From 10bfe2a1eef247493a0f836839cc01ceb0161a07 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?S=C3=A9bastien=20Doeraene?= <sjrdoeraene@gmail.com>
Date: Thu, 5 May 2022 11:50:52 +0200
Subject: [PATCH 1/2] Handle `Block` in `BCodeBodyBuilder.genCond`.

For `Block`s, we can push `genCond` inside the final `expr`,
reducing the number of cases where we need to go through
`loadAndTestBoolean`.

We explicitly exclude `Block`s whose `expr` is a `LabelDef`,
otherwise we short-circuit the match optimization done in
`genBlockTo`.

This is a backport of dotty's commit
https://github.com/lampepfl/dotty/commit/d260adf95b94f397a89543a8cd86122b50ecece9
---
 .../nsc/backend/jvm/BCodeBodyBuilder.scala    | 24 ++++++++++++++++---
 1 file changed, 21 insertions(+), 3 deletions(-)

diff --git a/src/compiler/scala/tools/nsc/backend/jvm/BCodeBodyBuilder.scala b/src/compiler/scala/tools/nsc/backend/jvm/BCodeBodyBuilder.scala
index e618d2359e18..794fce7045c1 100644
--- a/src/compiler/scala/tools/nsc/backend/jvm/BCodeBodyBuilder.scala
+++ b/src/compiler/scala/tools/nsc/backend/jvm/BCodeBodyBuilder.scala
@@ -977,11 +977,18 @@ abstract class BCodeBodyBuilder extends BCodeSkelBuilder {
           default()
       }
 
-      val end = currProgramPoint()
-      if (emitVars) { // add entries to LocalVariableTable JVM attribute
+      emitLocalVarScopes()
+      varsInScope = savedScope
+    }
+
+    /** Add entries to the `LocalVariableTable` JVM attribute for all the vars in
+     *  `varsInScope`, ending at the current program point.
+     */
+    def emitLocalVarScopes(): Unit = {
+      if (emitVars) {
+        val end = currProgramPoint()
         for ((sym, start) <- varsInScope.reverse) { emitLocalVarScope(sym, start, end) }
       }
-      varsInScope = savedScope
     }
 
     def adapt(from: BType, to: BType) {
@@ -1429,6 +1436,17 @@ abstract class BCodeBodyBuilder extends BCodeSkelBuilder {
                 loadAndTestBoolean()
           }
 
+        // Explicitly exclude LabelDef's here because we don't want to break the match optimization in genBlockTo.
+        // Anyway, we wouldn't be able to push `genCond` further inside a `LabelDef`, so we're not losing any opportunity.
+        case Block(stats, expr) if !expr.isInstanceOf[LabelDef] =>
+          // Push the decision further down the `expr`.
+          val savedScope = varsInScope
+          varsInScope = Nil
+          stats foreach genStat
+          genCond(expr, success, failure, targetIfNoJump)
+          emitLocalVarScopes()
+          varsInScope = savedScope
+
         case _ => loadAndTestBoolean()
       }
 

From a6e652408e7a5a131fe3f56cc992d3fb1c8a712d Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?S=C3=A9bastien=20Doeraene?= <sjrdoeraene@gmail.com>
Date: Thu, 5 May 2022 11:53:38 +0200
Subject: [PATCH 2/2] Handle `If` in `BCodeBodyBuilder.genCond()`.

This is more for completeness of `genCond()`. It can now push down
the jumps inside all the control structures for which it makes
sense to do so. It typically applies when the condition of an `if`
or `while` is an `if..else` expression.

It is however not always a win. If the result of the nested `if`s
must be converted to a boolean anyway, doing this will result in
duplication of the conversion to boolean.

This is a backport of dotty's commit
https://github.com/lampepfl/dotty/commit/32c3395233aff768a04685a328d6030b0dc0df07
---
 .../scala/tools/nsc/backend/jvm/BCodeBodyBuilder.scala   | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/src/compiler/scala/tools/nsc/backend/jvm/BCodeBodyBuilder.scala b/src/compiler/scala/tools/nsc/backend/jvm/BCodeBodyBuilder.scala
index 794fce7045c1..9e286358b962 100644
--- a/src/compiler/scala/tools/nsc/backend/jvm/BCodeBodyBuilder.scala
+++ b/src/compiler/scala/tools/nsc/backend/jvm/BCodeBodyBuilder.scala
@@ -1447,6 +1447,15 @@ abstract class BCodeBodyBuilder extends BCodeSkelBuilder {
           emitLocalVarScopes()
           varsInScope = savedScope
 
+        case If(condp, thenp, elsep) =>
+          val innerSuccess = new asm.Label
+          val innerFailure = new asm.Label
+          genCond(condp, innerSuccess, innerFailure, targetIfNoJump = innerSuccess)
+          markProgramPoint(innerSuccess)
+          genCond(thenp, success, failure, targetIfNoJump = innerFailure)
+          markProgramPoint(innerFailure)
+          genCond(elsep, success, failure, targetIfNoJump)
+
         case _ => loadAndTestBoolean()
       }