
+// height of a DSP column) with each DSP in each chunk being rewritten
+// to use [ABP]COUT -> [ABP]CIN cascading as appropriate
code
longest_chain.clear();
chain.emplace_back(first, -1, -1, -1);
arg first
arg next
+// (2.1) Match another DSP48E1 cell that (a) does not have the CREG enabled,
+// (b) has its Z multiplexer output set to the 'C' port, which is
+// driven by the 'P' output of the previous DSP cell, and (c) has its
+// 'PCIN' port unused
match nextP
select nextP->type.in(\DSP48E1)
select !param(nextP, \CREG, State::S1).as_bool()
semioptional
endmatch
+// (2.2) Same as (2.1) but with the 'C' port driven by the 'P' output of the
+// previous DSP cell right-shifted by 17 bits
match nextP_shift17
if !nextP
select nextP_shift17->type.in(\DSP48E1)
}
endcode
+// (3) For this subequent DSP48E1 match (i.e. PCOUT -> PCIN cascade exists)
+// if (a) the previous DSP48E1 uses either the A2REG or A1REG, (b) this
+// DSP48 does not use A2REG nor A1REG, (c) this DSP48E1 does not already
+// have an ACOUT -> ACIN cascade, (d) the previous DSP does not already
+// use its ACOUT port, then examine if an ACOUT -> ACIN cascade
+// opportunity exists by matching for a $dff-with-optional-clock-enable-
+// or-reset and checking that the 'D' input of this register is the same
+// as the 'A' input of the previous DSP
code argQ clock AREG
AREG = -1;
if (next) {
if (param(prev, \AREG, 2).as_int() > 0 &&
param(next, \AREG, 2).as_int() > 0 &&
param(next, \A_INPUT, Const("DIRECT")).decode_string() == "DIRECT" &&
- port(next, \ACIN, SigSpec()).is_fully_zero() &&
nusers(port(prev, \ACOUT, SigSpec())) <= 1) {
argQ = unextend(port(next, \A));
clock = port(prev, \CLK);
}
endcode
+// (4) Same as (3) but for BCOUT -> BCIN cascade
code argQ clock BREG
BREG = -1;
if (next) {
}
endcode
+// (5) Recursively go to (2.1) until no more matches possible, recording the
+// longest possible chain
code
if (next) {
chain.emplace_back(next, nextP_shift17 ? 17 : nextP ? 0 : -1, AREG, BREG);
SigSpec sigC = unextend(port(next, \C));
- // TODO: Cannot use 'reject' since semioptional
if (nextP_shift17) {
if (GetSize(sigC)+17 <= GetSize(port(std::get<0>(chain.back()), \P)) &&
port(std::get<0>(chain.back()), \P).extract(17, GetSize(sigC)) != sigC)
// #######################
+// Subpattern for matching against input registers, based on knowledge of the
+// 'Q' input.
+// At a high level:
+// (1) Starting from a $dff cell that (partially or fully) drives the given
+// 'Q' argument
+// (2) Match for a $mux cell implementing synchronous reset semantics ---
+// one that exclusively drives the 'D' input of the $dff, with one of its
+// $mux inputs being fully zero
+// (3) Match for a $mux cell implement clock enable semantics --- one that
+// exclusively drives the 'D' input of the $dff (or the other input of
+// the reset $mux) and where one of this $mux's inputs is connected to
+// the 'Q' output of the $dff
subpattern in_dffe
arg argD argQ clock
code
dff = nullptr;
- for (auto c : argQ.chunks()) {
+ for (const auto &c : argQ.chunks()) {
+ // Abandon matches when 'Q' is a constant
if (!c.wire)
reject;
+ // Abandon matches when 'Q' has the keep attribute set
if (c.wire->get_bool_attribute(\keep))
reject;
- Const init = c.wire->attributes.at(\init, State::Sx);
- if (!init.is_fully_undef() && !init.is_fully_zero())
- reject;
+ // Abandon matches when 'Q' has a non-zero init attribute set
+ // (not supported by DSP48E1)
+ Const init = c.wire->attributes.at(\init, Const());
+ for (auto b : init.extract(c.offset, c.width))
+ if (b != State::Sx && b != State::S0)
+ reject;
}
endcode
+// (1) Starting from a $dff cell that (partially or fully) drives the given
+// 'Q' argument
match ff
select ff->type.in($dff)
// DSP48E1 does not support clock inversion
filter GetSize(port(ff, \Q)) >= offset + GetSize(argQ)
filter port(ff, \Q).extract(offset, GetSize(argQ)) == argQ
+ filter clock == SigBit() || port(ff, \CLK) == clock
+
set ffoffset offset
endmatch
code argQ argD
-{
- if (clock != SigBit() && port(ff, \CLK) != clock)
- reject;
-
SigSpec Q = port(ff, \Q);
dff = ff;
dffclock = port(ff, \CLK);
// has two (ff, ffrstmux) users
if (nusers(dffD) > 2)
argD = SigSpec();
-}
endcode
+// (2) Match for a $mux cell implementing synchronous reset semantics ---
+// exclusively drives the 'D' input of the $dff, with one of the $mux
+// inputs being fully zero
match ffrstmux
if !argD.empty()
select ffrstmux->type.in($mux)
dffrstmux = nullptr;
endcode
+// (3) Match for a $mux cell implement clock enable semantics --- one that
+// exclusively drives the 'D' input of the $dff (or the other input of
+// the reset $mux) and where one of this $mux's inputs is connected to
+// the 'Q' output of the $dff
match ffcemux
if !argD.empty()
select ffcemux->type.in($mux)
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