From 59ed23a9e9dee42d884e35ebf9e9862ea3724c4b Mon Sep 17 00:00:00 2001
From: Boopathi Rajaa <me@boopathi.in>
Date: Wed, 25 Jan 2017 15:29:48 +0100
Subject: [PATCH] Moar optimizations for side-effecty if statements

---
 .../__tests__/dead-code-elimination-test.js   | 34 ++++++++++++++-
 .../src/index.js                              | 42 ++++++++++++++-----
 2 files changed, 64 insertions(+), 12 deletions(-)

diff --git a/packages/babel-plugin-minify-dead-code-elimination/__tests__/dead-code-elimination-test.js b/packages/babel-plugin-minify-dead-code-elimination/__tests__/dead-code-elimination-test.js
index 34952c60f..be4f9265c 100644
--- a/packages/babel-plugin-minify-dead-code-elimination/__tests__/dead-code-elimination-test.js
+++ b/packages/babel-plugin-minify-dead-code-elimination/__tests__/dead-code-elimination-test.js
@@ -2422,12 +2422,42 @@ describe("dce-plugin", () => {
     expect(transform(source)).toBe(expected);
   });
 
-  it("should deopt impure expressions in if statements", () => {
+  it("should impure expressions in confidently evaluated if statements", () => {
     const source = unpad(`
       if (a.b(), true) {
         foo();
       }
     `);
-    expect(transform(source)).toBe(source);
+    const expected = unpad(`
+      a.b();
+
+      foo();
+    `);
+    expect(transform(source)).toBe(expected);
+  });
+
+  it("should extract all necessary things from if statements", () => {
+    const source = unpad(`
+      if (a.b(), false) {
+        var foo = foo1;
+        foo();
+      } else if (b.c(), true) {
+        var bar = bar1;
+        bar();
+      } else {
+        var baz = baz1;
+        baz();
+      }
+    `);
+    const expected = unpad(`
+      a.b();
+      b.c();
+
+      var bar = bar1;
+      bar();
+      var baz;
+      var foo;
+    `);
+    expect(transform(source)).toBe(expected);
   });
 });
diff --git a/packages/babel-plugin-minify-dead-code-elimination/src/index.js b/packages/babel-plugin-minify-dead-code-elimination/src/index.js
index 7c4583314..d795912cd 100644
--- a/packages/babel-plugin-minify-dead-code-elimination/src/index.js
+++ b/packages/babel-plugin-minify-dead-code-elimination/src/index.js
@@ -640,9 +640,16 @@ module.exports = ({ types: t, traverse }) => {
           const alternate = path.get("alternate");
           const test = path.get("test");
 
+          const evalResult = test.evaluate();
           const isPure = test.isPure();
 
-          const evaluateTest = test.evaluateTruthy();
+          const replacements = [];
+
+          if (evalResult.confident && !isPure && test.isSequenceExpression()) {
+            replacements.push(
+              t.expressionStatement(extractSequenceImpure(test))
+            );
+          }
 
           // we can check if a test will be truthy 100% and if so then we can inline
           // the consequent and completely ignore the alternate
@@ -650,10 +657,10 @@ module.exports = ({ types: t, traverse }) => {
           //   if (true) { foo; } -> { foo; }
           //   if ("foo") { foo; } -> { foo; }
           //
-          if (isPure && evaluateTest === true) {
-            path.replaceWithMultiple(
-              [...toStatements(consequent), ...extractVars(alternate)]
-            );
+          if (evalResult.confident && evalResult.value) {
+            path.replaceWithMultiple([
+              ...replacements, ...toStatements(consequent), ...extractVars(alternate)
+            ]);
             return;
           }
 
@@ -663,14 +670,16 @@ module.exports = ({ types: t, traverse }) => {
           //   if ("") { bar; } else { foo; } -> { foo; }
           //   if ("") { bar; } ->
           //
-          if (isPure && evaluateTest === false) {
+          if (evalResult.confident && !evalResult.value) {
             if (alternate.node) {
-              path.replaceWithMultiple(
-                [...toStatements(alternate), ...extractVars(consequent)]
-              );
+              path.replaceWithMultiple([
+                ...replacements, ...toStatements(alternate), ...extractVars(consequent)
+              ]);
               return;
             } else {
-              path.replaceWithMultiple(extractVars(consequent));
+              path.replaceWithMultiple([
+                ...replacements, ...extractVars(consequent)
+              ]);
             }
           }
 
@@ -700,11 +709,13 @@ module.exports = ({ types: t, traverse }) => {
           }
         },
       },
+
       EmptyStatement(path) {
         if (path.parentPath.isBlockStatement() || path.parentPath.isProgram()) {
           path.remove();
         }
       },
+
       Program: {
         exit(path, {
           opts: {
@@ -1026,4 +1037,15 @@ module.exports = ({ types: t, traverse }) => {
     } while (parent = parent.parentPath);
     return false;
   }
+
+  function extractSequenceImpure(seq) {
+    const expressions = seq.get("expressions");
+    const result = [];
+    for (let i = 0; i < expressions.length; i++) {
+      if (!expressions[i].isPure()) {
+        result.push(expressions[i].node);
+      }
+    }
+    return t.sequenceExpression(result);
+  }
 };