Revert "Refactor proxies to nextpnr."

This reverts commit 9b17fe385cf7e8d3025747b5f7c7822ac2d99920.

10 files changed, 194 insertions, 218 deletions

diff --git a/common/nextpnr.cc b/common/nextpnr.cc
index 54df5de1..3861e5fe 100644
--- a/common/nextpnr.cc
+++ b/common/nextpnr.cc
@@ -21,17 +21,6 @@
 
 NEXTPNR_NAMESPACE_BEGIN
 
-MutateContext BaseCtx::rwproxy(void)
-{
-    return MutateContext(reinterpret_cast<Arch *>(this));
-}
-
-ReadContext BaseCtx::rproxy(void) const
-{
-    return ReadContext(reinterpret_cast<const Arch *>(this));
-}
-
-
 assertion_failure::assertion_failure(std::string msg, std::string expr_str, std::string filename, int line)
         : runtime_error("Assertion failure: " + msg + " (" + filename + ":" + std::to_string(line) + ")"), msg(msg),
           expr_str(expr_str), filename(filename), line(line)
diff --git a/common/nextpnr.h b/common/nextpnr.h
index c3fb913c..efcab9fc 100644
--- a/common/nextpnr.h
+++ b/common/nextpnr.h
@@ -26,7 +26,6 @@
 #include <unordered_map>
 #include <unordered_set>
 #include <vector>
-#include <boost/thread/shared_mutex.hpp>
 
 #ifndef NEXTPNR_H
 #define NEXTPNR_H
@@ -249,37 +248,21 @@ struct UIUpdatesRequired
     std::unordered_set<GroupId> groupUIReload;
 };
 
-class ReadContext;
-class MutateContext;
-class BaseReadCtx;
-class BaseMutateCtx;
-
-// Data that every architecture object should contain.
-class BaseCtx
+struct BaseCtx
 {
-    friend class ReadContext;
-    friend class MutateContext;
-    friend class BaseReadCtx;
-    friend class BaseMutateCtx;
-private:
-    mutable boost::shared_mutex mtx_;
+    // --------------------------------------------------------------
 
-    bool allUiReload = false;
-    bool frameUiReload = false;
-    std::unordered_set<BelId> belUiReload;
-    std::unordered_set<WireId> wireUiReload;
-    std::unordered_set<PipId> pipUiReload;
-    std::unordered_set<GroupId> groupUiReload;
+    mutable std::unordered_map<std::string, int> *idstring_str_to_idx;
+    mutable std::vector<const std::string *> *idstring_idx_to_str;
 
-public:
     IdString id(const std::string &s) const { return IdString(this, s); }
+
     IdString id(const char *s) const { return IdString(this, s); }
 
-    // TODO(q3k): These need to be made private.
+    // --------------------------------------------------------------
+
     std::unordered_map<IdString, std::unique_ptr<NetInfo>> nets;
     std::unordered_map<IdString, std::unique_ptr<CellInfo>> cells;
-    mutable std::unordered_map<std::string, int> *idstring_str_to_idx;
-    mutable std::vector<const std::string *> *idstring_idx_to_str;
 
     BaseCtx()
     {
@@ -303,83 +286,41 @@ public:
 
     // --------------------------------------------------------------
 
-    // Get a readwrite proxy to arch - this will keep a readwrite lock on the
-    // entire architecture until the proxy object goes out of scope.
-    MutateContext rwproxy(void);
-    // Get a read-only proxy to arch - this will keep a  read lock on the
-    // entire architecture until the proxy object goes out of scope. Other read
-    // locks can be taken while this one still exists. Ie., the UI can draw
-    // elements while the PnR is going a RO operation.
-    ReadContext rproxy(void) const;
-
-};
-
-// State-accessing read-only methods that every architecture object should
-// contain.
-class BaseReadCtx
-{
-protected:
-    const BaseCtx *base_;
-public:
-    BaseReadCtx(const BaseCtx *base) : base_(base) {}
-};
-
-// State-accesssing read/write methods that every architecture object should
-// contain.
-class BaseMutateCtx
-{
-protected:
-    BaseCtx *base_;
-
-public:
-    BaseMutateCtx(BaseCtx *base) : base_(base) {}
+    bool allUiReload = false;
+    bool frameUiReload = false;
+    std::unordered_set<BelId> belUiReload;
+    std::unordered_set<WireId> wireUiReload;
+    std::unordered_set<PipId> pipUiReload;
+    std::unordered_set<GroupId> groupUiReload;
 
