diff --git a/csharp/ql/lib/semmle/code/csharp/dataflow/internal/BaseSSA.qll b/csharp/ql/lib/semmle/code/csharp/dataflow/internal/BaseSSA.qll
index 4f37508f9a2c..63a9e782250f 100644
--- a/csharp/ql/lib/semmle/code/csharp/dataflow/internal/BaseSSA.qll
+++ b/csharp/ql/lib/semmle/code/csharp/dataflow/internal/BaseSSA.qll
@@ -7,12 +7,21 @@ module BaseSsa {
   private import AssignableDefinitions
   private import codeql.ssa.Ssa as SsaImplCommon
 
+  cached
+  private module BaseSsaStage {
+    cached
+    predicate ref() { any() }
+
+    cached
+    predicate backref() { (exists(any(SsaDefinition def).getARead()) implies any()) }
+  }
+
   /**
    * Holds if the `i`th node of basic block `bb` is assignable definition `def`,
    * targeting local scope variable `v`.
    */
   private predicate definitionAt(
-    AssignableDefinition def, BasicBlock bb, int i, SsaInput::SourceVariable v
+    AssignableDefinition def, BasicBlock bb, int i, SsaImplInput::SourceVariable v
   ) {
     bb.getNode(i) = def.getExpr().getControlFlowNode() and
     v = def.getTarget() and
@@ -24,7 +33,7 @@ module BaseSsa {
     )
   }
 
-  private predicate implicitEntryDef(Callable c, EntryBasicBlock bb, SsaInput::SourceVariable v) {
+  private predicate entryDef(Callable c, BasicBlock bb, SsaImplInput::SourceVariable v) {
     exists(EntryBasicBlock entry |
       c = entry.getEnclosingCallable() and
       // In case `c` has multiple bodies, we want each body to get its own implicit
@@ -79,16 +88,17 @@ module BaseSsa {
     }
   }
 
-  private module SsaInput implements SsaImplCommon::InputSig<Location, BasicBlock> {
+  private module SsaImplInput implements SsaImplCommon::InputSig<Location, BasicBlock> {
     class SourceVariable = SimpleLocalScopeVariable;
 
     predicate variableWrite(BasicBlock bb, int i, SourceVariable v, boolean certain) {
+      BaseSsaStage::ref() and
       exists(AssignableDefinition def |
         definitionAt(def, bb, i, v) and
         if def.isCertain() then certain = true else certain = false
       )
       or
-      implicitEntryDef(_, bb, v) and
+      entryDef(_, bb, v) and
       i = -1 and
       certain = true
     }
@@ -102,54 +112,35 @@ module BaseSsa {
     }
   }
 
-  private module SsaImpl = SsaImplCommon::Make<Location, Cfg, SsaInput>;
+  private module SsaImpl = SsaImplCommon::Make<Location, Cfg, SsaImplInput>;
 
-  class Definition extends SsaImpl::Definition {
-    final AssignableRead getARead() {
-      exists(BasicBlock bb, int i |
-        SsaImpl::ssaDefReachesRead(_, this, bb, i) and
-        result.getControlFlowNode() = bb.getNode(i)
-      )
-    }
+  private module SsaInput implements SsaImpl::SsaInputSig {
+    private import csharp as CS
 
-    final AssignableDefinition getDefinition() {
-      exists(BasicBlock bb, int i, SsaInput::SourceVariable v |
-        this.definesAt(v, bb, i) and
-        definitionAt(result, bb, i, v)
-      )
-    }
+    class Expr = CS::Expr;
 
-    final predicate isImplicitEntryDefinition(SsaInput::SourceVariable v) {
-      exists(BasicBlock bb |
-        this.definesAt(v, bb, -1) and
-        implicitEntryDef(_, bb, v)
-      )
-    }
+    class Parameter = CS::Parameter;
 
-    private Definition getAPhiInputOrPriorDefinition() {
-      result = this.(PhiNode).getAnInput() or
-      SsaImpl::uncertainWriteDefinitionInput(this, result)
-    }
+    class VariableWrite extends AssignableDefinition {
+      Expr asExpr() { result = this.getExpr() }
+
+      Expr getValue() { result = this.getSource() }
 
-    final Definition getAnUltimateDefinition() {
-      result = this.getAPhiInputOrPriorDefinition*() and
-      not result instanceof PhiNode
+      predicate isParameterInit(Parameter p) {
+        this.(ImplicitParameterDefinition).getParameter() = p
+      }
     }
 
