diff options
Diffstat (limited to 'passes/pmgen/xilinx_dsp_cascade.pmg')
| -rw-r--r-- | passes/pmgen/xilinx_dsp_cascade.pmg | 115 |
1 files changed, 103 insertions, 12 deletions
diff --git a/passes/pmgen/xilinx_dsp_cascade.pmg b/passes/pmgen/xilinx_dsp_cascade.pmg index 6f4ac5849..7a32df2b7 100644 --- a/passes/pmgen/xilinx_dsp_cascade.pmg +++ b/passes/pmgen/xilinx_dsp_cascade.pmg @@ -1,3 +1,46 @@ +// 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 @@ -6,7 +49,7 @@ state <Cell*> next state <SigSpec> clock state <int> AREG BREG -// subpattern +// Variables used for subpatterns state <SigSpec> argQ argD state <bool> ffcepol ffrstpol state <int> ffoffset @@ -19,12 +62,19 @@ code #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); @@ -106,6 +156,10 @@ subpattern tail 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() @@ -116,6 +170,8 @@ match nextP 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) @@ -145,6 +201,14 @@ code next } 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) { @@ -152,7 +216,6 @@ code argQ clock AREG 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); @@ -174,6 +237,7 @@ reject_AREG: ; } endcode +// (4) Same as (3) but for BCOUT -> BCIN cascade code argQ clock BREG BREG = -1; if (next) { @@ -203,13 +267,14 @@ reject_BREG: ; } 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) @@ -232,22 +297,44 @@ endcode // ####################### +// Subpattern for matching against input registers, based on knowledge of the +// 'Q' input. Typically, identifying registers with clock-enable and reset +// capability would be a task would be handled by other Yosys passes such as +// dff2dffe, but since DSP inference happens much before this, these patterns +// have to be manually identified. +// 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 @@ -260,14 +347,12 @@ match ff 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); @@ -279,9 +364,11 @@ code argQ argD // 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) @@ -313,6 +400,10 @@ code argD 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) |