-    void refreshUi(void)
-    {
-        base_->allUiReload = true;
-    }
+    void refreshUi() { allUiReload = true; }
 
-    void refreshUiFrame(void)
-    {
-        base_->frameUiReload = true;
-    }
+    void refreshUiFrame() { frameUiReload = true; }
 
-    void refreshUiBel(BelId bel)
-    {
-        base_->belUiReload.insert(bel);
-    }
+    void refreshUiBel(BelId bel) { belUiReload.insert(bel); }
 
-    void refreshUiWire(WireId wire)
-    {
-        base_->wireUiReload.insert(wire);
-    }
+    void refreshUiWire(WireId wire) { wireUiReload.insert(wire); }
 
-    void refreshUiPip(PipId pip)
-    {
-        base_->pipUiReload.insert(pip);
-    }
+    void refreshUiPip(PipId pip) { pipUiReload.insert(pip); }
 
-    void refreshUiGroup(GroupId group)
-    {
-        base_->groupUiReload.insert(group);
-    }
+    void refreshUiGroup(GroupId group) { groupUiReload.insert(group); }
 
     UIUpdatesRequired getUIUpdatesRequired(void)
     {
         UIUpdatesRequired req;
-        req.allUIReload = base_->allUiReload;
-        req.frameUIReload = base_->frameUiReload;
-        req.belUIReload = base_->belUiReload;
-        req.wireUIReload = base_->wireUiReload;
-        req.pipUIReload = base_->pipUiReload;
-        req.groupUIReload = base_->groupUiReload;
-
-        base_->allUiReload = false;
-        base_->frameUiReload = false;
-        base_->belUiReload.clear();
-        base_->wireUiReload.clear();
-        base_->pipUiReload.clear();
-        base_->groupUiReload.clear();
+        req.allUIReload = allUiReload;
+        req.frameUIReload = frameUiReload;
+        req.belUIReload = belUiReload;
+        req.wireUIReload = wireUiReload;
+        req.pipUIReload = pipUiReload;
+        req.groupUIReload = groupUiReload;
+
+        allUiReload = false;
+        frameUiReload = false;
+        belUiReload.clear();
+        wireUiReload.clear();
+        pipUiReload.clear();
+        groupUiReload.clear();
         return req;
     }
 };
@@ -390,53 +331,6 @@ NEXTPNR_NAMESPACE_END
 
 NEXTPNR_NAMESPACE_BEGIN
 
-// Read proxy to access ReadMethods while holding lock on underlying BaseCtx.
-class ReadContext : public ArchReadMethods
-{
-    friend class BaseCtx;
-private:
-    boost::shared_mutex *lock_;
-    ReadContext(const Arch *parent) : ArchReadMethods(parent), lock_(&parent->mtx_)
-    {
-        lock_->lock_shared();
-    }
-public:
-    ~ReadContext()
-    {
-        if (lock_ != nullptr) {
-            lock_->unlock_shared();
-        }
-    }
-    ReadContext(ReadContext &&other): ArchReadMethods(other), lock_(other.lock_)
-    {
-        other.lock_ = nullptr;
-    }
-};
-
-// Read proxy to access MutateMethods while holding lock on underlying BaseCtx.
-class MutateContext : public ArchReadMethods, public ArchMutateMethods
-{
-    friend class BaseCtx;
-private:
-    boost::shared_mutex *lock_;
-    MutateContext(Arch *parent) : ArchReadMethods(parent), ArchMutateMethods(parent), lock_(&parent->mtx_)
-    {
-        lock_->lock();
-    }
-public:
-    ~MutateContext()
-    {
-        if (lock_ != nullptr) {
-            lock_->unlock();
-        }
-    }
-    MutateContext(MutateContext &&other): ArchReadMethods(other), ArchMutateMethods(other), lock_(other.lock_)
-    {
-        other.lock_ = nullptr;
-    }
-};
-
-
 struct Context : Arch
 {
     bool verbose = false;
diff --git a/common/place_common.cc b/common/place_common.cc
index 9694b6fe..48416370 100644
--- a/common/place_common.cc
+++ b/common/place_common.cc
@@ -26,7 +26,7 @@
 NEXTPNR_NAMESPACE_BEGIN
 