-    override Location getLocation() {
-      result = this.getDefinition().getLocation()
+    predicate explicitWrite(VariableWrite w, BasicBlock bb, int i, SsaImplInput::SourceVariable v) {
+      definitionAt(w, bb, i, v)
       or
-      exists(Callable c, BasicBlock bb, SsaInput::SourceVariable v |
-        this.definesAt(v, bb, -1) and
-        implicitEntryDef(c, bb, v) and
-        result = c.getLocation()
-      )
+      entryDef(_, bb, v) and
+      i = -1 and
+      w.isParameterInit(v)
     }
   }
 
-  class PhiNode extends SsaImpl::PhiNode, Definition {
-    override Location getLocation() { result = this.getBasicBlock().getLocation() }
+  module Ssa = SsaImpl::MakeSsa<SsaInput>;
 
-    final Definition getAnInput() { SsaImpl::phiHasInputFromBlock(this, result, _) }
-  }
+  import Ssa
 }
diff --git a/csharp/ql/lib/semmle/code/csharp/dataflow/internal/Steps.qll b/csharp/ql/lib/semmle/code/csharp/dataflow/internal/Steps.qll
index 63cd0b65dc67..7e2709468d6a 100644
--- a/csharp/ql/lib/semmle/code/csharp/dataflow/internal/Steps.qll
+++ b/csharp/ql/lib/semmle/code/csharp/dataflow/internal/Steps.qll
@@ -15,8 +15,8 @@ module Steps {
    * Gets a read that may read the value assigned at definition `def`.
    */
   private AssignableRead getARead(AssignableDefinition def) {
-    exists(BaseSsa::Definition ssaDef |
-      ssaDef.getAnUltimateDefinition().getDefinition() = def and
+    exists(BaseSsa::SsaDefinition ssaDef |
+      ssaDef.getAnUltimateDefinition().(BaseSsa::SsaExplicitWrite).getDefinition() = def and
       result = ssaDef.getARead()
     )
     or
diff --git a/csharp/ql/lib/semmle/code/csharp/dispatch/Dispatch.qll b/csharp/ql/lib/semmle/code/csharp/dispatch/Dispatch.qll
index 10ea1c8eb524..6d535025a195 100644
--- a/csharp/ql/lib/semmle/code/csharp/dispatch/Dispatch.qll
+++ b/csharp/ql/lib/semmle/code/csharp/dispatch/Dispatch.qll
@@ -355,8 +355,8 @@ private module Internal {
       1 < strictcount(this.getADynamicTarget().getUnboundDeclaration()) and
       c = this.getCall().getEnclosingCallable().getUnboundDeclaration() and
       (
-        exists(BaseSsa::Definition def, Parameter p |
-          def.isImplicitEntryDefinition(p) and
+        exists(BaseSsa::SsaParameterInit def, Parameter p |
+          def.getParameter() = p and
           this.getSyntheticQualifier() = def.getARead() and
           p.getPosition() = i and
           c.getAParameter() = p and
diff --git a/csharp/ql/test/library-tests/dataflow/ssa/BaseSsaConsistency.ql b/csharp/ql/test/library-tests/dataflow/ssa/BaseSsaConsistency.ql
index 86e13215b0c4..4748f516114c 100644
--- a/csharp/ql/test/library-tests/dataflow/ssa/BaseSsaConsistency.ql
+++ b/csharp/ql/test/library-tests/dataflow/ssa/BaseSsaConsistency.ql
@@ -1,7 +1,9 @@
 import csharp
 import semmle.code.csharp.dataflow.internal.BaseSSA
 
-from AssignableRead ar, BaseSsa::Definition ssaDef, AssignableDefinition def, LocalScopeVariable v
+from
+  AssignableRead ar, BaseSsa::SsaExplicitWrite ssaDef, AssignableDefinition def,
+  LocalScopeVariable v
 where
   ar = ssaDef.getARead() and
   def = ssaDef.getDefinition() and
@@ -9,5 +11,9 @@ where
   not exists(Ssa::ExplicitDefinition edef |
     edef.getADefinition() = def and
     edef.getARead() = ar
+  ) and
+  not exists(Ssa::ImplicitParameterDefinition edef |
+    edef.getParameter() = def.(AssignableDefinitions::ImplicitParameterDefinition).getParameter() and
+    edef.getARead() = ar
   )
 select ar, def