From 4b33cd528a5952149ffb6db1bf271153cb9f7570 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Ji=C5=99=C3=AD=20Bene=C5=A1?= <mail@jiribenes.com>
Date: Sat, 5 Sep 2026 17:09:50 +0200
Subject: [PATCH] Drop unused pure calls

---
 .../src/test/scala/effekt/core/VMTests.scala  | 46 +++++++++----------
 .../effekt/core/optimizer/Deadcode.scala      |  9 ++++
 .../core/optimizer/StaticArguments.scala      |  9 ++--
 3 files changed, 37 insertions(+), 27 deletions(-)

diff --git a/effekt/jvm/src/test/scala/effekt/core/VMTests.scala b/effekt/jvm/src/test/scala/effekt/core/VMTests.scala
index 5ffb5417b7..238651e00a 100644
--- a/effekt/jvm/src/test/scala/effekt/core/VMTests.scala
+++ b/effekt/jvm/src/test/scala/effekt/core/VMTests.scala
@@ -382,13 +382,13 @@ class VMTests extends munit.FunSuite {
 
   val duality_of_compilation: Seq[(File, Option[Summary])] = Seq(
     examplesDir / "benchmarks" / "duality_of_compilation" / "erase_unused.effekt" -> Some(Summary(
-      staticDispatches = 21,
+      staticDispatches = 6,
       dynamicDispatches = 0,
       patternMatches = 0,
-      branches = 21,
-      pushedFrames = 6,
-      poppedFrames = 6,
-      allocations = 11,
+      branches = 6,
+      pushedFrames = 1,
+      poppedFrames = 1,
+      allocations = 0,
       closures = 0,
       variableReads = 0,
       variableWrites = 0,
@@ -1089,14 +1089,14 @@ class VMTests extends munit.FunSuite {
       resumes = 0
     )),
     examplesDir / "benchmarks" / "nofib" / "constraints.effekt" -> Some(Summary(
-      staticDispatches = 891409,
-      dynamicDispatches = 60355,
-      patternMatches = 1382034,
-      branches = 432810,
-      pushedFrames = 794941,
-      poppedFrames = 794941,
-      allocations = 852789,
-      closures = 10,
+      staticDispatches = 670319,
+      dynamicDispatches = 31053,
+      patternMatches = 1012640,
+      branches = 297136,
+      pushedFrames = 602935,
+      poppedFrames = 602935,
+      allocations = 642347,
+      closures = 6,
       variableReads = 0,
       variableWrites = 0,
       resets = 0,
@@ -1149,16 +1149,16 @@ class VMTests extends munit.FunSuite {
       resumes = 0
     )),
     examplesDir / "benchmarks" / "nofib" / "integer.effekt" -> Some(Summary(
-      staticDispatches = 4026,
-      dynamicDispatches = 792,
-      patternMatches = 2928,
-      branches = 1079,
-      pushedFrames = 3080,
-      poppedFrames = 3080,
-      allocations = 4532,
-      closures = 22,
-      variableReads = 924,
-      variableWrites = 792,
+      staticDispatches = 182,
+      dynamicDispatches = 0,
+      patternMatches = 135,
+      branches = 50,
+      pushedFrames = 104,
+      poppedFrames = 104,
+      allocations = 206,
+      closures = 0,
+      variableReads = 42,
+      variableWrites = 36,
       resets = 0,
       shifts = 0,
       resumes = 0
diff --git a/effekt/shared/src/main/scala/effekt/core/optimizer/Deadcode.scala b/effekt/shared/src/main/scala/effekt/core/optimizer/Deadcode.scala
index e2d306fae5..6d019fa584 100644
--- a/effekt/shared/src/main/scala/effekt/core/optimizer/Deadcode.scala
+++ b/effekt/shared/src/main/scala/effekt/core/optimizer/Deadcode.scala
@@ -13,11 +13,20 @@ class Deadcode(reachable: Map[Id, Usage])
 
   private def unused(id: Id): Boolean = !used(id)
 
+  /** A call that cannot be observed: the callee captures nothing, and neither does anything passed to it. */
+  private def isPureCall(stmt: Stmt): Boolean = stmt match {
+    case Stmt.App(callee, _, _, bargs) => callee.capt.isEmpty && bargs.forall(_.capt.isEmpty) && stmt.tpe != Type.TBottom
+    case _ => false
+  }
+
   override def rewrite(stmt: Stmt): Trampoline[Stmt] = stmt match {
     // Remove local unused definitions
     case Stmt.Def(id, block, body) if unused(id) => rewrite(body)
     case Stmt.Let(id, binding, body) if unused(id) => rewrite(body)
 
+    // Remove local unused 'val's as long as they are pure calls.
+    case Stmt.Val(id, binding, body) if unused(id) && isPureCall(binding) => rewrite(body)
+
     case Stmt.Reset(body) =>
       rewrite(body).map {
         case BlockLit(tparams, cparams, vparams, List(prompt), body) if unused(prompt.id) => body
diff --git a/effekt/shared/src/main/scala/effekt/core/optimizer/StaticArguments.scala b/effekt/shared/src/main/scala/effekt/core/optimizer/StaticArguments.scala
index bae4a51f1f..4b2a335fc3 100644
--- a/effekt/shared/src/main/scala/effekt/core/optimizer/StaticArguments.scala
+++ b/effekt/shared/src/main/scala/effekt/core/optimizer/StaticArguments.scala
@@ -155,17 +155,18 @@ object StaticArguments {
             case _ => false
           }
         }
+        lazy val read = body.free.freeIds // TODO: use Free.contains from #1453; reconsider free vs capt
         val isValueStatic = vparams.zipWithIndex.collect {
-          case (param, index) => vargs.nonEmpty && vargs.map(args => args(index)).forall {
+          case (param, index) => (vargs.nonEmpty && vargs.map(args => args(index)).forall {
             case ValueVar(other, _) => param.id == other
             case _ => false
-          }
+          }) || !read.contains(param.id) // vparam not read by body is vacuously static
         }
         val isBlockStatic = bparams.zipWithIndex.collect {
-          case (param, index) => bargs.nonEmpty && bargs.map(args => args(index)).forall {
+          case (param, index) => (bargs.nonEmpty && bargs.map(args => args(index)).forall {
             case BlockVar(other, _, _) => param.id == other
             case _ => false
-          }
+          }) || !read.contains(param.id) // bparam not read by body is vacuously static
         }
         id -> IsStatic(isTypeStatic, isValueStatic, isBlockStatic)
     }.toMap