 // Placing a single cell
-bool place_single_cell(MutateContext &proxy, Context *ctx, CellInfo *cell, bool require_legality)
+bool place_single_cell(ArchRWProxy &proxy, Context *ctx, CellInfo *cell, bool require_legality)
 {
     bool all_placed = false;
     int iters = 25;
diff --git a/common/place_common.h b/common/place_common.h
index 96ac48a9..57e82510 100644
--- a/common/place_common.h
+++ b/common/place_common.h
@@ -109,7 +109,7 @@ wirelen_t get_cell_wirelength_at_bel(const T &proxy, const Context *ctx, CellInf
 }
 
 // Place a single cell in the lowest wirelength Bel available, optionally requiring validity check
-bool place_single_cell(MutateContext &proxy, Context *ctx, CellInfo *cell, bool require_legality);
+bool place_single_cell(ArchRWProxy &proxy, Context *ctx, CellInfo *cell, bool require_legality);
 
 NEXTPNR_NAMESPACE_END
 
diff --git a/common/placer1.cc b/common/placer1.cc
index 78515ece..0e3a84f7 100644
--- a/common/placer1.cc
+++ b/common/placer1.cc
@@ -275,7 +275,7 @@ class SAPlacer
 
   private:
     // Initial random placement
-    void place_initial(MutateContext &proxy, CellInfo *cell)
+    void place_initial(ArchRWProxy &proxy, CellInfo *cell)
     {
         bool all_placed = false;
         int iters = 25;
@@ -325,7 +325,7 @@ class SAPlacer
     }
 
     // Attempt a SA position swap, return true on success or false on failure
-    bool try_swap_position(MutateContext &proxy, CellInfo *cell, BelId newBel)
+    bool try_swap_position(ArchRWProxy &proxy, CellInfo *cell, BelId newBel)
     {
         static std::unordered_set<NetInfo *> update;
         static std::vector<std::pair<IdString, wirelen_t>> new_lengths;
diff --git a/common/router1.cc b/common/router1.cc
index f7a7e8a2..0d26a36d 100644
--- a/common/router1.cc
+++ b/common/router1.cc
@@ -74,7 +74,7 @@ struct RipupScoreboard
     std::unordered_map<std::pair<IdString, PipId>, int, hash_id_pip> netPipScores;
 };
 
-void ripup_net(MutateContext &proxy, Context *ctx, IdString net_name)
+void ripup_net(ArchRWProxy &proxy, Context *ctx, IdString net_name)
 {
     auto net_info = ctx->nets.at(net_name).get();
     std::vector<PipId> pips;
@@ -115,7 +115,7 @@ struct Router
     delay_t maxDelay = 0.0;
     WireId failedDest;
 
-    void route(MutateContext &proxy, const std::unordered_map<WireId, delay_t> &src_wires, WireId dst_wire)
+    void route(ArchRWProxy &proxy, const std::unordered_map<WireId, delay_t> &src_wires, WireId dst_wire)
     {
         std::priority_queue<QueuedWire, std::vector<QueuedWire>, QueuedWire::Greater> queue;
 
diff --git a/ice40/arch.cc b/ice40/arch.cc
index 790167e9..547dbcd6 100644
--- a/ice40/arch.cc
+++ b/ice40/arch.cc
@@ -29,6 +29,18 @@
 
 NEXTPNR_NAMESPACE_BEGIN
 
+ArchRWProxy Arch::rwproxy(void) {
+    ArchRWProxy res(this);
+    return res;
+}
+
+ArchRProxy Arch::rproxy(void) const {
+    ArchRProxy res(this);
+    return res;
+}
+
+// -----------------------------------------------------------------------
+
 IdString Arch::belTypeToId(BelType type) const
 {
     if (type == TYPE_ICESTORM_LC)
@@ -522,7 +534,7 @@ DecalXY Arch::getGroupDecal(GroupId group) const
     return decalxy;
 };
 
-std::vector<GraphicElement> ArchReadMethods::getDecalGraphics(DecalId decal) const
+std::vector<GraphicElement> ArchRProxyMethods::getDecalGraphics(DecalId decal) const
 {
     std::vector<GraphicElement> ret;
 
