Add getBelPinsForCellPin to Arch API

This is a basic implementation, without considering "M of N"
arrangements (e.g. for LUT permuation where you only want to route to 1
out of 4/6 sinks) or using a type other than IdString to identify bel
pins.

But this is also enough to start working out where in nextpnr will break
due to removing the 1:1 cell:bel pin cardinality, as a next step.

Signed-off-by: gatecat <gatecat@ds0.me>

7 files changed, 26 insertions, 0 deletions

diff --git a/common/nextpnr.h b/common/nextpnr.h
index c43b9dc4..cf04f831 100644
--- a/common/nextpnr.h
+++ b/common/nextpnr.h
@@ -1093,6 +1093,7 @@ template <typename R> struct ArchAPI : BaseCtx
     virtual WireId getBelPinWire(BelId bel, IdString pin) const = 0;
     virtual PortType getBelPinType(BelId bel, IdString pin) const = 0;
     virtual typename R::BelPinsRangeT getBelPins(BelId bel) const = 0;
+    virtual typename R::CellBelPinRangeT getBelPinsForCellPin(CellInfo *cell_info, IdString pin) const = 0;
     // Wire methods
     virtual typename R::AllWiresRangeT getWires() const = 0;
     virtual WireId getWireByName(IdStringList name) const = 0;
@@ -1176,6 +1177,8 @@ template <typename R> struct ArchAPI : BaseCtx
 // This contains the relevant range types for the default implementations of Arch functions
 struct BaseArchRanges
 {
+    // Bels
+    using CellBelPinRangeT = std::array<IdString, 1>;
     // Attributes
     using BelAttrsRangeT = std::vector<std::pair<IdString, std::string>>;
     using WireAttrsRangeT = std::vector<std::pair<IdString, std::string>>;
@@ -1241,6 +1244,11 @@ template <typename R> struct BaseArch : ArchAPI<R>
         return empty_if_possible<typename R::BelAttrsRangeT>();
     }
 
+    virtual typename R::CellBelPinRangeT getBelPinsForCellPin(CellInfo *cell_info, IdString pin) const override
+    {
+        return return_if_match<std::array<IdString, 1>, typename R::CellBelPinRangeT>({pin});
+    }
+
     // Wire methods
     virtual IdString getWireType(WireId wire) const override { return IdString(); }
     virtual typename R::WireAttrsRangeT getWireAttrs(WireId) const override
diff --git a/docs/archapi.md b/docs/archapi.md
index f6f184e0..2a38502c 100644
--- a/docs/archapi.md
+++ b/docs/archapi.md
@@ -13,6 +13,7 @@ The contents of `ArchRanges` is as follows:
 |`TileBelsRangeT`           | `BelId`                          |
 |`BelAttrsRangeT`           | std::pair<IdString, std::string> |
 |`BelPinsRangeT`            | `IdString`                       |
+|`CellBelPinRangeT`         | `IdString`                       |
 |`AllWiresRangeT`           | `WireId`                         |
 |`DownhillPipRangeT`        | `PipId`                          |
 |`UphillPipRangeT`          | `PipId`                          |
@@ -244,6 +245,12 @@ Return the type (input/output/inout) of the given bel pin.
 
 Return a list of all pins on that bel.
 
+### CellBelPinRangeT getBelPinsForCellPin(CellInfo *cell_info, IdString pin) const
+
+Return the list of bel pin names that a given cell pin should be routed to. In most cases there will be a single bel pin for each cell pin; and output pins must _always_ have only one bel pin associated with them.
+
+*BaseArch default: returns a one-element array containing `pin`*
+
 Wire Methods
 ------------
 
diff --git a/fpga_interchange/arch.h b/fpga_interchange/arch.h
index fd2d16a2..e5c7551b 100644
--- a/fpga_interchange/arch.h
+++ b/fpga_interchange/arch.h
@@ -660,6 +660,7 @@ struct ArchRanges
     using TileBelsRangeT = BelRange;
     using BelAttrsRangeT = std::vector<std::pair<IdString, std::string>>;
     using BelPinsRangeT = IdStringRange;
+    using CellBelPinRangeT = std::array<IdString, 1>;
     // Wires
     using AllWiresRangeT = WireRange;
     using DownhillPipRangeT = DownhillPipRange;
@@ -866,6 +867,8 @@ struct Arch : ArchAPI<ArchRanges>
         return str_range;
     }
 
