Python: use the shared SSA facade in the ESSA adapter

The adapter already instantiates Impl::MakeSsa<SsaInput>, so consume its cached public API for ordinary reads and phi inputs.

Synthetic normal-exit uses cannot be represented as SsaInput::Expr, so retain a narrow cached raw projection filtered to normal exits. Adjacency likewise remains a cached adapter boundary because MakeSsa does not expose it. This avoids adding broad cache policy to shared SSA.

Exact #156 to #158 DCA #39117 completed 48/48 with exact outputs, -0.748% aggregate query time, -0.541% median, and -3.29% DIL.

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>
Copilot-Session: f3f2a493-1e3f-402e-b70d-4b558ac6bf29

Author: yoff

python/ql/lib/semmle/python/dataflow/new/internal/SsaImpl.qll

@@ -334,13 +334,13 @@ private module SsaInput implements Impl::SsaInputSig {
 
 module Ssa = Impl::MakeSsa<SsaInput>;
 
-final class Definition = Impl::Definition;
+final class Definition = Ssa::SsaDefinition;
 
-final class WriteDefinition = Impl::WriteDefinition;
+final class WriteDefinition = Ssa::SsaWriteDefinition;
 
-final class UncertainWriteDefinition = Impl::UncertainWriteDefinition;
+final class UncertainWriteDefinition = Ssa::SsaUncertainWrite;
 
-final class PhiNode = Impl::PhiNode;
+final class PhiNode = Ssa::SsaPhiDefinition;
 
 // ===========================================================================
 // ESSA-shaped adapter layer
@@ -523,7 +523,7 @@ class PhiFunction extends PhiNode {
    * the phi from one of its predecessor blocks). Mirrors legacy
    * ESSA's `PhiFunction.getAnInput()`.
    */
-  Ssa::SsaDefinition getAnInput() { Impl::phiHasInputFromBlock(this, result, _) }
+  Ssa::SsaDefinition getAnInput() { result = this.(Ssa::SsaPhiDefinition).getAnInput() }
 }
 
 /** An ESSA definition (legacy-shaped). */
@@ -537,6 +537,19 @@ class EssaVariable extends Ssa::SsaDefinition {
   /** Gets the underlying SSA definition (legacy name). */
   Ssa::SsaDefinition getDefinition() { result = this }
 
+  /**
+   * Gets a synthetic normal-exit use of this definition. These uses have no
+   * `SsaInput::Expr`, so they cannot be exposed by `SsaDefinition.getARead()`.
+   */
+  cached
+  private Cfg::ControlFlowNode getASyntheticExitUse() {
+    exists(CfgImpl::BasicBlock bb, int i |
+      Impl::ssaDefReachesRead(this.getSourceVariable(), this, bb, i) and
+      bb.getNode(i) = result and
+      result.isNormalExit()
+    )
+  }
+
   /**
    * Gets a CFG node where this definition is used. Includes regular
    * `Name` reads as well as the synthetic scope-exit "use" registered
@@ -545,10 +558,9 @@ class EssaVariable extends Ssa::SsaDefinition {
    * from `SsaSourceVariable`.
    */
   Cfg::ControlFlowNode getAUse() {
-    exists(CfgImpl::BasicBlock bb, int i |
-      Impl::ssaDefReachesRead(this.getSourceVariable(), this, bb, i) and
-      bb.getNode(i) = result
-    )
+    result.getNode() = this.(Ssa::SsaDefinition).getARead().asExpr()
+    or
+    result = this.getASyntheticExitUse()
   }
 
   /** Gets the (textual) name of the underlying variable. */
@@ -563,8 +575,10 @@ class EssaVariable extends Ssa::SsaDefinition {
  * library. Provides the same interface as legacy
  * `semmle.python.essa.SsaCompute::AdjacentUses`.
  */
+cached
 module AdjacentUses {
   /** Holds if `nodeFrom` and `nodeTo` are adjacent uses of the same SSA variable. */
+  cached
   predicate adjacentUseUse(Cfg::NameNode nodeFrom, Cfg::NameNode nodeTo) {
     exists(CfgImpl::BasicBlock bb1, int i1, CfgImpl::BasicBlock bb2, int i2 |
       Impl::adjacentUseUse(bb1, i1, bb2, i2, _, _) and
@@ -574,6 +588,7 @@ module AdjacentUses {
   }
 
   /** Holds if `use` is a first use of definition `def`. */
+  cached
   predicate firstUse(Ssa::SsaDefinition def, Cfg::NameNode use) {
     exists(CfgImpl::BasicBlock bb, int i |
       Impl::firstUse(def, bb, i, _) and
@@ -585,6 +600,7 @@ module AdjacentUses {
    * Holds if `use` is any reachable use of definition `def`. Combines
    * `firstUse` with transitive use-use adjacency.
    */
+  cached
   predicate useOfDef(Ssa::SsaDefinition def, Cfg::NameNode use) {
     firstUse(def, use)
     or