@@ -720,25 +732,25 @@ bool Arch::isGlobalNet(const NetInfo *net) const
 
 // -----------------------------------------------------------------------
 
-bool ArchReadMethods::checkBelAvail(BelId bel) const
+bool ArchRProxyMethods::checkBelAvail(BelId bel) const
 {
     NPNR_ASSERT(bel != BelId());
     return bel_to_cell[bel.index] == IdString();
 }
 
-IdString ArchReadMethods::getBoundBelCell(BelId bel) const
+IdString ArchRProxyMethods::getBoundBelCell(BelId bel) const
 {
     NPNR_ASSERT(bel != BelId());
     return bel_to_cell[bel.index];
 }
 
-IdString ArchReadMethods::getConflictingBelCell(BelId bel) const
+IdString ArchRProxyMethods::getConflictingBelCell(BelId bel) const
 {
     NPNR_ASSERT(bel != BelId());
     return bel_to_cell[bel.index];
 }
 
-WireId ArchReadMethods::getWireBelPin(BelId bel, PortPin pin) const
+WireId ArchRProxyMethods::getWireBelPin(BelId bel, PortPin pin) const
 {
     WireId ret;
 
@@ -756,7 +768,7 @@ WireId ArchReadMethods::getWireBelPin(BelId bel, PortPin pin) const
     return ret;
 }
 
-WireId ArchReadMethods::getWireByName(IdString name) const
+WireId ArchRProxyMethods::getWireByName(IdString name) const
 {
     WireId ret;
 
@@ -772,25 +784,25 @@ WireId ArchReadMethods::getWireByName(IdString name) const
     return ret;
 }
 
-bool ArchReadMethods::checkWireAvail(WireId wire) const
+bool ArchRProxyMethods::checkWireAvail(WireId wire) const
 {
     NPNR_ASSERT(wire != WireId());
     return wire_to_net[wire.index] == IdString();
 }
 
-IdString ArchReadMethods::getBoundWireNet(WireId wire) const
+IdString ArchRProxyMethods::getBoundWireNet(WireId wire) const
 {
     NPNR_ASSERT(wire != WireId());
     return wire_to_net[wire.index];
 }
 
-IdString ArchReadMethods::getConflictingWireNet(WireId wire) const
+IdString ArchRProxyMethods::getConflictingWireNet(WireId wire) const
 {
     NPNR_ASSERT(wire != WireId());
     return wire_to_net[wire.index];
 }
 
-PipId ArchReadMethods::getPipByName(IdString name) const
+PipId ArchRProxyMethods::getPipByName(IdString name) const
 {
     PipId ret;
 
@@ -809,25 +821,25 @@ PipId ArchReadMethods::getPipByName(IdString name) const
     return ret;
 }
 
-bool ArchReadMethods::checkPipAvail(PipId pip) const
+bool ArchRProxyMethods::checkPipAvail(PipId pip) const
 {
     NPNR_ASSERT(pip != PipId());
     return switches_locked[chip_info->pip_data[pip.index].switch_index] == IdString();
 }
 
-IdString ArchReadMethods::getBoundPipNet(PipId pip) const
+IdString ArchRProxyMethods::getBoundPipNet(PipId pip) const
 {
     NPNR_ASSERT(pip != PipId());
     return pip_to_net[pip.index];
 }
 
-IdString ArchReadMethods::getConflictingPipNet(PipId pip) const
+IdString ArchRProxyMethods::getConflictingPipNet(PipId pip) const
 {
     NPNR_ASSERT(pip != PipId());
     return switches_locked[chip_info->pip_data[pip.index].switch_index];
 }
 
-BelId ArchReadMethods::getBelByName(IdString name) const
+BelId ArchRProxyMethods::getBelByName(IdString name) const
 {
     BelId ret;
 
@@ -845,27 +857,27 @@ BelId ArchReadMethods::getBelByName(IdString name) const
 
 // -----------------------------------------------------------------------
 
-void ArchMutateMethods::bindBel(BelId bel, IdString cell, PlaceStrength strength)
+void ArchRWProxyMethods::bindBel(BelId bel, IdString cell, PlaceStrength strength)
 {
     NPNR_ASSERT(bel != BelId());
     NPNR_ASSERT(bel_to_cell[bel.index] == IdString());
     bel_to_cell[bel.index] = cell;
     parent_->cells[cell]->bel = bel;
     parent_->cells[cell]->belStrength = strength;
-    refreshUiBel(bel);
+    parent_->refreshUiBel(bel);
 }
 