+    std::array<IdString, 1> getBelPinsForCellPin(CellInfo *cell_info, IdString pin) const override { return {pin}; }
+
     // -------------------------------------------------
 
     WireId getWireByName(IdStringList name) const override;
diff --git a/generic/arch.cc b/generic/arch.cc
index 912f8a53..c51fbb84 100644
--- a/generic/arch.cc
+++ b/generic/arch.cc
@@ -339,6 +339,8 @@ std::vector<IdString> Arch::getBelPins(BelId bel) const
     return ret;
 }
 
+std::array<IdString, 1> Arch::getBelPinsForCellPin(CellInfo *cell_info, IdString pin) const { return {pin}; }
+
 // ---------------------------------------------------------------
 
 WireId Arch::getWireByName(IdStringList name) const
diff --git a/generic/arch.h b/generic/arch.h
index 09fd8e34..e7d204ef 100644
--- a/generic/arch.h
+++ b/generic/arch.h
@@ -124,6 +124,7 @@ struct ArchRanges
     using TileBelsRangeT = const std::vector<BelId> &;
     using BelAttrsRangeT = const std::map<IdString, std::string> &;
     using BelPinsRangeT = std::vector<IdString>;
+    using CellBelPinRangeT = std::array<IdString, 1>;
     // Wires
     using AllWiresRangeT = const std::vector<WireId> &;
     using DownhillPipRangeT = const std::vector<PipId> &;
@@ -241,6 +242,7 @@ struct Arch : ArchAPI<ArchRanges>
     WireId getBelPinWire(BelId bel, IdString pin) const override;
     PortType getBelPinType(BelId bel, IdString pin) const override;
     std::vector<IdString> getBelPins(BelId bel) const override;
+    std::array<IdString, 1> getBelPinsForCellPin(CellInfo *cell_info, IdString pin) const override;
 
     WireId getWireByName(IdStringList name) const override;
     IdStringList getWireName(WireId wire) const override;
diff --git a/gowin/arch.cc b/gowin/arch.cc
index d4e17a8a..8aeccfba 100644
--- a/gowin/arch.cc
+++ b/gowin/arch.cc
@@ -822,6 +822,8 @@ std::vector<IdString> Arch::getBelPins(BelId bel) const
     return ret;
 }
 
+std::array<IdString, 1> Arch::getBelPinsForCellPin(CellInfo *cell_info, IdString pin) const { return {pin}; }
+
 // ---------------------------------------------------------------
 
 WireId Arch::getWireByName(IdStringList name) const
diff --git a/gowin/arch.h b/gowin/arch.h
index c5ef0a2e..eab75899 100644
--- a/gowin/arch.h
+++ b/gowin/arch.h
@@ -251,6 +251,7 @@ struct ArchRanges
     using TileBelsRangeT = const std::vector<BelId> &;
     using BelAttrsRangeT = const std::map<IdString, std::string> &;
     using BelPinsRangeT = std::vector<IdString>;
+    using CellBelPinRangeT = std::array<IdString, 1>;
     // Wires
     using AllWiresRangeT = const std::vector<WireId> &;
     using DownhillPipRangeT = const std::vector<PipId> &;
@@ -375,6 +376,7 @@ struct Arch : BaseArch<ArchRanges>
     WireId getBelPinWire(BelId bel, IdString pin) const override;
     PortType getBelPinType(BelId bel, IdString pin) const override;
     std::vector<IdString> getBelPins(BelId bel) const override;
+    std::array<IdString, 1> getBelPinsForCellPin(CellInfo *cell_info, IdString pin) const override;
 
     WireId getWireByName(IdStringList name) const override;
     IdStringList getWireName(WireId wire) const override;