diff options
Diffstat (limited to 'passes/pmgen/xilinx_dsp_CREG.pmg')
| -rw-r--r-- | passes/pmgen/xilinx_dsp_CREG.pmg | 81 |
1 files changed, 67 insertions, 14 deletions
diff --git a/passes/pmgen/xilinx_dsp_CREG.pmg b/passes/pmgen/xilinx_dsp_CREG.pmg index a31dc80bf..a57043009 100644 --- a/passes/pmgen/xilinx_dsp_CREG.pmg +++ b/passes/pmgen/xilinx_dsp_CREG.pmg @@ -1,3 +1,26 @@ +// This file describes the second 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) doesn't have a CREG already, +// and (b) uses the 'C' port +// (2) Match the driver of the 'C' input to a possible $dff cell (CREG) +// (attached to at most two $mux cells that implement clock-enable or +// reset functionality, using a subpattern discussed below) +// Notes: +// - Running CREG packing after xilinx_dsp_pack is necessary since there is no +// guarantee that the cell ordering corresponds to the "expected" case (i.e. +// the order in which they appear in the source) thus the possiblity existed +// that a register got packed as a CREG into a downstream DSP that should +// have otherwise been a PREG of an upstream DSP that had not been visited +// yet +// - The reason this is separated out from the xilinx_dsp.pmg file is +// for efficiency --- each *.pmg file creates a class of the same basename, +// which when constructed, creates a custom database tailored to the +// pattern(s) contained within. Since the pattern in this file must be +// executed after the pattern contained in xilinx_dsp.pmg, it is necessary +// to reconstruct this database. Separating the two patterns into +// independent files causes two smaller, more specific, databases. + pattern xilinx_dsp_packC udata <std::function<SigSpec(const SigSpec&)>> unextend @@ -6,7 +29,7 @@ state <SigSpec> sigC sigP state <bool> ffCcepol ffCrstpol state <Cell*> ffC ffCcemux ffCrstmux -// subpattern +// Variables used for subpatterns state <SigSpec> argQ argD state <bool> ffcepol ffrstpol state <int> ffoffset @@ -15,13 +38,15 @@ udata <SigBit> dffclock udata <Cell*> dff dffcemux dffrstmux udata <bool> dffcepol dffrstpol +// (1) Starting from a DSP48E1 cell that (a) doesn't have a CREG already, +// and (b) uses the 'C' port match dsp select dsp->type.in(\DSP48E1) select param(dsp, \CREG, 1).as_int() == 0 select nusers(port(dsp, \C, SigSpec())) > 1 endmatch -code argQ ffC ffCcemux ffCrstmux ffCcepol ffCrstpol sigC sigP clock +code sigC sigP clock unextend = [](const SigSpec &sig) { int i; for (i = GetSize(sig)-1; i > 0; i--) @@ -48,11 +73,13 @@ code argQ ffC ffCcemux ffCrstmux ffCcepol ffCrstpol sigC sigP clock else sigP = P; - if (sigC == sigP) - reject; - clock = port(dsp, \CLK, SigBit()); +endcode +// (2) Match the driver of the 'C' input to a possible $dff cell (CREG) +// (attached to at most two $mux cells that implement clock-enable or +// reset functionality, using the in_dffe subpattern) +code argQ ffC ffCcemux ffCrstmux ffCcepol ffCrstpol sigC clock argQ = sigC; subpattern(in_dffe); if (dff) { @@ -77,22 +104,44 @@ 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 @@ -105,14 +154,12 @@ 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); @@ -124,9 +171,11 @@ 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) @@ -158,6 +207,10 @@ 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) |