-void ArchMutateMethods::unbindBel(BelId bel)
+void ArchRWProxyMethods::unbindBel(BelId bel)
 {
     NPNR_ASSERT(bel != BelId());
     NPNR_ASSERT(bel_to_cell[bel.index] != IdString());
     parent_->cells[bel_to_cell[bel.index]]->bel = BelId();
     parent_->cells[bel_to_cell[bel.index]]->belStrength = STRENGTH_NONE;
     bel_to_cell[bel.index] = IdString();
-    refreshUiBel(bel);
+    parent_->refreshUiBel(bel);
 }
 
-void ArchMutateMethods::bindWire(WireId wire, IdString net, PlaceStrength strength)
+void ArchRWProxyMethods::bindWire(WireId wire, IdString net, PlaceStrength strength)
 {
     NPNR_ASSERT(wire != WireId());
     NPNR_ASSERT(wire_to_net[wire.index] == IdString());
@@ -873,10 +885,10 @@ void ArchMutateMethods::bindWire(WireId wire, IdString net, PlaceStrength streng
     wire_to_net[wire.index] = net;
     parent_->nets[net]->wires[wire].pip = PipId();
     parent_->nets[net]->wires[wire].strength = strength;
-    refreshUiWire(wire);
+    parent_->refreshUiWire(wire);
 }
 
-void ArchMutateMethods::unbindWire(WireId wire)
+void ArchRWProxyMethods::unbindWire(WireId wire)
 {
     NPNR_ASSERT(wire != WireId());
     NPNR_ASSERT(wire_to_net[wire.index] != IdString());
@@ -889,15 +901,15 @@ void ArchMutateMethods::unbindWire(WireId wire)
     if (pip != PipId()) {
         pip_to_net[pip.index] = IdString();
         switches_locked[chip_info->pip_data[pip.index].switch_index] = IdString();
-        refreshUiPip(pip);
+        parent_->refreshUiPip(pip);
     }
 
     net_wires.erase(it);
     wire_to_net[wire.index] = IdString();
-    refreshUiWire(wire);
+    parent_->refreshUiWire(wire);
 }
 
-void ArchMutateMethods::bindPip(PipId pip, IdString net, PlaceStrength strength)
+void ArchRWProxyMethods::bindPip(PipId pip, IdString net, PlaceStrength strength)
 {
     NPNR_ASSERT(pip != PipId());
     NPNR_ASSERT(pip_to_net[pip.index] == IdString());
@@ -913,11 +925,11 @@ void ArchMutateMethods::bindPip(PipId pip, IdString net, PlaceStrength strength)
     parent_->nets[net]->wires[dst].pip = pip;
     parent_->nets[net]->wires[dst].strength = strength;
 
-    refreshUiPip(pip);
-    refreshUiWire(dst);
+    parent_->refreshUiPip(pip);
+    parent_->refreshUiWire(dst);
 }
 
-void ArchMutateMethods::unbindPip(PipId pip)
+void ArchRWProxyMethods::unbindPip(PipId pip)
 {
     NPNR_ASSERT(pip != PipId());
     NPNR_ASSERT(pip_to_net[pip.index] != IdString());
@@ -932,13 +944,18 @@ void ArchMutateMethods::unbindPip(PipId pip)
     pip_to_net[pip.index] = IdString();
     switches_locked[chip_info->pip_data[pip.index].switch_index] = IdString();
 
-    refreshUiPip(pip);
-    refreshUiWire(dst);
+    parent_->refreshUiPip(pip);
+    parent_->refreshUiWire(dst);
 }
 
-CellInfo *ArchMutateMethods::getCell(IdString cell)
+CellInfo *ArchRWProxyMethods::getCell(IdString cell)
 {
     return parent_->cells.at(cell).get();
 }
 
+UIUpdatesRequired ArchRWProxyMethods::getUIUpdatesRequired(void)
+{
+    return parent_->getUIUpdatesRequired();
+}
+
 NEXTPNR_NAMESPACE_END
diff --git a/ice40/arch.h b/ice40/arch.h
index 5d4eaedf..4311f4a5 100644
--- a/ice40/arch.h
+++ b/ice40/arch.h
@@ -22,6 +22,9 @@
 #error Include "arch.h" via "nextpnr.h" only.
 #endif
 
+#include <boost/thread/shared_lock_guard.hpp>
+#include <boost/thread/shared_mutex.hpp>
+
 NEXTPNR_NAMESPACE_BEGIN
 
 /**** Everything in this section must be kept in sync with chipdb.py ****/
@@ -327,8 +330,11 @@ struct ArchArgs
 
 /// Forward declare proxy classes for Arch.
 
-class ArchMutateMethods;
-class ArchReadMethods;
+class ArchRWProxyMethods;
+class ArchRProxyMethods;
+class ArchRWProxy;
+class ArchRProxy;
+
 
 /// Arch/Context
 // Arch is the main state class of the PnR algorithms. It keeps note of mapped
@@ -341,8 +347,11 @@ class ArchReadMethods;
 class Arch : public BaseCtx
 {
     // We let proxy methods access our state.
-    friend class ArchMutateMethods;
-    friend class ArchReadMethods;
+    friend class ArchRWProxyMethods;
+    friend class ArchRProxyMethods;
+    // We let proxy objects access our mutex.
+    friend class ArchRWProxy;
+    friend class ArchRProxy;
 private:
     // All of the following...
     std::vector<IdString> bel_to_cell;
@@ -353,6 +362,9 @@ private:
     mutable std::unordered_map<IdString, int> wire_by_name;
     mutable std::unordered_map<IdString, int> pip_by_name;
 
+    // ... are guarded by the following lock:
+    mutable boost::shared_mutex mtx_;
+
 public:
     const ChipInfoPOD *chip_info;
     const PackageInfoPOD *package_info;
@@ -360,6 +372,15 @@ public:
     ArchArgs args;
     Arch(ArchArgs args);
 
+    // Get a readwrite proxy to arch - this will keep a readwrite lock on the
+    // entire architecture until the proxy object goes out of scope.
+    ArchRWProxy rwproxy(void);
+    // Get a read-only proxy to arch - this will keep a  read lock on the
+    // entire architecture until the proxy object goes out of scope. Other read
+    // locks can be taken while this one still exists. Ie., the UI can draw
+    // elements while the PnR is going a RO operation.
+    ArchRProxy rproxy(void) const;
+
     std::string getChipName();
 
     IdString archId() const { return id("ice40"); }
@@ -611,9 +632,22 @@ public:
 };
 
 // Read-only methods on Arch that require state access.
-class ArchReadMethods : public BaseReadCtx {
+class ArchRProxyMethods {
+    // We let proxy objects access our private constructors.
+    friend class ArchRProxy;
+    friend class ArchRWProxy;
 private:
     const Arch *parent_;
+    ArchRProxyMethods(const Arch *parent) : parent_(parent), chip_info(parent->chip_info),
+        bel_to_cell(parent->bel_to_cell), wire_to_net(parent->wire_to_net),
+        pip_to_net(parent->pip_to_net), switches_locked(parent->switches_locked),
+        bel_by_name(parent->bel_by_name), wire_by_name(parent->wire_by_name),
+        pip_by_name(parent->pip_by_name) {}
+    ArchRProxyMethods(ArchRProxyMethods &&other) noexcept : ArchRProxyMethods(other.parent_) {}
+    ArchRProxyMethods(const ArchRProxyMethods &other) : ArchRProxyMethods(other.parent_) {}
+
+    // Let methods access hot members directly without having to go through
+    // parent_.
     const ChipInfoPOD *chip_info;
     const std::vector<IdString> &bel_to_cell;
     const std::vector<IdString> &wire_to_net;
@@ -622,17 +656,8 @@ private:
     std::unordered_map<IdString, int> &bel_by_name;
     std::unordered_map<IdString, int> &wire_by_name;
     std::unordered_map<IdString, int> &pip_by_name;
-
 public:
-    ~ArchReadMethods() noexcept { }
-    ArchReadMethods(const Arch *parent) : BaseReadCtx(parent), parent_(parent),
-        chip_info(parent->chip_info), bel_to_cell(parent->bel_to_cell), 
-        wire_to_net(parent->wire_to_net), pip_to_net(parent->pip_to_net),
-        switches_locked(parent->switches_locked),
-        bel_by_name(parent->bel_by_name), wire_by_name(parent->wire_by_name),
-        pip_by_name(parent->pip_by_name) {}
-    ArchReadMethods(ArchReadMethods &&other) noexcept : ArchReadMethods(other.parent_) {}
-    ArchReadMethods(const ArchReadMethods &other) : ArchReadMethods(other.parent_) {}
+    ~ArchRProxyMethods() noexcept { }
 
     /// Perform placement validity checks, returning false on failure (all implemented in arch_place.cc)
 
@@ -668,11 +693,44 @@ public:
     std::vector<GraphicElement> getDecalGraphics(DecalId decal) const;
 };
 
+// A proxy object that keeps an Arch shared/readonly lock until it goes out
+// of scope. All const/read-only ArchRProxyMethods are available on it.
+class ArchRProxy : public ArchRProxyMethods {
+    friend class Arch;
+    friend class ArchRWProxy;
+private:
+    boost::shared_mutex *lock_;
+    ArchRProxy(const Arch *parent) : ArchRProxyMethods(parent), lock_(&parent->mtx_)
+    {
+        lock_->lock_shared();
+    }
+
+public:
+    ~ArchRProxy() {
+        if (lock_ != nullptr) {
+            lock_->unlock_shared();
+        }
+    }
+    ArchRProxy(ArchRProxy &&other) : ArchRProxyMethods(other), lock_(other.lock_)
+    {
+        other.lock_ = nullptr;
+    }
+};
+
 // State mutating methods on Arch.
-class ArchMutateMethods : public BaseMutateCtx {
-    friend class MutateContext;
+class ArchRWProxyMethods {
+    // We let proxy objects access our private constructors.
+    friend class ArchRWProxy;
 private:
     Arch *parent_;
+    ArchRWProxyMethods(Arch *parent) : parent_(parent), chip_info(parent->chip_info),
+        bel_to_cell(parent->bel_to_cell), wire_to_net(parent->wire_to_net),
+        pip_to_net(parent->pip_to_net), switches_locked(parent->switches_locked),
+        bel_by_name(parent->bel_by_name), wire_by_name(parent->wire_by_name),
+        pip_by_name(parent->pip_by_name) {}
+    ArchRWProxyMethods(ArchRWProxyMethods &&other) : ArchRWProxyMethods(other.parent_) {}
+    ArchRWProxyMethods(const ArchRWProxyMethods &other) : ArchRWProxyMethods(other.parent_) {}
+
     const ChipInfoPOD *chip_info;
     std::vector<IdString> &bel_to_cell;
     std::vector<IdString> &wire_to_net;
@@ -681,17 +739,8 @@ private:
     std::unordered_map<IdString, int> &bel_by_name;
     std::unordered_map<IdString, int> &wire_by_name;
     std::unordered_map<IdString, int> &pip_by_name;
-
 public:
-    ArchMutateMethods(Arch *parent) : BaseMutateCtx(parent), parent_(parent),
-        chip_info(parent->chip_info), bel_to_cell(parent->bel_to_cell), 
-        wire_to_net(parent->wire_to_net), pip_to_net(parent->pip_to_net),
-        switches_locked(parent->switches_locked),
-        bel_by_name(parent->bel_by_name), wire_by_name(parent->wire_by_name),
-        pip_by_name(parent->pip_by_name) {}
-    ArchMutateMethods(ArchMutateMethods &&other) : ArchMutateMethods(other.parent_) {}
-    ArchMutateMethods(const ArchMutateMethods &other) : ArchMutateMethods(other.parent_) {}
-    ~ArchMutateMethods() {}
+    ~ArchRWProxyMethods() {}
 
     void unbindWire(WireId wire);
     void unbindPip(PipId pip);
@@ -701,6 +750,33 @@ public:
     void bindBel(BelId bel, IdString cell, PlaceStrength strength);
     // Returned pointer is valid as long as Proxy object exists.
     CellInfo *getCell(IdString cell);
+
+
+    // Methods to be used by UI for detecting whether we need to redraw.
+    UIUpdatesRequired getUIUpdatesRequired(void);
+};
+
+// A proxy object that keeps an Arch readwrite lock until it goes out of scope.
+// All ArchRProxyMethods and ArchRWProxyMethods are available on it.
+class ArchRWProxy : public ArchRProxyMethods, public ArchRWProxyMethods {
+    friend class Arch;
+private:
+    boost::shared_mutex *lock_;
+    ArchRWProxy(Arch *parent) : ArchRProxyMethods(parent), ArchRWProxyMethods(parent), lock_(&parent->mtx_) {
+        lock_->lock();
+    }
+
+public:
+    ArchRWProxy(ArchRWProxy &&other) : ArchRProxyMethods(other), ArchRWProxyMethods(other), lock_(other.lock_)
+    {
+        other.lock_ = nullptr;
+    }
+    ~ArchRWProxy()
+    {
+        if (lock_ != nullptr) {
+            lock_->unlock();
+        }
+    }
 };
 
 NEXTPNR_NAMESPACE_END
diff --git a/ice40/arch_place.cc b/ice40/arch_place.cc
index 42efceab..cb7c44b8 100644
--- a/ice40/arch_place.cc
+++ b/ice40/arch_place.cc
@@ -25,7 +25,7 @@
 
 NEXTPNR_NAMESPACE_BEGIN
 
-bool ArchReadMethods::logicCellsCompatible(const std::vector<const CellInfo *> &cells) const
+bool ArchRProxyMethods::logicCellsCompatible(const std::vector<const CellInfo *> &cells) const
 {
     bool dffs_exist = false, dffs_neg = false;
     const NetInfo *cen = nullptr, *clk = nullptr, *sr = nullptr;
@@ -76,7 +76,7 @@ bool ArchReadMethods::logicCellsCompatible(const std::vector<const CellInfo *> &
     return locals_count <= 32;
 }
 
-bool ArchReadMethods::isBelLocationValid(BelId bel) const
+bool ArchRProxyMethods::isBelLocationValid(BelId bel) const
 {
     if (parent_->getBelType(bel) == TYPE_ICESTORM_LC) {
         std::vector<const CellInfo *> bel_cells;
@@ -97,7 +97,7 @@ bool ArchReadMethods::isBelLocationValid(BelId bel) const
     }
 }
 
-bool ArchReadMethods::isValidBelForCell(CellInfo *cell, BelId bel) const
+bool ArchRProxyMethods::isValidBelForCell(CellInfo *cell, BelId bel) const
 {
     if (cell->type == parent_->id_icestorm_lc) {
         NPNR_ASSERT(parent_->getBelType(bel) == TYPE_ICESTORM_LC);
diff --git a/ice40/place_legaliser.cc b/ice40/place_legaliser.cc
index fcb47cfd..10a6f3ff 100644
--- a/ice40/place_legaliser.cc
+++ b/ice40/place_legaliser.cc
@@ -253,7 +253,7 @@ class PlacementLegaliser
     }
 
     // Find Bel closest to a location, meeting chain requirements
-    std::tuple<int, int, int> find_closest_bel(MutateContext &proxy, float target_x, float target_y, CellChain &chain)
+    std::tuple<int, int, int> find_closest_bel(ArchRWProxy &proxy, float target_x, float target_y, CellChain &chain)
     {
         std::tuple<int, int, int> best_origin = std::make_tuple(-1, -1, -1);
         wirelen_t best_wirelength = std::numeric_limits<wirelen_t>::max();
@@ -283,7 +283,7 @@ class PlacementLegaliser
     }
 
     // Split a carry chain into multiple legal chains
-    std::vector<CellChain> split_carry_chain(const ReadContext &proxy, CellChain &carryc)
+    std::vector<CellChain> split_carry_chain(const ArchRProxy &proxy, CellChain &carryc)
     {
         bool start_of_chain = true;
         std::vector<CellChain> chains;
@@ -335,7 +335,7 @@ class PlacementLegaliser
     }
 
     // Place a logic cell at a given grid location, handling rip-up etc
-    void place_lc(MutateContext &proxy, CellInfo *cell, int x, int y, int z)
+    void place_lc(ArchRWProxy &proxy, CellInfo *cell, int x, int y, int z)
     {
         auto &loc = logic_bels.at(x).at(y).at(z);
         NPNR_ASSERT(!loc.second);